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A Very Simple Example

The following command runs a simple awk program that searches the input file `BBS-list' for the string of characters: `foo'. (A string of characters is usually called a string. The term string is perhaps based on similar usage in English, such as "a string of pearls," or, "a string of cars in a train.")

awk '/foo/ { print $0 }' BBS-list

When lines containing `foo' are found, they are printed, because `print $0' means print the current line. (Just `print' by itself means the same thing, so we could have written that instead.)

You will notice that slashes, `/', surround the string `foo' in the awk program. The slashes indicate that `foo' is a pattern to search for. This type of pattern is called a regular expression, and is covered in more detail later (see section Regular Expressions). The pattern is allowed to match parts of words. There are single-quotes around the awk program so that the shell won't interpret any of it as special shell characters.

Here is what this program prints:

$ awk '/foo/ { print $0 }' BBS-list
-| fooey        555-1234     2400/1200/300     B
-| foot         555-6699     1200/300          B
-| macfoo       555-6480     1200/300          A
-| sabafoo      555-2127     1200/300          C

In an awk rule, either the pattern or the action can be omitted, but not both. If the pattern is omitted, then the action is performed for every input line. If the action is omitted, the default action is to print all lines that match the pattern.

Thus, we could leave out the action (the print statement and the curly braces) in the above example, and the result would be the same: all lines matching the pattern `foo' would be printed. By comparison, omitting the print statement but retaining the curly braces makes an empty action that does nothing; then no lines would be printed.

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