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The gawk Command The gawk command takes as its arguments a Gawk instruction and a list of filenames The Gawk instruction is encased in single quotes and is read as one argument The Gawk instruction itself consists of two segments: a pattern and an action The action is enclosed in brackets The term "pattern" can be misleading It is perhaps clearer to think of the pattern segment as a condition The pattern segment can be either a pattern search or a test condition of the type found in programming languages The Gawk utility has a full set of operators with which to construct complex conditions You can think of a pattern search as just one other kind of condition for retrieving records Instead of simply matching patterns as in the case of grep, the user specifies a condition Records that meet that condition are then retrieved.

The actions in the action segment are then applied to the record The next example shows the syntax of a Gawk instruction, which you can think of as condition {action}: pattern {action} The Gawk utility operates on either files or the standard input You can list filenames on the command line after the instruction If there are no filenames listed, input is taken from the.

Expression rBumper( ) = 0 rBumper( ) =15 rBumper( ) rBumper( ) = 4 rBumper( ) = 12

standard input. The example below shows the structure of the entire Gawk instruction. The invocation of Gawk consists of the gawk keyword followed by a Gawk instruction and filenames. As with the sed commands, the instruction should be placed within single quotes to avoid interpretation by the shell. Since the condition and action are not separate arguments for Gawk, you need to enclose them both in one set of quotes. The next example shows the syntax of a gawk command: $ gawk 'pattern action { }' filenames You can think of the pattern in a Gawk instruction as referencing a line. The Gawk action is then performed on that line. The next two examples below print all lines with the pattern "Penguin". The pattern segment is a pattern search. A pattern search is denoted by a pattern enclosed in slashes. All records with this pattern are retrieved. The action segment in the first example contains the print command. The print command outputs the line to the standard output. books Tempest Shakespeare 15.75 Penguin Christmas Dickens 3.50 Academic Iliad Homer 10.25 Random Raven Poe 2.50 Penguin $ gawk '/Penguin/{print}' books Tempest Shakespeare 15.75 Penguin Raven Poe 2.50 Penguin

Tip Both the action and pattern have defaults that allow you to leave either of them out. The print action is the default action. If an action is not specified, the line is printed. The default pattern is the selection of every line in the text. If the pattern is not specified, the action is applied to all lines. In the second example, there is no action segment. The default action is then used, the print action. $ gawk '/Penguin/' books Tempest Shakespeare 15.75 Penguin Raven Poe 2.50 Penguin Pattern Searches and Special Characters Gawk can retrieve lines using a pattern search that contains special characters. The pattern is designated with a beginning and ending slash, and placed in the pattern segment of the Gawk instruction. /pattern/ {action}

The pattern search is performed on all the lines in the file If the pattern is found, the action is performed on the line In this respect, Gawk performs very much like an editing operation Like sed, a line is treated as a line of text and the pattern is searched for throughout the line In the next example, Gawk searches for any line with the pattern "Poe" When a match is found, the line is output $ gawk '/Poe/{print}' books Raven Poe 250 Penguin You can use the same special characters for Gawk that are used for regular expressions in the sed filter and the Ed editor The first example below searches for a pattern at the beginning of the line The special character ^ references the beginning of a line.

if all bumpers are not pressed if all bumpers are pressed if any bumper is pressed if only the front bumper is pressed if both the front and the left are pressed

The second example searches for a pattern at the end of a line using the special character $: $ gawk '/^Christmas/{print}' books Christmas Dickens 350 Academic $ gawk '/Random$/{print}' books Iliad Homer 1025 Random As in Ed and sed, you can use special characters to specify variations on a pattern The period matches any character, the asterisk matches repeated characters, and the brackets match a class of characters: , *, and [] In the first example below, the period is used to match any pattern in which a single character is followed by the characters "en": $ gawk '/en/{print}' books Tempest Shakespeare 1575 Penguin Christmas Dickens 350 Academic Raven Poe 250 Penguin The next example uses the brackets and asterisk special characters to specify a sequence of numbers.