This text is a brief description of the features that are present in the Bash shell. This is Edition 2.2, last updated 1 April 1998, of `The GNU Bash Reference Manual', for `Bash', Version 2.02. Copyright (C) 1991, 1993, 1996, 1997 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Free Software Foundation. Bash Features ************* This text is a brief description of the features that are present in the Bash shell. This is Edition 2.2, last updated 1 April 1998, of `The GNU Bash Reference Manual', for `Bash', Version 2.02. Copyright (C) 1991, 1993, 1996 Free Software Foundation, Inc. Bash contains features that appear in other popular shells, and some features that only appear in Bash. Some of the shells that Bash has borrowed concepts from are the Bourne Shell (`sh'), the Korn Shell (`ksh'), and the C-shell (`csh' and its successor, `tcsh'). The following menu breaks the features up into categories based upon which one of these other shells inspired the feature. This manual is meant as a brief introduction to features found in Bash. The Bash manual page should be used as the definitive reference on shell behavior. Introduction ************ What is Bash? ============= Bash is the shell, or command language interpreter, that will appear in the GNU operating system. The name is an acronym for the `Bourne-Again SHell', a pun on Steve Bourne, the author of the direct ancestor of the current Unix shell `/bin/sh', which appeared in the Seventh Edition Bell Labs Research version of Unix. Bash is an `sh'-compatible shell that incorporates useful features from the Korn shell `ksh' and the C shell `csh'. It is intended to be a conformant implementation of the IEEE POSIX Shell and Tools specification (IEEE Working Group 1003.2). It offers functional improvements over `sh' for both interactive and programming use. While the GNU operating system will include a version of `csh', Bash will be the default shell. Like other GNU software, Bash is quite portable. It currently runs on nearly every version of Unix and a few other operating systems - independently-supported ports exist for MS-DOS, OS/2, Windows 95, and Windows NT. What is a shell? ================ At its base, a shell is simply a macro processor that executes commands. A Unix shell is both a command interpreter, which provides the user interface to the rich set of Unix utilities, and a programming language, allowing these utilitites to be combined. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories like `/bin', allowing users or groups to establish custom environments. A shell allows execution of Unix commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asynchronous commands continue to execute in parallel with the shell while it reads and executes additional commands. The "redirection" constructs permit fine-grained control of the input and output of those commands, and the shell allows control over the contents of their environment. Unix shells also provide a small set of built-in commands ("builtins") implementing functionality impossible (e.g., `cd', `break', `continue', and `exec'), or inconvenient (`history', `getopts', `kill', or `pwd', for example) to obtain via separate utilities. Shells may be used interactively or non-interactively: they accept input typed from the keyboard or from a file. All of the shell builtins are described in subsequent sections. While executing commands is essential, most of the power (and complexity) of shells is due to their embedded programming languages. Like any high-level language, the shell provides variables, flow control constructs, quoting, and functions. Shells have begun offering features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, history and aliases. Each of these features is described in this manual. Definitions *********** These definitions are used throughout the remainder of this manual. `POSIX' A family of open system standards based on Unix. Bash is concerned with POSIX 1003.2, the Shell and Tools Standard. `blank' A space or tab character. `builtin' A command that is implemented internally by the shell itself, rather than by an executable program somewhere in the file system. `control operator' A `word' that performs a control function. It is a `newline' or one of the following: `||', `&&', `&', `;', `;;', `|', `(', or `)'. `exit status' The value returned by a command to its caller. `field' A unit of text that is the result of one of the shell expansions. After expansion, when executing a command, the resulting fields are used as the command name and arguments. `filename' A string of characters used to identify a file. `job' A set of processes comprising a pipeline, and any processes descended from it, that are all in the same process group. `job control' A mechanism by which users can selectively stop (suspend) and restart (resume) execution of processes. `metacharacter' A character that, when unquoted, separates words. A metacharacter is a `blank' or one of the following characters: `|', `&', `;', `(', `)', `<', or `>'. `name' A `word' consisting solely of letters, numbers, and underscores, and beginning with a letter or underscore. `Name's are used as shell variable and function names. Also referred to as an `identifier'. `operator' A `control operator' or a `redirection operator'. *Note Redirections::, for a list of redirection operators. `process group' A collection of related processes each having the same process group ID. `process group ID' A unique identifer that represents a `process group' during its lifetime. `reserved word' A `word' that has a special meaning to the shell. Most reserved words introduce shell flow control constructs, such as `for' and `while'. `return status' A synonym for `exit status'. `signal' A mechanism by which a process may be notified by the kernal of an event occurring in the system. `special builtin' A shell builtin command that has been classified as special by the POSIX.2 standard. `token' A sequence of characters considered a single unit by the shell. It is either a `word' or an `operator'. `word' A `token' that is not an `operator'. Basic Shell Features ******************** Bash is an acronym for `Bourne-Again SHell'. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, and the rules for evaluation and quoting are taken from the POSIX 1003.2 specification for the `standard' Unix shell. This chapter briefly summarizes the shell's `building blocks': commands, control structures, shell functions, shell parameters, shell expansions, redirections, which are a way to direct input and output from and to named files, and how the shell executes commands. Shell Syntax ============ Shell Operation --------------- The following is a brief description of the shell's operation when it reads and executes a command. Basically, the shell does the following: 1. Reads its input from a file (*note Shell Scripts::.), from a string supplied as an argument to the `-c' invocation option (*note Invoking Bash::.), or from the user's terminal. 2. Breaks the input into words and operators, obeying the quoting rules described in *Note Quoting::. These tokens are separated by `metacharacters'. Alias expansion is performed by this step (*note Aliases::.). 3. Parses the tokens into simple and compound commands (*note Shell Commands::.). 4. Performs the various shell expansions (*note Shell Expansions::.), breaking the expanded tokens into lists of filenames (*note Filename Expansion::.) and commands and arguments. 5. Performs any necessary redirections (*note Redirections::.) and removes the redirection operators and their operands from the argument list. 6. Executes the command (*note Executing Commands::.). 7. Optionally waits for the command to complete and collects its exit status (*note Exit Status::.). Quoting ------- Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion. Each of the shell metacharacters (*note Definitions::.) has special meaning to the shell and must be quoted if it is to represent itself. There are three quoting mechanisms: the ESCAPE CHARACTER, single quotes, and double quotes. Escape Character ................ A non-quoted backslash `\' is the Bash escape character. It preserves the literal value of the next character that follows, with the exception of `newline'. If a `\newline' pair appears, and the backslash itself is not quoted, the `\newline' is treated as a line continuation (that is, it is removed from the input stream and effectively ignored). Single Quotes ............. Enclosing characters in single quotes preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash. Double Quotes ............. Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception of `$', ``', and `\'. The characters `$' and ``' retain their special meaning within double quotes (*note Shell Expansions::.). The backslash retains its special meaning only when followed by one of the following characters: `$', ``', `"', `\', or `newline'. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. The special parameters `*' and `@' have special meaning when in double quotes (*note Shell Parameter Expansion::.). ANSI-C Quoting .............. Words of the form `$'STRING'' are treated specially. The word expands to STRING, with backslash-escaped characters replaced as specifed by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows: `\a' alert (bell) `\b' backspace `\e' an escape character (not ANSI C) `\f' form feed `\n' newline `\r' carriage return `\t' horizontal tab `\v' vertical tab `\\' backslash `\NNN' the character whose `ASCII' code is the octal value NNN (one to three digits) `\xNNN' the character whose `ASCII' code is the hexadecimal value NNN (one to three digits) The result is single-quoted, as if the dollar sign had not been present. Locale-Specific Translation ........................... A double-quoted string preceded by a dollar sign (`$') will cause the string to be translated according to the current locale. If the current locale is `C' or `POSIX', the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted. Comments -------- In a non-interactive shell, or an interactive shell in which the `interactive_comments' option to the `shopt' builtin is enabled (*note Bash Builtins::.), a word beginning with `#' causes that word and all remaining characters on that line to be ignored. An interactive shell without the `interactive_comments' option enabled does not allow comments. The `interactive_comments' option is on by default in interactive shells. *Note Is This Shell Interactive?::, for a description of what makes a shell interactive. Shell Commands ============== Simple Commands --------------- A simple command is the kind of command encountered most often. It's just a sequence of words separated by `blank's, terminated by one of the shell's control operators (*note Definitions::.). The first word generally specifies a command to be executed. The return status (*note Exit Status::.) of a simple command is its exit status as provided by the POSIX.1 `waitpid' function, or 128+N if the command was terminated by signal N. Pipelines --------- A `pipeline' is a sequence of simple commands separated by `|'. The format for a pipeline is [`time' [`-p']] [`!'] COMMAND1 [`|' COMMAND2 ...] The output of each command in the pipeline is connected to the input of the next command. That is, each command reads the previous command's output. The reserved word `time' causes timing statistics to be printed for the pipeline once it finishes. The statistics currently consist of elapsed (wall-clock) time and user and system time consumed by the command's execution. The `-p' option changes the output format to that specified by POSIX. The `TIMEFORMAT' variable may be set to a format string that specifies how the timing information should be displayed. *Note Bash Variables::, for a description of the available formats. The use of `time' as a reserved word permits the timing of shell builtins, shell functions, and pipelines. An external `time' command cannot time these easily. If the pipeline is not executed asynchronously (*note Lists::.), the shell waits for all commands in the pipeline to complete. Each command in a pipeline is executed in its own subshell (*note Command Execution Environment::.). The exit status of a pipeline is the exit status of the last command in the pipeline. If the reserved word `!' precedes the pipeline, the exit status is the logical negation of the exit status of the last command. Lists of Commands ----------------- A `list' is a sequence of one or more pipelines separated by one of the operators `;', `&', `&&', or `||', and optionally terminated by one of `;', `&', or a `newline'. Of these list operators, `&&' and `||' have equal precedence, followed by `;' and `&', which have equal precedence. If a command is terminated by the control operator `&', the shell executes the command asynchronously in a subshell. This is known as executing the command in the BACKGROUND. The shell does not wait for the command to finish, and the return status is 0 (true). The standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from `/dev/null'. Commands separated by a `;' are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed. The control operators `&&' and `||' denote AND lists and OR lists, respectively. An AND list has the form COMMAND && COMMAND2 COMMAND2 is executed if, and only if, COMMAND returns an exit status of zero. An OR list has the form COMMAND || COMMAND2 COMMAND2 is executed if, and only if, COMMAND returns a non-zero exit status. The return status of AND and OR lists is the exit status of the last command executed in the list. Looping Constructs ------------------ Bash supports the following looping constructs. Note that wherever you see a `;' in the description of a command's syntax, it may be replaced with one or more newlines. `until' The syntax of the `until' command is: until TEST-COMMANDS; do CONSEQUENT-COMMANDS; done Execute CONSEQUENT-COMMANDS as long as TEST-COMMANDS has an exit status which is not zero. The return status is the exit status of the last command executed in CONSEQUENT-COMMANDS, or zero if none was executed. `while' The syntax of the `while' command is: while TEST-COMMANDS; do CONSEQUENT-COMMANDS; done Execute CONSEQUENT-COMMANDS as long as TEST-COMMANDS has an exit status of zero. The return status is the exit status of the last command executed in CONSEQUENT-COMMANDS, or zero if none was executed. `for' The syntax of the `for' command is: for NAME [in WORDS ...]; do COMMANDS; done Expand WORDS, and execute COMMANDS once for each member in the resultant list, with NAME bound to the current member. If `in WORDS' is not present, `in "$@"' is assumed. The return status is the exit status of the last command that executes. If there are no items in the expansion of WORDS, no commands are executed, and the return status is zero. The `break' and `continue' builtins (*note Bourne Shell Builtins::.) may be used to control loop execution. Conditional Constructs ---------------------- `if' The syntax of the `if' command is: if TEST-COMMANDS; then CONSEQUENT-COMMANDS; [elif MORE-TEST-COMMANDS; then MORE-CONSEQUENTS;] [else ALTERNATE-CONSEQUENTS;] fi The TEST-COMMANDS list is executed, and if its return status is zero, the CONSEQUENT-COMMANDS list is executed. If TEST-COMMANDS returns a non-zero status, each `elif' list is executed in turn, and if its exit status is zero, the corresponding MORE-CONSEQUENTS is executed and the command completes. If `else ALTERNATE-CONSEQUENTS' is present, and the final command in the final `if' or `elif' clause has a non-zero exit status, then ALTERNATE-CONSEQUENTS is executed. The return status is the exit status of the last command executed, or zero if no condition tested true. `case' The syntax of the `case' command is: `case WORD in [ [(] PATTERN [| PATTERN]...) COMMAND-LIST ;;]... esac' `case' will selectively execute the COMMAND-LIST corresponding to the first PATTERN that matches WORD. The `|' is used to separate multiple patterns, and the `)' operator terminates a pattern list. A list of patterns and an associated command-list is known as a CLAUSE. Each clause must be terminated with `;;'. The WORD undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before matching is attempted. Each PATTERN undergoes tilde expansion, parameter expansion, command substitution, and arithmetic expansion. There may be an arbitrary number of `case' clauses, each terminated by a `;;'. The first pattern that matches determines the command-list that is executed. Here is an example using `case' in a script that could be used to describe one interesting feature of an animal: echo -n "Enter the name of an animal: " read ANIMAL echo -n "The $ANIMAL has " case $ANIMAL in horse | dog | cat) echo -n "four";; man | kangaroo ) echo -n "two";; *) echo -n "an unknown number of";; esac echo " legs." The return status is zero if no PATTERN is matched. Otherwise, the return status is the exit status of the COMMAND-LIST executed. `select' The `select' construct allows the easy generation of menus. It has almost the same syntax as the `for' command: select NAME [in WORDS ...]; do COMMANDS; done The list of words following `in' is expanded, generating a list of items. The set of expanded words is printed on the standard error output stream, each preceded by a number. If the `in WORDS' is omitted, the positional parameters are printed, as if `in "$@"' had been specifed. The `PS3' prompt is then displayed and a line is read from the standard input. If the line consists of a number corresponding to one of the displayed words, then the value of NAME is set to that word. If the line is empty, the words and prompt are displayed again. If `EOF' is read, the `select' command completes. Any other value read causes NAME to be set to null. The line read is saved in the variable `REPLY'. The COMMANDS are executed after each selection until a `break' or `return' command is executed, at which point the `select' command completes. Here is an example that allows the user to pick a filename from the current directory, and displays the name and index of the file selected. select fname in *; do echo you picked $fname \($REPLY\) break; done `((...))' (( EXPRESSION )) The arithmetic EXPRESSION is evaluated according to the rules described below (*note Shell Arithmetic::.). If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to let "EXPRESSION" *Note Bash Builtins::, for a full description of the `let' builtin. `[[...]]' [[ EXPRESSION ]] Return a status of 0 or 1 depending on the evaluation of the conditional expression EXPRESSION. Expressions are composed of the primaries described below in *Note Bash Conditional Expressions::. Word splitting and filename expansion are not performed on the words between the `[[' and `]]'; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. When the `==' and `!=' operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in *Note Pattern Matching::. The return value is 0 if the string matches or does not match the pattern, respectively, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string. Expressions may be combined using the following operators, listed in decreasing order of precedence: `( EXPRESSION )' Returns the value of EXPRESSION. This may be used to override the normal precedence of operators. `! EXPRESSION' True if EXPRESSION is false. `EXPRESSION1 && EXPRESSION2' True if both EXPRESSION1 and EXPRESSION2 are true. `EXPRESSION1 || EXPRESSION2' True if either EXPRESSION1 or EXPRESSION2 is true. The && and || commands do not execute EXPRESSION2 if the value of EXPRESSION1 is sufficient to determine the return value of the entire conditional expression. Grouping Commands ----------------- Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream. `()' ( LIST ) Placing a list of commands between parentheses causes a subshell to be created, and each of the commands in LIST to be executed in that subshell. Since the LIST is executed in a subshell, variable assignments do not remain in effect after the subshell completes. `{}' { LIST; } Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The semicolon (or newline) following LIST is required. In addition to the creation of a subshell, there is a subtle difference between these two constructs due to historical reasons. The braces are `reserved words', so they must be separated from the LIST by `blank's. The parentheses are `operators', and are recognized as separate tokens by the shell even if they are not separated from the LIST by whitespace. The exit status of both of these constructs is the exit status of LIST. Shell Functions =============== Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. Shell functions are executed in the current shell context; no new process is created to interpret them. Functions are declared using this syntax: [ `function' ] NAME () { COMMAND-LIST; } This defines a shell function named NAME. The reserved word `function' is optional. If the `function' reserved word is supplied, the parentheses are optional. The BODY of the function is the COMMAND-LIST between { and }. This list is executed whenever NAME is specified as the name of a command. The exit status of a function is the exit status of the last command executed in the body. When a function is executed, the arguments to the function become the positional parameters during its execution (*note Positional Parameters::.). The special parameter `#' that expands to the number of positional parameters is updated to reflect the change. Positional parameter `0' is unchanged. If the builtin command `return' is executed in a function, the function completes and execution resumes with the next command after the function call. When a function completes, the values of the positional parameters and the special parameter `#' are restored to the values they had prior to the function's execution. If a numeric argument is given to `return', that is the function's return status; otherwise the functions's return status is the exit status of the last command executed before the `return'. Variables local to the function may be declared with the `local' builtin. These variables are visible only to the function and the commands it invokes. Functions may be recursive. No limit is placed on the number of recursive calls. Shell Parameters ================ A PARAMETER is an entity that stores values. It can be a `name', a number, or one of the special characters listed below. For the shell's purposes, a VARIABLE is a parameter denoted by a `name'. A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the `unset' builtin command. A variable may be assigned to by a statement of the form NAME=[VALUE] If VALUE is not given, the variable is assigned the null string. All VALUEs undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (detailed below). If the variable has its `integer' attribute set (see the description of the `declare' builtin in *Note Bash Builtins::), then VALUE is subject to arithmetic expansion even if the `$((...))' expansion is not used (*note Arithmetic Expansion::.). Word splitting is not performed, with the exception of `"$@"' as explained below. Filename expansion is not performed. Positional Parameters --------------------- A POSITIONAL PARAMETER is a parameter denoted by one or more digits, other than the single digit `0'. Positional parameters are assigned from the shell's arguments when it is invoked, and may be reassigned using the `set' builtin command. Positional parameter `N' may be referenced as `${N}'. Positional parameters may not be assigned to with assignment statements. The positional parameters are temporarily replaced when a shell function is executed (*note Shell Functions::.). When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces. Special Parameters ------------------ The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed. `*' Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the `IFS' special variable. That is, `"$*"' is equivalent to `"$1C$2C..."', where C is the first character of the value of the `IFS' variable. If `IFS' is unset, the parameters are separated by spaces. If `IFS' is null, the parameters are joined without intervening separators. `@' Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands to a separate word. That is, `"$@"' is equivalent to `"$1" "$2" ...'. When there are no positional parameters, `"$@"' and `$@' expand to nothing (i.e., they are removed). `#' Expands to the number of positional parameters in decimal. `?' Expands to the exit status of the most recently executed foreground pipeline. `-' Expands to the current option flags as specified upon invocation, by the `set' builtin command, or those set by the shell itself (such as the `-i' option). `$' Expands to the process ID of the shell. In a `()' subshell, it expands to the process ID of the invoking shell, not the subshell. `!' Expands to the process ID of the most recently executed background (asynchronous) command. `0' Expands to the name of the shell or shell script. This is set at shell initialization. If Bash is invoked with a file of commands (*note Shell Scripts::.), `$0' is set to the name of that file. If Bash is started with the `-c' option (*note Invoking Bash::.), then `$0' is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the filename used to invoke Bash, as given by argument zero. `_' At shell startup, set to the absolute filename of the shell or shell script being executed as passed in the argument list. Subsequently, expands to the last argument to the previous command, after expansion. Also set to the full pathname of each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file. Shell Expansions ================ Expansion is performed on the command line after it has been split into `token's. There are seven kinds of expansion performed: * brace expansion * tilde expansion * parameter and variable expansion * command substitution * arithmetic expansion * word splitting * filename expansion The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion. On systems that can support it, there is an additional expansion available: PROCESS SUBSTITUTION. This is performed at the same time as parameter, variable, and arithmetic expansion and command substitution. Only brace expansion, word splitting, and filename expansion can change the number of words of the expansion; other expansions expand a single word to a single word. The only exceptions to this are the expansions of `"$@"' (*note Special Parameters::.) and `"${NAME[@]}"' (*note Arrays::.). After all expansions, `quote removal' (*note Quote Removal::.) is performed. Brace Expansion --------------- Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to FILENAME EXPANSION (*note Filename Expansion::.), but the file names generated need not exist. Patterns to be brace expanded take the form of an optional PREAMBLE, followed by a series of comma-separated strings between a pair of braces, followed by an optional POSTSCRIPT. The preamble is prepended to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right. Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example, bash$ echo a{d,c,b}e ade ace abe Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces. A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma. Any incorrectly formed brace expansion is left unchanged. This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example: mkdir /usr/local/src/bash/{old,new,dist,bugs} or chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}} Tilde Expansion --------------- If a word begins with an unquoted tilde character (`~'), all of the characters up to the first unquoted slash (or all characters, if there is no unquoted slash) are considered a TILDE-PREFIX. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following the tilde are treated as a possible LOGIN NAME. If this login name is the null string, the tilde is replaced with the value of the `HOME' shell variable. If `HOME' is unset, the home directory of the user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name. If the tilde-prefix is `~+', the value of the shell variable `PWD' replaces the tilde-prefix. If the tilde-prefix is `~-', the value of the shell variable `OLDPWD', if it is set, is substituted. If the characters following the tilde in the tilde-prefix consist of a number N, optionally prefixed by a `+' or a `-', the tilde-prefix is replaced with the corresponding element from the directory stack, as it would be displayed by the `dirs' builtin invoked with the characters following tilde in the tilde-prefix as an argument (*note The Directory Stack::.). If the tilde-prefix, sans the tilde, consists of a number without a leading `+' or `-', `+' is assumed. If the login name is invalid, or the tilde expansion fails, the word is left unchanged. Each variable assignment is checked for unquoted tilde-prefixes immediately following a `:' or `='. In these cases, tilde expansion is also performed. Consequently, one may use file names with tildes in assignments to `PATH', `MAILPATH', and `CDPATH', and the shell assigns the expanded value. The following table shows how Bash treats unquoted tilde-prefixes: `~' The value of `$HOME' `~/foo' `$HOME/foo' `~fred/foo' The subdirectory `foo' of the home directory of the user `fred' `~+/foo' `$PWD/foo' `~-/foo' `${OLDPWD-'~-'}/foo' `~N' The string that would be displayed by `dirs +N' `~+N' The string that would be displayed by `dirs +N' `~-N' The string that would be displayed by `dirs -N' Shell Parameter Expansion ------------------------- The `$' character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name. When braces are used, the matching ending brace is the first `}' not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion. The basic form of parameter expansion is ${PARAMETER}. The value of PARAMETER is substituted. The braces are required when PARAMETER is a positional parameter with more than one digit, or when PARAMETER is followed by a character that is not to be interpreted as part of its name. If the first character of PARAMETER is an exclamation point, a level of variable indirection is introduced. Bash uses the value of the variable formed from the rest of PARAMETER as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of PARAMETER itself. This is known as `indirect expansion'. In each of the cases below, WORD is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. When not performing substring expansion, Bash tests for a parameter that is unset or null; omitting the colon results in a test only for a parameter that is unset. `${PARAMETER:-WORD}' If PARAMETER is unset or null, the expansion of WORD is substituted. Otherwise, the value of PARAMETER is substituted. `${PARAMETER:=WORD}' If PARAMETER is unset or null, the expansion of WORD is assigned to PARAMETER. The value of PARAMETER is then substituted. Positional parameters and special parameters may not be assigned to in this way. `${PARAMETER:?WORD}' If PARAMETER is null or unset, the expansion of WORD (or a message to that effect if WORD is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of PARAMETER is substituted. `${PARAMETER:+WORD}' If PARAMETER is null or unset, nothing is substituted, otherwise the expansion of WORD is substituted. `${PARAMETER:OFFSET}' `${PARAMETER:OFFSET:LENGTH}' Expands to up to LENGTH characters of PARAMETER, starting at the character specified by OFFSET. If LENGTH is omitted, expands to the substring of PARAMETER, starting at the character specified by OFFSET. LENGTH and OFFSET are arithmetic expressions (*note Shell Arithmetic::.). This is referred to as Substring Expansion. LENGTH must evaluate to a number greater than or equal to zero. If OFFSET evaluates to a number less than zero, the value is used as an offset from the end of the value of PARAMETER. If PARAMETER is `@', the result is LENGTH positional parameters beginning at OFFSET. If PARAMETER is an array name indexed by `@' or `*', the result is the LENGTH members of the array beginning with `${PARAMETER[OFFSET]}'. Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1. `${#PARAMETER}' The length in characters of the expanded value of PARAMETER is substituted. If PARAMETER is `*' or `@', the value substituted is the number of positional parameters. If PARAMETER is an array name subscripted by `*' or `@', the value substituted is the number of elements in the array. `${PARAMETER#WORD}' `${PARAMETER##WORD}' The WORD is expanded to produce a pattern just as in filename expansion (*note Filename Expansion::.). If the pattern matches the beginning of the expanded value of PARAMETER, then the result of the expansion is the expanded value of PARAMETER with the shortest matching pattern (the `#' case) or the longest matching pattern (the `##' case) deleted. If PARAMETER is `@' or `*', the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If PARAMETER is an array variable subscripted with `@' or `*', the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list. `${PARAMETER%WORD}' `${PARAMETER%%WORD}' The WORD is expanded to produce a pattern just as in filename expansion. If the pattern matches a trailing portion of the expanded value of PARAMETER, then the result of the expansion is the value of PARAMETER with the shortest matching pattern (the `%' case) or the longest matching pattern (the `%%' case) deleted. If PARAMETER is `@' or `*', the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If PARAMETER is an array variable subscripted with `@' or `*', the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list. `${PARAMETER/PATTERN/STRING}' `${PARAMETER//PATTERN/STRING}' The PATTERN is expanded to produce a pattern just as in filename expansion. PARAMETER is expanded and the longest match of PATTERN against its value is replaced with STRING. In the first form, only the first match is replaced. The second form causes all matches of PATTERN to be replaced with STRING. If PATTERN begins with `#', it must match at the beginning of STRING. If PATTERN begins with `%', it must match at the end of STRING. If STRING is null, matches of PATTERN are deleted and the `/' following PATTERN may be omitted. If PARAMETER is `@' or `*', the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If PARAMETER is an array variable subscripted with `@' or `*', the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list. Command Substitution -------------------- Command substitution allows the output of a command to replace the command name. There are two forms: $(COMMAND) or `COMMAND` Bash performs the expansion by executing COMMAND and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting. The command substitution `$(cat FILE)' can be replaced by the equivalent but faster `$(< FILE)'. When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by `$', ``', or `\'. The first backquote not preceded by a backslash terminates the command substitution. When using the `$(COMMAND)' form, all characters between the parentheses make up the command; none are treated specially. Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes. If the substitution appears within double quotes, word splitting and filename expansion are not performed on the results. Arithmetic Expansion -------------------- Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is: $(( EXPRESSION )) The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated specially. All tokens in the expression undergo parameter expansion, command substitution, and quote removal. Arithmetic substitutions may be nested. The evaluation is performed according to the rules listed below (*note Shell Arithmetic::.). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs. Process Substitution -------------------- Process substitution is supported on systems that support named pipes (FIFOs) or the `/dev/fd' method of naming open files. It takes the form of <(LIST) or >(LIST) The process LIST is run with its input or output connected to a FIFO or some file in `/dev/fd'. The name of this file is passed as an argument to the current command as the result of the expansion. If the `>(LIST)' form is used, writing to the file will provide input for LIST. If the `<(LIST)' form is used, the file passed as an argument should be read to obtain the output of LIST. When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion. Word Splitting -------------- The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting. The shell treats each character of `$IFS' as a delimiter, and splits the results of the other expansions into words on these characters. If `IFS' is unset, or its value is exactly `', the default, then any sequence of `IFS' characters serves to delimit words. If `IFS' has a value other than the default, then sequences of the whitespace characters `space' and `tab' are ignored at the beginning and end of the word, as long as the whitespace character is in the value of `IFS' (an `IFS' whitespace character). Any character in `IFS' that is not `IFS' whitespace, along with any adjacent `IFS' whitespace characters, delimits a field. A sequence of `IFS' whitespace characters is also treated as a delimiter. If the value of `IFS' is null, no word splitting occurs. Explicit null arguments (`""' or `''') are retained. Unquoted implicit null arguments, resulting from the expansion of PARAMETERs that have no values, are removed. If a parameter with no value is expanded within double quotes, a null argument results and is retained. Note that if no expansion occurs, no splitting is performed. Filename Expansion ------------------ After word splitting, unless the `-f' option has been set (*note The Set Builtin::.), Bash scans each word for the characters `*', `?', `(', and `['. If one of these characters appears, then the word is regarded as a PATTERN, and replaced with an alphabetically sorted list of file names matching the pattern. If no matching file names are found, and the shell option `nullglob' is disabled, the word is left unchanged. If the `nullglob' option is set, and no matches are found, the word is removed. If the shell option `nocaseglob' is enabled, the match is performed without regard to the case of alphabetic characters. When a pattern is used for filename generation, the character `.' at the start of a filename or immediately following a slash must be matched explicitly, unless the shell option `dotglob' is set. When matching a file name, the slash character must always be matched explicitly. In other cases, the `.' character is not treated specially. See the description of `shopt' in *Note Bash Builtins::, for a description of the `nocaseglob', `nullglob', and `dotglob' options. The `GLOBIGNORE' shell variable may be used to restrict the set of filenames matching a pattern. If `GLOBIGNORE' is set, each matching filename that also matches one of the patterns in `GLOBIGNORE' is removed from the list of matches. The filenames `.' and `..' are always ignored, even when `GLOBIGNORE' is set. However, setting `GLOBIGNORE' has the effect of enabling the `dotglob' shell option, so all other filenames beginning with a `.' will match. To get the old behavior of ignoring filenames beginning with a `.', make `.*' one of the patterns in `GLOBIGNORE'. The `dotglob' option is disabled when `GLOBIGNORE' is unset. Pattern Matching ................ Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The NUL character may not occur in a pattern. The special pattern characters must be quoted if they are to be matched literally. The special pattern characters have the following meanings: `*' Matches any string, including the null string. `?' Matches any single character. `[...]' Matches any one of the enclosed characters. A pair of characters separated by a minus sign denotes a RANGE; any character lexically between those two characters, inclusive, is matched. If the first character following the `[' is a `!' or a `^' then any character not enclosed is matched. A `-' may be matched by including it as the first or last character in the set. A `]' may be matched by including it as the first character in the set. Within `[' and `]', CHARACTER CLASSES can be specified using the syntax `[:'CLASS`:]', where CLASS is one of the following classes defined in the POSIX.2 standard: alnum alpha ascii blank cntrl digit graph lower print punct space upper xdigit A character class matches any character belonging to that class. Within `[' and `]', an EQUIVALENCE CLASS can be specified using the syntax `[='C`=]', which matches all characters with the same collation weight (as defined by the current locale) as the character C. Within `[' and `]', the syntax `[.'SYMBOL`.]' matches the collating symbol SYMBOL. If the `extglob' shell option is enabled using the `shopt' builtin, several extended pattern matching operators are recognized. In the following description, a PATTERN-LIST is a list of one or more patterns separated by a `|'. Composite patterns may be formed using one or more of the following sub-patterns: `?(PATTERN-LIST)' Matches zero or one occurrence of the given patterns. `*(PATTERN-LIST)' Matches zero or more occurrences of the given patterns. `+(PATTERN-LIST)' Matches one or more occurrences of the given patterns. `@(PATTERN-LIST)' Matches exactly one of the given patterns. `!(PATTERN-LIST)' Matches anything except one of the given patterns. Quote Removal ------------- After the preceding expansions, all unquoted occurrences of the characters `\', `'', and `"' that did not result from one of the above expansions are removed. Redirections ============ Before a command is executed, its input and output may be REDIRECTED using a special notation interpreted by the shell. Redirection may also be used to open and close files for the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right. In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is `<', the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is `>', the redirection refers to the standard output (file descriptor 1). The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, and filename expansion. If it expands to more than one word, Bash reports an error. Note that the order of redirections is significant. For example, the command ls > DIRLIST 2>&1 directs both standard output and standard error to the file DIRLIST, while the command ls 2>&1 > DIRLIST directs only the standard output to file DIRLIST, because the standard error was duplicated as standard output before the standard output was redirected to DIRLIST. A failure to open or create a file causes the redirection to fail. Redirecting Input ----------------- Redirection of input causes the file whose name results from the expansion of WORD to be opened for reading on file descriptor `n', or the standard input (file descriptor 0) if `n' is not specified. The general format for redirecting input is: [n][|]WORD If the redirection operator is `>', and the `noclobber' option to the `set' builtin has been enabled, the redirection will fail if the filename whose name results from the expansion of WORD exists and is a regular file. If the redirection operator is `>|', or the redirection operator is `>' and the `noclobber' option is not enabled, the redirection is attempted even if the file named by WORD exists. Appending Redirected Output --------------------------- Redirection of output in this fashion causes the file whose name results from the expansion of WORD to be opened for appending on file descriptor `n', or the standard output (file descriptor 1) if `n' is not specified. If the file does not exist it is created. The general format for appending output is: [n]>>WORD Redirecting Standard Output and Standard Error ---------------------------------------------- Bash allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of WORD with this construct. There are two formats for redirecting standard output and standard error: &>WORD and >&WORD Of the two forms, the first is preferred. This is semantically equivalent to >WORD 2>&1 Here Documents -------------- This type of redirection instructs the shell to read input from the current source until a line containing only WORD (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command. The format of here-documents is as follows: <<[-]WORD HERE-DOCUMENT DELIMITER No parameter expansion, command substitution, filename expansion, or arithmetic expansion is performed on WORD. If any characters in WORD are quoted, the DELIMITER is the result of quote removal on WORD, and the lines in the here-document are not expanded. If WORD is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion. In the latter case, the pair `\newline' is ignored, and `\' must be used to quote the characters `\', `$', and ``'. If the redirection operator is `<<-', then all leading tab characters are stripped from input lines and the line containing DELIMITER. This allows here-documents within shell scripts to be indented in a natural fashion. Duplicating File Descriptors ---------------------------- The redirection operator [n]<&WORD is used to duplicate input file descriptors. If WORD expands to one or more digits, the file descriptor denoted by `n' is made to be a copy of that file descriptor. If the digits in WORD do not specify a file descriptor open for input, a redirection error occurs. If WORD evaluates to `-', file descriptor `n' is closed. If `n' is not specified, the standard input (file descriptor 0) is used. The operator [n]>&WORD is used similarly to duplicate output file descriptors. If `n' is not specified, the standard output (file descriptor 1) is used. If the digits in WORD do not specify a file descriptor open for output, a redirection error occurs. As a special case, if `n' is omitted, and WORD does not expand to one or more digits, the standard output and standard error are redirected as described previously. Opening File Descriptors for Reading and Writing ------------------------------------------------ The redirection operator [n]<>WORD causes the file whose name is the expansion of WORD to be opened for both reading and writing on file descriptor `n', or on file descriptor 0 if `n' is not specified. If the file does not exist, it is created. Executing Commands ================== Simple Command Expansion ------------------------ When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right. 1. The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing. 2. The words that are not variable assignments or redirections are expanded (*note Shell Expansions::.). If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments. 3. Redirections are performed as described above (*note Redirections::.). 4. The text after the `=' in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable. If no command name results, the variable assignments affect the current shell environment. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status. If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status. If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a status of zero. Command Search and Execution ---------------------------- After a command has been split into words, if it results in a simple command and an optional list of arguments, the following actions are taken. 1. If the command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name, that function is invoked as described above in *Note Shell Functions::. 2. If the name does not match a function, the shell searches for it in the list of shell builtins. If a match is found, that builtin is invoked. 3. If the name is neither a shell function nor a builtin, and contains no slashes, Bash searches each element of `$PATH' for a directory containing an executable file by that name. Bash uses a hash table to remember the full pathnames of executable files to avoid multiple `PATH' searches (see the description of `hash' in *Note Bourne Shell Builtins::). A full search of the directories in `$PATH' is performed only if the command is not found in the hash table. If the search is unsuccessful, the shell prints an error message and returns an exit status of 127. 4. If the search is successful, or if the command name contains one or more slashes, the shell executes the named program in a separate execution environment. Argument 0 is set to the name given, and the remaining arguments to the command are set to the arguments supplied, if any. 5. If this execution fails because the file is not in executable format, and the file is not a directory, it is assumed to be a SHELL SCRIPT and the shell executes it as described in *Note Shell Scripts::. 6. If the command was not begun asynchronously, the shell waits for the command to complete and collects its exit status. Command Execution Environment ----------------------------- The shell has an EXECUTION ENVIRONMENT, which consists of the following: * open files inherited by the shell at invocation, as modified by redirections supplied to the `exec' builtin * the current working directory as set by `cd', `pushd', or `popd', or inherited by the shell at invocation * the file creation mode mask as set by `umask' or inherited from the shell's parent * current traps set by `trap' * shell parameters that are set by variable assignment or with `set' or inherited from the shell's parent in the environment * shell functions defined during execution or inherited from the shell's parent in the environment * options enabled at invocation (either by default or with command-line arguments) or by `set' * options enabled by `shopt' * shell aliases defined with `alias' (*note Aliases::.) * various process IDs, including those of background jobs (*note Lists::.), the value of `$$', and the value of `$PPID' When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell. * the shell's open files, plus any modifications and additions specified by redirections to the command * the current working directory * the file creation mode mask * shell variables marked for export, along with variables exported for the command, passed in the environment (*note Environment::.) * traps caught by the shell are reset to the values inherited from the shell's parent, and traps ignored by the shell are ignored A command invoked in this separate environment cannot affect the shell's execution environment. Command substitution and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell's execution environment. Environment ----------- When a program is invoked it is given an array of strings called the ENVIRONMENT. This is a list of name-value pairs, of the form `name=value'. Bash allows you to manipulate the environment in several ways. On invocation, the shell scans its own environment and creates a parameter for each name found, automatically marking it for EXPORT to child processes. Executed commands inherit the environment. The `export' and `declare -x' commands allow parameters and functions to be added to and deleted from the environment. If the value of a parameter in the environment is modified, the new value becomes part of the environment, replacing the old. The environment inherited by any executed command consists of the shell's initial environment, whose values may be modified in the shell, less any pairs removed by the `unset' and `export -n' commands, plus any additions via the `export' and `declare -x' commands. The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described in *Note Shell Parameters::. These assignment statements affect only the environment seen by that command. If the `-k' option is set (*note The Set Builtin::.), then all parameter assignments are placed in the environment for a command, not just those that precede the command name. When Bash invokes an external command, the variable `$_' is set to the full path name of the command and passed to that command in its environment. Exit Status ----------- For the shell's purposes, a command which exits with a zero exit status has succeeded. A non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes. When a command terminates on a fatal signal whose number is N, Bash uses the value 128+N as the exit status. If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126. If a command fails because of an error during expansion or redirection, the exit status is greater than zero. The exit status is used by the Bash conditional commands (*note Conditional Constructs::.) and some of the list constructs (*note Lists::.). All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure, so they may be used by the conditional and list constructs. All builtins return an exit status of 2 to indicate incorrect usage. Signals ------- When Bash is interactive, in the absence of any traps, it ignores `SIGTERM' (so that `kill 0' does not kill an interactive shell), and `SIGINT' is caught and handled (so that the `wait' builtin is interruptible). When Bash receives a `SIGINT', it breaks out of any executing loops. In all cases, Bash ignores `SIGQUIT'. If job control is in effect (*note Job Control::.), Bash ignores `SIGTTIN', `SIGTTOU', and `SIGTSTP'. Commands started by Bash have signal handlers set to the values inherited by the shell from its parent. When job control is not in effect, asynchronous commands ignore `SIGINT' and `SIGQUIT' as well. Commands run as a result of command substitution ignore the keyboard-generated job control signals `SIGTTIN', `SIGTTOU', and `SIGTSTP'. The shell exits by default upon receipt of a `SIGHUP'. Before exiting, it resends the `SIGHUP' to all jobs, running or stopped. Stopped jobs are sent `SIGCONT' to ensure that they receive the `SIGHUP'. To prevent the shell from sending the `SIGHUP' signal to a particular job, it should be removed from the jobs table with the `disown' builtin (*note Job Control Builtins::.) or marked to not receive `SIGHUP' using `disown -h'. If the `huponexit' shell option has been set with `shopt' (*note Bash Builtins::.), Bash sends a `SIGHUP' to all jobs when an interactive login shell exits. When Bash receives a signal for which a trap has been set while waiting for a command to complete, the trap will not be executed until the command completes. When Bash is waiting for an asynchronous command via the `wait' builtin, the reception of a signal for which a trap has been set will cause the `wait' builtin to return immediately with an exit status greater than 128, immediately after which the trap is executed. Shell Scripts ============= A shell script is a text file containing shell commands. When such a file is used as the first non-option argument when invoking Bash, and neither the `-c' nor `-s' option is supplied (*note Invoking Bash::.), Bash reads and executes commands from the file, then exits. This mode of operation creates a non-interactive shell. When Bash runs a shell script, it sets the special parameter `0' to the name of the file, rather than the name of the shell, and the positional parameters are set to the remaining arguments, if any are given. If no additional arguments are supplied, the positional parameters are unset. A shell script may be made executable by using the `chmod' command to turn on the execute bit. When Bash finds such a file while searching the `$PATH' for a command, it spawns a subshell to execute it. In other words, executing filename ARGUMENTS is equivalent to executing bash filename ARGUMENTS if `filename' is an executable shell script. This subshell reinitializes itself, so that the effect is as if a new shell had been invoked to interpret the script, with the exception that the locations of commands remembered by the parent (see the description of `hash' in *Note Bourne Shell Builtins::) are retained by the child. Most versions of Unix make this a part of the kernel's command execution mechanism. If the first line of a script begins with the two characters `#!', the remainder of the line specifies an interpreter for the program. The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the script file, followed by the name of the script file, followed by the rest of the arguments. Bash will perform this action on operating systems that do not handle it themselves. Note that some older versions of Unix limit the interpreter name and argument to a maximum of 32 characters. Bourne Shell Style Features *************************** This section briefly summarizes things which Bash inherits from the Bourne Shell: builtins, variables, and other features. It also lists the significant differences between Bash and the Bourne Shell. Many of the builtins have been extended by POSIX or Bash. Bourne Shell Builtins ===================== The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX 1003.2 standard. `:' : [ARGUMENTS] Do nothing beyond expanding ARGUMENTS and performing redirections. The return status is zero. `.' . FILENAME Read and execute commands from the FILENAME argument in the current shell context. If FILENAME does not contain a slash, the `$PATH' variable is used to find FILENAME. The current directory is searched if FILENAME is not found in `$PATH'. The return status is the exit status of the last command executed, or zero if no commands are executed. If FILENAME is not found, or cannot be read, the return status is non-zero. `break' break [N] Exit from a `for', `while', `until', or `select' loop. If N is supplied, the Nth enclosing loop is exited. N must be greater than or equal to 1. The return status is zero unless N is not greater than or equal to 1. `cd' cd [-LP] [DIRECTORY] Change the current working directory to DIRECTORY. If DIRECTORY is not given, the value of the `HOME' shell variable is used. If the shell variable `CDPATH' exists, it is used as a search path. If DIRECTORY begins with a slash, `CDPATH' is not used. The `-P' option means to not follow symbolic links; symbolic links are followed by default or with the `-L' option. If DIRECTORY is `-', it is equivalent to `$OLDPWD'. The return status is zero if the directory is successfully changed, non-zero otherwise. `continue' continue [N] Resume the next iteration of an enclosing `for', `while', `until', or `select' loop. If N is supplied, the execution of the Nth enclosing loop is resumed. N must be greater than or equal to 1. The return status is zero unless N is not greater than or equal to 1. `eval' eval [ARGUMENTS] The arguments are concatenated together into a single command, which is then read and executed, and its exit status returned as the exit status of `eval'. If there are no arguments or only empty arguments, the return status is zero. `exec' exec [-cl] [-a NAME] [COMMAND [ARGUMENTS]] If COMMAND is supplied, it replaces the shell without creating a new process. If the `-l' option is supplied, the shell places a dash in the zeroth arg passed to COMMAND. This is what the `login' program does. The `-c' option causes COMMAND to be executed with an empty environment. If `-a' is supplied, the shell passes NAME as the zeroth argument to COMMAND. If no COMMAND is specified, redirections may be used to affect the current shell environment. If there are no redirection errors, the return status is zero; otherwise the return status is non-zero. `exit' exit [N] Exit the shell, returning a status of N to the shell's parent. Any trap on `EXIT' is executed before the shell terminates. `export' export [-fn] [-p] [NAME[=VALUE]] Mark each NAME to be passed to child processes in the environment. If the `-f' option is supplied, the NAMEs refer to shell functions; otherwise the names refer to shell variables. The `-n' option means to no longer mark each NAME for export. If no NAMES are supplied, or if the `-p' option is given, a list of exported names is displayed. The `-p' option displays output in a form that may be reused as input. The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or `-f' is supplied with a name that is not a shell function. `getopts' getopts OPTSTRING NAME [ARGS] `getopts' is used by shell scripts to parse positional parameters. OPTSTRING contains the option letters to be recognized; if a letter is followed by a colon, the option is expected to have an argument, which should be separated from it by white space. Each time it is invoked, `getopts' places the next option in the shell variable NAME, initializing NAME if it does not exist, and the index of the next argument to be processed into the variable `OPTIND'. `OPTIND' is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, `getopts' places that argument into the variable `OPTARG'. The shell does not reset `OPTIND' automatically; it must be manually reset between multiple calls to `getopts' within the same shell invocation if a new set of parameters is to be used. When the end of options is encountered, `getopts' exits with a return value greater than zero. `OPTIND' is set to the index of the first non-option argument, and `name' is set to `?'. `getopts' normally parses the positional parameters, but if more arguments are given in ARGS, `getopts' parses those instead. `getopts' can report errors in two ways. If the first character of OPTSTRING is a colon, SILENT error reporting is used. In normal operation diagnostic messages are printed when invalid options or missing option arguments are encountered. If the variable `OPTERR' is set to 0, no error messages will be displayed, even if the first character of `optstring' is not a colon. If an invalid option is seen, `getopts' places `?' into NAME and, if not silent, prints an error message and unsets `OPTARG'. If `getopts' is silent, the option character found is placed in `OPTARG' and no diagnostic message is printed. If a required argument is not found, and `getopts' is not silent, a question mark (`?') is placed in NAME, `OPTARG' is unset, and a diagnostic message is printed. If `getopts' is silent, then a colon (`:') is placed in NAME and `OPTARG' is set to the option character found. `hash' hash [-r] [-p FILENAME] [NAME] Remember the full pathnames of commands specified as NAME arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in `$PATH'. The `-p' option inhibits the path search, and FILENAME is used as the location of NAME. The `-r' option causes the shell to forget all remembered locations. If no arguments are given, information about remembered commands is printed. The return status is zero unless a NAME is not found or an invalid option is supplied. `pwd' pwd [-LP] Print the current working directory. If the `-P' option is supplied, the path printed will not contain symbolic links. If the `-L' option is supplied, the path printed may contain symbolic links. The return status is zero unless an error is encountered while determining the name of the current directory or an invalid option is supplied. `readonly' readonly [-apf] [NAME] ... Mark each NAME as readonly. The values of these names may not be changed by subsequent assignment. If the `-f' option is supplied, each NAME refers to a shell function. The `-a' option means each NAME refers to an array variable. If no NAME arguments are given, or if the `-p' option is supplied, a list of all readonly names is printed. The `-p' option causes output to be displayed in a format that may be reused as input. The return status is zero unless an invalid option is supplied, one of the NAME arguments is not a valid shell variable or function name, or the `-f' option is supplied with a name that is not a shell function. `return' return [N] Cause a shell function to exit with the return value N. This may also be used to terminate execution of a script being executed with the `.' builtin, returning either N or the exit status of the last command executed within the script as the exit status of the script. The return status is false if `return' is used outside a function and not during the execution of a script by `.'. `shift' shift [N] Shift the positional parameters to the left by N. The positional parameters from N+1 ... `$#' are renamed to `$1' ... `$#'-N+1. Parameters represented by the numbers `$#' to N+1 are unset. N must be a non-negative number less than or equal to `$#'. If N is zero or greater than `$#', the positional parameters are not changed. The return status is zero unless N is greater than `$#' or less than zero, non-zero otherwise. `test' `[' Evaluate a conditional expression EXPR. Each operator and operand must be a separate argument. Expressions are composed of the primaries described below in *Note Bash Conditional Expressions::. Expressions may be combined using the following operators, listed in decreasing order of precedence. `! EXPR' True if EXPR is false. `( EXPR )' Returns the value of EXPR. This may be used to override the normal precedence of operators. `EXPR1 -a EXPR2' True if both EXPR1 and EXPR2 are true. `EXPR1 -o EXPR2' True if either EXPR1 or EXPR2 is true. The `test' and `[' builtins evaluate conditional expressions using a set of rules based on the number of arguments. 0 arguments The expression is false. 1 argument The expression is true if and only if the argument is not null. 2 arguments If the first argument is `!', the expression is true if and only if the second argument is null. If the first argument is one of the unary conditional operators (*note Bash Conditional Expressions::.), the expression is true if the unary test is true. If the first argument is not a valid unary operator, the expression is false. 3 arguments If the second argument is one of the binary conditional operators (*note Bash Conditional Expressions::.), the result of the expression is the result of the binary test using the first and third arguments as operands. If the first argument is `!', the value is the negation of the two-argument test using the second and third arguments. If the first argument is exactly `(' and the third argument is exactly `)', the result is the one-argument test of the second argument. Otherwise, the expression is false. The `-a' and `-o' operators are considered binary operators in this case. 4 arguments If the first argument is `!', the result is the negation of the three-argument expression composed of the remaining arguments. Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above. 5 or more arguments The expression is parsed and evaluated according to precedence using the rules listed above. `times' times Print out the user and system times used by the shell and its children. The return status is zero. `trap' trap [-lp] [ARG] [SIGSPEC ...] The commands in ARG are to be read and executed when the shell receives signal SIGSPEC. If ARG is absent or equal to `-', all specified signals are reset to the values they had when the shell was started. If ARG is the null string, then the signal specified by each SIGSPEC is ignored by the shell and commands it invokes. If ARG is `-p', the shell displays the trap commands associated with each SIGSPEC. If no arguments are supplied, or only `-p' is given, `trap' prints the list of commands associated with each signal number in a form that may be reused as shell input. Each SIGSPEC is either a signal name such as `SIGINT' (with or without the `SIG' prefix) or a signal number. If a SIGSPEC is `0' or `EXIT', ARG is executed when the shell exits. If a SIGSPEC is `DEBUG', the command ARG is executed after every simple command. The `-l' option causes the shell to print a list of signal names and their corresponding numbers. Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals are reset to their original values in a child process when it is created. The return status is zero unless a SIGSPEC does not specify a valid signal. `umask' umask [-p] [-S] [MODE] Set the shell process's file creation mask to MODE. If MODE begins with a digit, it is interpreted as an octal number; if not, it is interpreted as a symbolic mode mask similar to that accepted by the `chmod' command. If MODE is omitted, the current value of the mask is printed. If the `-S' option is supplied without a MODE argument, the mask is printed in a symbolic format. If the `-p' option is supplied, and MODE is omitted, the output is in a form that may be reused as input. The return status is zero if the mode is successfully changed or if no MODE argument is supplied, and non-zero otherwise. `unset' unset [-fv] [NAME] Each variable or function NAME is removed. If no options are supplied, or the `-v' option is given, each NAME refers to a shell variable. If the `-f' option is given, the NAMEs refer to shell functions, and the function definition is removed. Readonly variables and functions may not be unset. The return status is zero unless a NAME does not exist or is readonly. Bourne Shell Variables ====================== Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable. `CDPATH' A colon-separated list of directories used as a search path for the `cd' builtin command. `HOME' The current user's home directory; the default for the `cd' builtin command. The value of this variable is also used by tilde expansion (*note Tilde Expansion::.). `IFS' A list of characters that separate fields; used when the shell splits words as part of expansion. `MAIL' If this parameter is set to a filename and the `MAILPATH' variable is not set, Bash informs the user of the arrival of mail in the specified file. `MAILPATH' A colon-separated list of filenames which the shell periodically checks for new mail. Each list entry can specify the message that is printed when new mail arrives in the mail file by separating the file name from the message with a `?'. When used in the text of the message, `$_' expands to the name of the current mail file. `OPTARG' The value of the last option argument processed by the `getopts' builtin. `OPTIND' The index of the last option argument processed by the `getopts' builtin. `PATH' A colon-separated list of directories in which the shell looks for commands. `PS1' The primary prompt string. The default value is `\s-\v\$ '. `PS2' The secondary prompt string. The default value is `> '. Other Bourne Shell Features =========================== Bash implements essentially the same grammar, parameter and variable expansion, redirection, and quoting as the Bourne Shell. Bash uses the POSIX 1003.2 standard as the specification of how these features are to be implemented. There are some differences between the traditional Bourne shell and Bash; this section quickly details the differences of significance. A number of these differences are explained in greater depth in subsequent sections. Major Differences From The SVR4.2 Bourne Shell ---------------------------------------------- * Bash is POSIX-conformant, even where the POSIX specification differs from traditional `sh' behavior. * Bash has multi-character invocation options (*note Invoking Bash::.). * Bash has command-line editing (*note Command Line Editing::.) and the `bind' builtin. * Bash has command history (*note Bash History Facilities::.) and the `history' and `fc' builtins to manipulate it. * Bash implements `csh'-like history expansion (*note History Interaction::.). * Bash has one-dimensional array variables (*note Arrays::.), and the appropriate variable expansions and assignment syntax to use them. Several of the Bash builtins take options to act on arrays. Bash provides a number of built-in array variables. * The `$'...'' quoting syntax, which expands ANSI-C backslash-escaped characters in the text between the single quotes, is supported (*note ANSI-C Quoting::.). * Bash supports the `$"..."' quoting syntax to do locale-specific translation of the characters between the double quotes. The `-D', `--dump-strings', and `--dump-po-strings' invocation options list the translatable strings found in a script (*note Locale Translation::.). * Bash implements the `!' keyword to negate the return value of a pipeline (*note Pipelines::.). Very useful when an `if' statement needs to act only if a test fails. * Bash has the `time' reserved word and command timing (*note Pipelines::.). The display of the timing statistics may be controlled with the `TIMEFORMAT' variable. * Bash includes the `select' compound command, which allows the generation of simple menus (*note Conditional Constructs::.). * Bash includes the `[[' compound command, which makes conditional testing part of the shell grammar (*note Conditional Constructs::.). * Bash includes brace expansion (*note Brace Expansion::.) and tilde expansion (*note Tilde Expansion::.). * Bash implements command aliases and the `alias' and `unalias' builtins (*note Aliases::.). * Bash provides shell arithmetic, the `((' compound command (*note Conditional Constructs::.), and arithmetic expansion (*note Shell Arithmetic::.). * Variables present in the shell's initial environment are automatically exported to child processes. The Bourne shell does not normally do this unless the variables are explicitly marked using the `export' command. * Bash includes the POSIX pattern removal `%', `#', `%%' and `##' expansions to remove leading or trailing substrings from variable values (*note Shell Parameter Expansion::.). * The expansion `${#xx}', which returns the length of `${xx}', is supported (*note Shell Parameter Expansion::.). * The expansion `${var:'OFFSET`[:'LENGTH`]}', which expands to the substring of `var''s value of length LENGTH, beginning at OFFSET, is present (*note Shell Parameter Expansion::.). * The expansion `${var/[/]'PATTERN`[/'REPLACEMENT`]}', which matches PATTERN and replaces it with REPLACEMENT in the value of `var', is available (*note Shell Parameter Expansion::.). * Bash has INDIRECT variable expansion using `${!word}' (*note Shell Parameter Expansion::.). * Bash can expand positional parameters beyond `$9' using `${NUM}'. * The POSIX `$()' form of command substitution is implemented (*note Command Substitution::.), and preferred to the Bourne shell's ```' (which is also implemented for backwards compatibility). * Bash has process substitution (*note Process Substitution::.). * Bash automatically assigns variables that provide information about the current user (`UID', `EUID', and `GROUPS'), the current host (`HOSTTYPE', `OSTYPE', `MACHTYPE', and `HOSTNAME'), and the instance of Bash that is running (`BASH', `BASH_VERSION', and `BASH_VERSINFO'). *Note Bash Variables::, for details. * The `IFS' variable is used to split only the results of expansion, not all words (*note Word Splitting::.). This closes a longstanding shell security hole. * Bash implements the full set of POSIX.2 filename expansion operators, including CHARACTER CLASSES, EQUIVALENCE CLASSES, and COLLATING SYMBOLS (*note Filename Expansion::.). * Bash implements extended pattern matching features when the `extglob' shell option is enabled (*note Pattern Matching::.). * It is possible to have a variable and a function with the same name; `sh' does not separate the two name spaces. * Bash functions are permitted to have local variables using the `local' builtin, and thus useful recursive functions may be written. * Variable assignments preceding commands affect only that command, even builtins and functions (*note Environment::.). In `sh', all variable assignments preceding commands are global unless the command is executed from the file system. * Bash performs filename expansion on filenames specified as operands to input and output redirection operators. * Bash contains the `<>' redirection operator, allowing a file to be opened for both reading and writing, and the `&>' redirection operator, for directing standard output and standard error to the same file (*note Redirections::.). * The `noclobber' option is available to avoid overwriting existing files with output redirection (*note The Set Builtin::.). The `>|' redirection operator may be used to override `noclobber'. * The Bash `cd' and `pwd' builtins (*note Bourne Shell Builtins::.) each take `-L' and `-P' builtins to switch between logical and physical modes. * Bash allows a function to override a builtin with the same name, and provides access to that builtin's functionality within the function via the `builtin' and `command' builtins (*note Bash Builtins::.). * The `command' builtin allows selective disabling of functions when command lookup is performed (*note Bash Builtins::.). * Individual builtins may be enabled or disabled using the `enable' builtin (*note Bash Builtins::.). * The Bash `exec' builtin takes additional options that allow users to control the contents of the environment passed to the executed command, and what the zeroth argument to the command is to be (*note Bourne Shell Builtins::.). * Shell functions may be exported to children via the environment using `export -f' (*note Shell Functions::.). * The Bash `export', `readonly', and `declare' builtins can take a `-f' option to act on shell functions, a `-p' option to display variables with various attributes set in a format that can be used as shell input, a `-n' option to remove various variable attributes, and `name=value' arguments to set variable attributes and values simultaneously. * The Bash `hash' builtin allows a name to be associated with an arbitrary filename, even when that filename cannot be found by searching the `$PATH', using `hash -p' (*note Bourne Shell Builtins::.). * Bash includes a `help' builtin for quick reference to shell facilities (*note Bash Builtins::.). * The `printf' builtin is available to display formatted output (*note Bash Builtins::.). * The Bash `read' builtin (*note Bash Builtins::.) will read a line ending in `\' with the `-r' option, and will use the `REPLY' variable as a default if no arguments are supplied. The Bash `read' builtin also accepts a prompt string with the `-p' option and will use Readline to obtain the line when given the `-e' option. * The `return' builtin may be used to abort execution of scripts executed with the `.' or `source' builtins (*note Bourne Shell Builtins::.). * Bash includes the `shopt' builtin, for finer control of shell optional capabilities (*note Bash Builtins::.). * Bash has much more optional behavior controllable with the `set' builtin (*note The Set Builtin::.). * The `test' builtin (*note Bourne Shell Builtins::.) is slightly different, as it implements the POSIX algorithm, which specifies the behavior based on the number of arguments. * The `trap' builtin (*note Bourne Shell Builtins::.) allows a `DEBUG' pseudo-signal specification, similar to `EXIT'. Commands specified with a `DEBUG' trap are executed after every simple command. The `DEBUG' trap is not inherited by shell functions. * The Bash `type' builtin is more extensive and gives more information about the names it finds (*note Bash Builtins::.). * The Bash `umask' builtin permits a `-p' option to cause the output to be displayed in the form of a `umask' command that may be reused as input (*note Bourne Shell Builtins::.). * Bash implements a `csh'-like directory stack, and provides the `pushd', `popd', and `dirs' builtins to manipulate it (*note The Directory Stack::.). Bash also makes the directory stack visible as the value of the `DIRSTACK' shell variable. * Bash interprets special backslash-escaped characters in the prompt strings when interactive (*note Printing a Prompt::.). * The Bash restricted mode is more useful (*note The Restricted Shell::.); the SVR4.2 shell restricted mode is too limited. * The `disown' builtin can remove a job from the internal shell job table (*note Job Control Builtins::.) or suppress the sending of `SIGHUP' to a job when the shell exits as the result of a `SIGHUP'. * The SVR4.2 shell has two privilege-related builtins (`mldmode' and `priv') not present in Bash. * Bash does not have the `stop' or `newgrp' builtins. * Bash does not use the `SHACCT' variable or perform shell accounting. * The SVR4.2 `sh' uses a `TIMEOUT' variable like Bash uses `TMOUT'. More features unique to Bash may be found in *Note Bash Features::. Implementation Differences From The SVR4.2 Shell ------------------------------------------------ Since Bash is a completely new implementation, it does not suffer from many of the limitations of the SVR4.2 shell. For instance: * Bash does not fork a subshell when redirecting into or out of a shell control structure such as an `if' or `while' statement. * Bash does not allow unbalanced quotes. The SVR4.2 shell will silently insert a needed closing quote at `EOF' under certain circumstances. This can be the cause of some hard-to-find errors. * The SVR4.2 shell uses a baroque memory management scheme based on trapping `SIGSEGV'. If the shell is started from a process with `SIGSEGV' blocked (e.g., by using the `system()' C library function call), it misbehaves badly. * In a questionable attempt at security, the SVR4.2 shell, when invoked without the `-p' option, will alter its real and effective UID and GID if they are less than some magic threshold value, commonly 100. This can lead to unexpected results. * The SVR4.2 shell does not allow users to trap `SIGSEGV', `SIGALRM', or `SIGCHLD'. * The SVR4.2 shell does not allow the `IFS', `MAILCHECK', `PATH', `PS1', or `PS2' variables to be unset. * The SVR4.2 shell treats `^' as the undocumented equivalent of `|'. * Bash allows multiple option arguments when it is invoked (`-x -v'); the SVR4.2 shell allows only one option argument (`-xv'). In fact, some versions of the shell dump core if the second argument begins with a `-'. * The SVR4.2 shell exits a script if any builtin fails; Bash exits a script only if one of the POSIX.2 special builtins fails, and only for certain failures, as enumerated in the POSIX.2 standard. * The SVR4.2 shell behaves differently when invoked as `jsh' (it turns on job control). Bash Features ************* This section describes features unique to Bash. Invoking Bash ============= bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o OPTION] [ARGUMENT ...] bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o OPTION] -c STRING [ARGUMENT ...] bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o OPTION] [ARGUMENT ...] In addition to the single-character shell command-line options (*note The Set Builtin::.), there are several multi-character options that you can use. These options must appear on the command line before the single-character options in order for them to be recognized. `--dump-po-strings' Equivalent to `-D', but the output is in the GNU `gettext' PO (portable object) file format. `--dump-strings' Equivalent to `-D'. `--help' Display a usage message on standard output and exit sucessfully. `--login' Make this shell act as if it were directly invoked by login. This is equivalent to `exec -l bash' but can be issued from another shell, such as `csh'. `exec bash --login' will replace the current shell with a Bash login shell. `--noediting' Do not use the GNU Readline library (*note Command Line Editing::.) to read interactive command lines. `--noprofile' Don't load the system-wide startup file `/etc/profile' or any of the personal initialization files `~/.bash_profile', `~/.bash_login', or `~/.profile' when Bash is invoked as a login shell. `--norc' Don't read the `~/.bashrc' initialization file in an interactive shell. This is on by default if the shell is invoked as `sh'. `--posix' Change the behavior of Bash where the default operation differs from the POSIX 1003.2 standard to match the standard. This is intended to make Bash behave as a strict superset of that standard. *Note Bash POSIX Mode::, for a description of the Bash POSIX mode. `--rcfile FILENAME' Execute commands from FILENAME (instead of `~/.bashrc') in an interactive shell. `--restricted' Make the shell a restricted shell (*note The Restricted Shell::.). `--verbose' Equivalent to `-v'. `--version' Show version information for this instance of Bash on the standard output and exit successfully. There are several single-character options that may be supplied at invocation which are not available with the `set' builtin. `-c STRING' Read and execute commands from STRING after processing the options, then exit. Any remaining arguments are assigned to the positional parameters, starting with `$0'. `-i' Force the shell to run interactively. `-r' Make the shell a restricted shell (*note The Restricted Shell::.). `-s' If this option is present, or if no arguments remain after option processing, then commands are read from the standard input. This option allows the positional parameters to be set when invoking an interactive shell. `-D' A list of all double-quoted strings preceded by `$' is printed on the standard ouput. These are the strings that are subject to language translation when the current locale is not `C' or `POSIX' (*note Locale Translation::.). This implies the `-n' option; no commands will be executed. `--' A `--' signals the end of options and disables further option processing. Any arguments after the `--' are treated as filenames and arguments. An *interactive* shell is one whose input and output are both connected to terminals (as determined by `isatty(3)'), or one started with the `-i' option. If arguments remain after option processing, and neither the `-c' nor the `-s' option has been supplied, the first argument is assumed to be the name of a file containing shell commands (*note Shell Scripts::.). When Bash is invoked in this fashion, `$0' is set to the name of the file, and the positional parameters are set to the remaining arguments. Bash reads and executes commands from this file, then exits. Bash's exit status is the exit status of the last command executed in the script. If no commands are executed, the exit status is 0. Bash Startup Files ================== This section describs how Bash executes its startup files. If any of the files exist but cannot be read, Bash reports an error. Tildes are expanded in file names as described above under Tilde Expansion (*note Tilde Expansion::.). When Bash is invoked as an interactive login shell, it first reads and executes commands from the file `/etc/profile', if that file exists. After reading that file, it looks for `~/.bash_profile', `~/.bash_login', and `~/.profile', in that order, and reads and executes commands from the first one that exists and is readable. The `--noprofile' option may be used when the shell is started to inhibit this behavior. When a login shell exits, Bash reads and executes commands from the file `~/.bash_logout', if it exists. When an interactive shell that is not a login shell is started, Bash reads and executes commands from `~/.bashrc', if that file exists. This may be inhibited by using the `--norc' option. The `--rcfile FILE' option will force Bash to read and execute commands from FILE instead of `~/.bashrc'. So, typically, your `~/.bash_profile' contains the line `if [ -f `~/.bashrc' ]; then . `~/.bashrc'; fi' after (or before) any login-specific initializations. When Bash is started non-interactively, to run a shell script, for example, it looks for the variable `BASH_ENV' in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed: `if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi' but the value of the `PATH' variable is not used to search for the file name. If Bash is invoked with the name `sh', it tries to mimic the startup behavior of historical versions of `sh' as closely as possible, while conforming to the POSIX standard as well. When invoked as an interactive login shell, it first attempts to read and execute commands from `/etc/profile' and `~/.profile', in that order. The `--noprofile' option may be used to inhibit this behavior. When invoked as an interactive shell with the name `sh', Bash looks for the variable `ENV', expands its value if it is defined, and uses the expanded value as the name of a file to read and execute. Since a shell invoked as `sh' does not attempt to read and execute commands from any other startup files, the `--rcfile' option has no effect. A non-interactive shell invoked with the name `sh' does not attempt to read any startup files. When invoked as `sh', Bash enters POSIX mode after the startup files are read. When Bash is started in POSIX mode, as with the `--posix' command line option, it follows the POSIX standard for startup files. In this mode, interactive shells expand the `ENV' variable and commands are read and executed from the file whose name is the expanded value. No other startup files are read. Bash attempts to determine when it is being run by the remote shell daemon, usually `rshd'. If Bash determines it is being run by rshd, it reads and executes commands from `~/.bashrc', if that file exists and is readable. It will not do this if invoked as `sh'. The `--norc' option may be used to inhibit this behavior, and the `--rcfile' option may be used to force another file to be read, but `rshd' does not generally invoke the shell with those options or allow them to be specified. Is This Shell Interactive? ========================== As defined in *Note Invoking Bash::, an interactive shell is one whose input and output are both connected to terminals (as determined by `isatty(3)'), or one started with the `-i' option. To determine within a startup script whether Bash is running interactively or not, examine the variable `$PS1'; it is unset in non-interactive shells, and set in interactive shells. Thus: if [ -z "$PS1" ]; then echo This shell is not interactive else echo This shell is interactive fi Alternatively, startup scripts may test the value of the `-' special parameter. It contains `i' when the shell is interactive. For example: case "$-" in *i*) echo This shell is interactive ;; *) echo This shell is not interactive ;; esac Bash Builtin Commands ===================== This section describes builtin commands which are unique to or have been extended in Bash. `bind' bind [-m KEYMAP] [-lpsvPSV] bind [-m KEYMAP] [-q FUNCTION] [-u FUNCTION] [-r KEYSEQ] bind [-m KEYMAP] -f FILENAME bind [-m KEYMAP] KEYSEQ:FUNCTION-NAME Display current Readline (*note Command Line Editing::.) key and function bindings, or bind a key sequence to a Readline function or macro. The binding syntax accepted is identical to that of `.inputrc' (*note Readline Init File::.), but each binding must be passed as a separate argument: e.g., `"\C-x\C-r":re-read-init-file'. Options, if supplied, have the following meanings: `-m KEYMAP' Use KEYMAP as the keymap to be affected by the subsequent bindings. Acceptable KEYMAP names are `emacs', `emacs-standard', `emacs-meta', `emacs-ctlx', `vi', `vi-command', and `vi-insert'. `vi' is equivalent to `vi-command'; `emacs' is equivalent to `emacs-standard'. `-l' List the names of all Readline functions. `-p' Display Readline function names and bindings in such a way that they can be re-read. `-P' List current Readline function names and bindings. `-v' Display Readline variable names and values in such a way that they can be re-read. `-V' List current Readline variable names and values. `-s' Display Readline key sequences bound to macros and the strings they output in such a way that they can be re-read. `-S' Display Readline key sequences bound to macros and the strings they output. `-f FILENAME' Read key bindings from FILENAME. `-q FUNCTION' Query about which keys invoke the named FUNCTION. `-u FUNCTION' Unbind all keys bound to the named FUNCTION. `-r KEYSEQ' Remove any current binding for KEYSEQ. The return status is zero unless an invalid option is supplied or an error occurs. `builtin' builtin [SHELL-BUILTIN [ARGS]] Run a shell builtin, passing it ARGS, and return its exit status. This is useful when defining a shell function with the same name as a shell builtin, retaining the functionality of the builtin within the function. The return status is non-zero if SHELL-BUILTIN is not a shell builtin command. `command' command [-pVv] COMMAND [ARGUMENTS ...] Runs COMMAND with ARGUMENTS ignoring any shell function named COMMAND. Only shell builtin commands or commands found by searching the `PATH' are executed. If there is a shell function named `ls', running `command ls' within the function will execute the external command `ls' instead of calling the function recursively. The `-p' option means to use a default value for `$PATH' that is guaranteed to find all of the standard utilities. The return status in this case is 127 if COMMAND cannot be found or an error occurred, and the exit status of COMMAND otherwise. If either the `-V' or `-v' option is supplied, a description of COMMAND is printed. The `-v' option causes a single word indicating the command or file name used to invoke COMMAND to be displayed; the `-V' option produces a more verbose description. In this case, the return status is zero if COMMAND is found, and non-zero if not. `declare' declare [-afFrxi] [-p] [NAME[=VALUE]] Declare variables and give them attributes. If no NAMEs are given, then display the values of variables instead. The `-p' option will display the attributes and values of each NAME. When `-p' is used, additional options are ignored. The `-F' option inhibits the display of function definitions; only the function name and attributes are printed. `-F' implies `-f'. The following options can be used to restrict output to variables with the specified attributes or to give variables attributes: `-a' Each NAME is an array variable (*note Arrays::.). `-f' Use function names only. `-i' The variable is to be treated as an integer; arithmetic evaluation (*note Shell Arithmetic::.) is performed when the variable is assigned a value. `-r' Make NAMEs readonly. These names cannot then be assigned values by subsequent assignment statements or unset. `-x' Mark each NAME for export to subsequent commands via the environment. Using `+' instead of `-' turns off the attribute instead. When used in a function, `declare' makes each NAME local, as with the `local' command. The return status is zero unless an invalid option is encountered, an attempt is made to define a function using `-f foo=bar', an attempt is made to assign a value to a readonly variable, an attempt is made to assign a value to an array variable without using the compound assignment syntax (*note Arrays::.), one of the NAMES is not a valid shell variable name, an attempt is made to turn off readonly status for a readonly variable, an attempt is made to turn off array status for an array variable, or an attempt is made to display a non-existent function with `-f'. `echo' echo [-neE] [ARG ...] Output the ARGs, separated by spaces, terminated with a newline. The return status is always 0. If `-n' is specified, the trailing newline is suppressed. If the `-e' option is given, interpretation of the following backslash-escaped characters is enabled. The `-E' option disables the interpretation of these escape characters, even on systems where they are interpreted by default. `echo' interprets the following escape sequences: `\a' alert (bell) `\b' backspace `\c' suppress trailing newline `\e' escape `\f' form feed `\n' new line `\r' carriage return `\t' horizontal tab `\v' vertical tab `\\' backslash `\NNN' the character whose `ASCII' code is the octal value NNN (one to three digits) `\xNNN' the character whose `ASCII' code is the hexadecimal value NNN (one to three digits) `enable' enable [-n] [-p] [-f FILENAME] [-ads] [NAME ...] Enable and disable builtin shell commands. Disabling a builtin allows a disk command which has the same name as a shell builtin to be executed with specifying a full pathname, even though the shell normally searches for builtins before disk commands. If `-n' is used, the NAMEs become disabled. Otherwise NAMEs are enabled. For example, to use the `test' binary found via `$PATH' instead of the shell builtin version, type `enable -n test'. If the `-p' option is supplied, or no NAME arguments appear, a list of shell builtins is printed. With no other arguments, the list consists of all enabled shell builtins. The `-a' option means to list each builtin with an indication of whether or not it is enabled. The `-f' option means to load the new builtin command NAME from shared object FILENAME, on systems that support dynamic loading. The `-d' option will delete a builtin loaded with `-f'. If there are no options, a list of the shell builtins is displayed. The `-s' option restricts `enable' to the POSIX special builtins. If `-s' is used with `-f', the new builtin becomes a special builtin. The return status is zero unless a NAME is not a shell builtin or there is an error loading a new builtin from a shared object. `help' help [PATTERN] Display helpful information about builtin commands. If PATTERN is specified, `help' gives detailed help on all commands matching PATTERN, otherwise a list of the builtins is printed. The return status is zero unless no command matches PATTERN. `let' let EXPRESSION [EXPRESSION] The `let' builtin allows arithmetic to be performed on shell variables. Each EXPRESSION is evaluated according to the rules given below in *Note Shell Arithmetic::. If the last EXPRESSION evaluates to 0, `let' returns 1; otherwise 0 is returned. `local' local NAME[=VALUE] For each argument, a local variable named NAME is created, and assigned VALUE. `local' can only be used within a function; it makes the variable NAME have a visible scope restricted to that function and its children. The return status is zero unless `local' is used outside a function or an invalid NAME is supplied. `logout' logout [N] Exit a login shell, returning a status of N to the shell's parent. `printf' `printf' FORMAT [ARGUMENTS] Write the formatted ARGUMENTS to the standard output under the control of the FORMAT. The FORMAT is a character string which contains three types of objects: plain characters, which are simply copied to standard output, character escape sequences, which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successive ARGUMENT. In addition to the standard `printf(1)' formats, `%b' causes `printf' to expand backslash escape sequences in the corresponding ARGUMENT, and `%q' causes `printf' to output the corresponding ARGUMENT in a format that can be reused as shell input. The FORMAT is reused as necessary to consume all of the ARGUMENTS. If the FORMAT requires more ARGUMENTS than are supplied, the extra format specifications behave as if a zero value or null string, as appropriate, had been supplied. `read' read [-a ANAME] [-p PROMPT] [-er] [NAME ...] One line is read from the standard input, and the first word is assigned to the first NAME, the second word to the second NAME, and so on, with leftover words and their intervening separators assigned to the last NAME. If there are fewer words read from the standard input than names, the remaining names are assigned empty values. The characters in the value of the `IFS' variable are used to split the line into words. If no names are supplied, the line read is assigned to the variable `REPLY'. The return code is zero, unless end-of-file is encountered. Options, if supplied, have the following meanings: `-r' If this option is given, a backslash-newline pair is not ignored, and the backslash is considered to be part of the line. `-p PROMPT' Display PROMPT, without a trailing newline, before attempting to read any input. The prompt is displayed only if input is coming from a terminal. `-a ANAME' The words are assigned to sequential indices of the array variable ANAME, starting at 0. All elements are removed from ANAME before the assignment. Other NAME arguments are ignored. `-e' Readline (*note Command Line Editing::.) is used to obtain the line. `shopt' shopt [-pqsu] [-o] [OPTNAME ...] Toggle the values of variables controlling optional shell behavior. With no options, or with the `-p' option, a list of all settable options is displayed, with an indication of whether or not each is set. The `-p' option causes output to be displayed in a form that may be reused as input. Other options have the following meanings: `-s' Enable (set) each OPTNAME. `-u' Disable (unset) each OPTNAME. `-q' Suppresses normal output; the return status indicates whether the OPTNAME is set or unset. If multiple OPTNAME arguments are given with `-q', the return status is zero if all OPTNAMES are enabled; non-zero otherwise. `-o' Restricts the values of OPTNAME to be those defined for the `-o' option to the `set' builtin (*note The Set Builtin::.). If either `-s' or `-u' is used with no OPTNAME arguments, the display is limited to those options which are set or unset, respectively. Unless otherwise noted, the `shopt' options are disabled (off) by default. The return status when listing options is zero if all OPTNAMES are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an OPTNAME is not a valid shell option. The list of `shopt' options is: `cdable_vars' If this is set, an argument to the `cd' builtin command that is not a directory is assumed to be the name of a variable whose value is the directory to change to. `cdspell' If set, minor errors in the spelling of a directory component in a `cd' command will be corrected. The errors checked for are transposed characters, a missing character, and a character too many. If a correction is found, the corrected path is printed, and the command proceeds. This option is only used by interactive shells. `checkhash' If this is set, Bash checks that a command found in the hash table exists before trying to execute it. If a hashed command no longer exists, a normal path search is performed. `checkwinsize' If set, Bash checks the window size after each command and, if necessary, updates the values of `LINES' and `COLUMNS'. `cmdhist' If set, Bash attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multi-line commands. `dotglob' If set, Bash includes filenames beginning with a `.' in the results of filename expansion. `execfail' If this is set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the `exec' builtin command. An interactive shell does not exit if `exec' fails. `expand_aliases' If set, aliases are expanded as described below< under Aliases (*note Aliases::.). This option is enabled by default for interactive shells. `extglob' If set, the extended pattern matching features described above (*note Pattern Matching::.) are enabled. `histappend' If set, the history list is appended to the file named by the value of the `HISTFILE' variable when the shell exits, rather than overwriting the file. `histreedit' If set, and Readline is being used, a user is given the opportunity to re-edit a failed history substitution. `histverify' If set, and Readline is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the Readline editing buffer, allowing further modification. `hostcomplete' If set, and Readline is being used, Bash will attempt to perform hostname completion when a word containing a `@' is being completed (*note Commands For Completion::.). This option is enabled by default. `huponexit' If set, Bash will send `SIGHUP' to all jobs when an interactive login shell exits (*note Signals::.). `interactive_comments' Allow a word beginning with `#' to cause that word and all remaining characters on that line to be ignored in an interactive shell. This option is enabled by default. `lithist' If enabled, and the `cmdhist' option is enabled, multi-line commands are saved to the history with embedded newlines rather than using semicolon separators where possible. `mailwarn' If set, and a file that Bash is checking for mail has been accessed since the last time it was checked, the message `"The mail in MAILFILE has been read"' is displayed. `nocaseglob' If set, Bash matches filenames in a case-insensitive fashion when performing filename expansion. `nullglob' If set, Bash allows filename patterns which match no files to expand to a null string, rather than themselves. `promptvars' If set, prompt strings undergo variable and parameter expansion after being expanded (*note Printing a Prompt::.). This option is enabled by default. `shift_verbose' If this is set, the `shift' builtin prints an error message when the shift count exceeds the number of positional parameters. `sourcepath' If set, the `source' builtin uses the value of `PATH' to find the directory containing the file supplied as an argument. This option is enabled by default. The return status when listing options is zero if all OPTNAMES are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an OPTNAME is not a valid shell option. `source' source FILENAME A synonym for `.' (*note Bourne Shell Builtins::.). `type' type [-atp] [NAME ...] For each NAME, indicate how it would be interpreted if used as a command name. If the `-t' option is used, `type' prints a single word which is one of `alias', `function', `builtin', `file' or `keyword', if NAME is an alias, shell function, shell builtin, disk file, or shell reserved word, respectively. If the NAME is not found, then nothing is printed, and `type' returns a failure status. If the `-p' option is used, `type' either returns the name of the disk file that would be executed, or nothing if `-t' would not return `file'. If the `-a' option is used, `type' returns all of the places that contain an executable named FILE. This includes aliases and functions, if and only if the `-p' option is not also used. The return status is zero if any of the NAMES are found, non-zero if none are found. `typeset' typeset [-afFrxi] [-p] [NAME[=VALUE]] The `typeset' command is supplied for compatibility with the Korn shell; however, it has been deprecated in favor of the `declare' builtin command. `ulimit' ulimit [-acdflmnpstuvSH] [LIMIT] `ulimit' provides control over the resources available to processes started by the shell, on systems that allow such control. If an option is given, it is interpreted as follows: `-S' Change and report the soft limit associated with a resource. `-H' Change and report the hard limit associated with a resource. `-a' All current limits are reported. `-c' The maximum size of core files created. `-d' The maximum size of a process's data segment. `-f' The maximum size of files created by the shell. `-l' The maximum size that may be locked into memory. `-m' The maximum resident set size. `-n' The maximum number of open file descriptors. `-p' The pipe buffer size. `-s' The maximum stack size. `-t' The maximum amount of cpu time in seconds. `-u' The maximum number of processes available to a single user. `-v' The maximum amount of virtual memory available to the process. If LIMIT is given, it is the new value of the specified resource. Otherwise, the current value of the soft limit for the specified resource is printed, unless the `-H' option is supplied. When setting new limits, if neither `-H' nor `-S' is supplied, both the hard and soft limits are set. If no option is given, then `-f' is assumed. Values are in 1024-byte increments, except for `-t', which is in seconds, `-p', which is in units of 512-byte blocks, and `-n' and `-u', which are unscaled values. The return status is zero unless an invalid option is supplied, a non-numeric argument other than `unlimited' is supplied as a LIMIT, or an error occurs while setting a new limit. The Set Builtin =============== This builtin is so complicated that it deserves its own section. `set' set [--abefhkmnptuvxBCHP] [-o OPTION] [ARGUMENT ...] If no options or arguments are supplied, `set' displays the names and values of all shell variables and functions, sorted according to the current locale, in a format that may be reused as input. When options are supplied, they set or unset shell attributes. Options, if specified, have the following meanings: `-a' Mark variables which are modified or created for export. `-b' Cause the status of terminated background jobs to be reported immediately, rather than before printing the next primary prompt. `-e' Exit immediately if a simple command (*note Simple Commands::.) exits with a non-zero status, unless the command that fails is part of an `until' or `while' loop, part of an `if' statement, part of a `&&' or `||' list, or if the command's return status is being inverted using `!'. `-f' Disable file name generation (globbing). `-h' Locate and remember (hash) commands as they are looked up for execution. This option is enabled by default. `-k' All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name. `-m' Job control is enabled (*note Job Control::.). `-n' Read commands but do not execute them; this may be used to check a script for syntax errors. This option is ignored by interactive shells. `-o OPTION-NAME' Set the option corresponding to OPTION-NAME: `allexport' Same as `-a'. `braceexpand' Same as `-B'. `emacs' Use an `emacs'-style line editing interface (*note Command Line Editing::.). `errexit' Same as `-e'. `hashall' Same as `-h'. `histexpand' Same as `-H'. `history' Enable command history, as described in *Note Bash History Facilities::. This option is on by default in interactive shells. `ignoreeof' An interactive shell will not exit upon reading EOF. `keyword' Same as `-k'. `monitor' Same as `-m'. `noclobber' Same as `-C'. `noexec' Same as `-n'. `noglob' Same as `-f'. `notify' Same as `-b'. `nounset' Same as `-u'. `onecmd' Same as `-t'. `physical' Same as `-P'. `posix' Change the behavior of Bash where the default operation differs from the POSIX 1003.2 standard to match the standard (*note Bash POSIX Mode::.). This is intended to make Bash behave as a strict superset of that standard. `privileged' Same as `-p'. `verbose' Same as `-v'. `vi' Use a `vi'-style line editing interface. `xtrace' Same as `-x'. `-p' Turn on privileged mode. In this mode, the `$BASH_ENV' and `$ENV' files are not processed, shell functions are not inherited from the environment, and the `SHELLOPTS' variable, if it appears in the environment, is ignored. This is enabled automatically on startup if the effective user (group) id is not equal to the real user (group) id. Turning this option off causes the effective user and group ids to be set to the real user and group ids. `-t' Exit after reading and executing one command. `-u' Treat unset variables as an error when performing parameter expansion. An error message will be written to the standard error, and a non-interactive shell will exit. `-v' Print shell input lines as they are read. `-x' Print a trace of simple commands and their arguments after they are expanded and before they are executed. `-B' The shell will perform brace expansion (*note Brace Expansion::.). This option is on by default. `-C' Prevent output redirection using `>', `>&', and `<>' from overwriting existing files. `-H' Enable `!' style history substitution (*note History Interaction::.). This option is on by default for interactive shells. `-P' If set, do not follow symbolic links when performing commands such as `cd' which change the current directory. The physical directory is used instead. By default, Ba