Running programs conventionally have three input and output streams already available to them for reading and writing. These are known as the standard input, standard output, and standard error output. These streams are, by default, connected to your terminal, but they are often redirected with the shell, via the `<', `<<', `>', `>>', `>&' and `|' operators. Standard error is typically used for writing error messages; the reason we have two separate streams, standard output and standard error, is so that they can be redirected separately.
In other implementations of
awk, the only way to write an error
message to standard error in an
awk program is as follows:
print "Serious error detected!" | "cat 1>&2"
This works by opening a pipeline to a shell command which can access the
standard error stream which it inherits from the
This is far from elegant, and is also inefficient, since it requires a
separate process. So people writing
awk programs often
neglect to do this. Instead, they send the error messages to the
terminal, like this:
print "Serious error detected!" > "/dev/tty"
This usually has the same effect, but not always: although the
standard error stream is usually the terminal, it can be redirected, and
when that happens, writing to the terminal is not correct. In fact, if
awk is run from a background job, it may not have a terminal at all.
Then opening `/dev/tty' will fail.
gawk provides special file names for accessing the three standard
streams. When you redirect input or output in
gawk, if the file name
matches one of these special names, then
gawk directly uses the
stream it stands for.
awkexecution (typically the shell). Unless you take special pains in the shell from which you invoke
gawk, only descriptors 0, 1 and 2 are available.
The file names `/dev/stdin', `/dev/stdout', and `/dev/stderr' are aliases for `/dev/fd/0', `/dev/fd/1', and `/dev/fd/2', respectively, but they are more self-explanatory.
The proper way to write an error message in a
is to use `/dev/stderr', like this:
print "Serious error detected!" > "/dev/stderr"
gawk also provides special file names that give access to information
about the running
gawk process. Each of these "files" provides
a single record of information. To read them more than once, you must
first close them with the
(see section Closing Input and Output Files and Pipes).
The filenames are:
getuidsystem call (the real user ID number).
geteuidsystem call (the effective user ID number).
getgidsystem call (the real group ID number).
getegidsystem call (the effective group ID number).
getgroupssystem call. (Multiple groups may not be supported on all systems.)
These special file names may be used on the command line as data
files, as well as for I/O redirections within an
They may not be used as source files with the `-f' option.
Recognition of these special file names is disabled if
gawk is in
compatibility mode (see section Command Line Options).
Caution: Unless your system actually has a `/dev/fd' directory
(or any of the other above listed special files),
the interpretation of these file names is done by
For example, using `/dev/fd/4' for output will actually write on
file descriptor 4, and not on a new file descriptor that was
from file descriptor 4. Most of the time this does not matter; however, it
is important to not close any of the files related to file descriptors
0, 1, and 2. If you do close one of these files, unpredictable behavior
The special files that provide process-related information may disappear
in a future version of
See section Probable Future Extensions.
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