gawk (gnu awk)

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      awk <options> 'Program' Input-File1 Input-File2 ...

      awk -f PROGRAM-FILE <options> Input-File1 Input-File2 ...

`-F FS'
`--field-separator FS'
     Use FS for the input field separator (the value of the `FS'
     predefined variable).

     Read the `awk' program source from the file PROGRAM-FILE, instead
     of from the first command line argument.

`-mf NNN'
`-mr NNN'
     The `f' flag sets the maximum number of fields, and the `r' flag
     sets the maximum record size.  These options are ignored by
     `gawk', since `gawk' has no predefined limits; they are only for
     compatibility with the Bell Labs research version of Unix `awk'.

`-v VAR=VAL'
`--assign VAR=VAL'
     Assign the variable VAR the value VAL before program execution

`-W traditional'
`-W compat'
     Use compatibility mode, in which `gawk' extensions are turned off.

`-W lint'
     Give warnings about dubious or non-portable `awk' constructs.

`-W lint-old'
     Warn about constructs that are not available in the original
     Version 7 Unix version of `awk'.

`-W posix'
     Use POSIX compatibility mode, in which `gawk' extensions are
     turned off and additional restrictions apply.

`-W re-interval'
     Allow interval expressions, in regexps.

`-W source=PROGRAM-TEXT'
`--source PROGRAM-TEXT'
     Use PROGRAM-TEXT as `awk' program source code.  This option allows
     mixing command line source code with source code from files, and is
     particularly useful for mixing command line programs with library

     Signal the end of options.  This is useful to allow further
     arguments to the `awk' program itself to start with a `-'.  This
     is mainly for consistency with POSIX argument parsing conventions.

     A series of patterns and actions: see below

     If no Input-File is specified then `awk' applies the Program to 
     "standard input", (piped output of some other command or the terminal.
     Typed input will continue until end-of-file (typing `Control-d')

Basic functions

The basic function of awk is to search files for lines (or other units of text) that contain a pattern. When a line matches, awk performs a specific action on that line.

The Program statement that tells `awk' what to do; consists of a series of "rules". Each rule specifies one pattern to search for, and one action to perform when that pattern is found.

For ease of reading, each line in an `awk' program is normally a separate Program statement , like this:

     pattern { action }
     pattern { action }

e.g. Display lines from my_file containing the string "123" or "abc" or "some text":

awk '/123/ { print $0 } 
     /abc/ { print $0 }
     /some text/ { print $0 }' my_file

A regular expression enclosed in slashes (`/') is an `awk' pattern that matches every input record whose text belongs to that set. e.g. the pattern /foo/ matches any input record containing the three characters `foo', *anywhere* in the record.

`awk' patterns may be one of the following:

/Regular Expression/        - Match =
Pattern && Pattern          - AND
Pattern || Pattern          - OR
! Pattern                   - NOT
Pattern ? Pattern : Pattern - If, Then, Else
Pattern1, Pattern2          - Range Start - end
BEGIN                       - Perform action BEFORE input file is read
END                         - Perform action AFTER input file is read

In addition to simple pattern matching `awk' has a huge range of text and arithmetic Functions, Variables and Operators.

`gawk' will ignore newlines after any of the following:

    , { ? : || && do else

Comments - start with a `#', and continue to the end of the line:

 # This program prints a nice friendly message


This program prints the length of the longest input line:

 awk '{ if (length($0) > max) max = length($0) }
      END { print max }' data

This program prints every line that has at least one field. This is an easy way to delete blank lines from a file (or rather, to
create a new file similar to the old file but from which the blank lines have been deleted)

 awk 'NF > 0' data

This program prints seven random numbers from zero to 100, inclusive.

 awk 'BEGIN { for (i = 1; i <= 7; i++)
                print int(101 * rand()) }'

This program prints the total number of bytes used by FILES.

 ls -lg FILES | awk '{ x += $5 } ; END { print "total bytes: " x }'

This program prints a sorted list of the login names of all users.

 awk -F: '{ print $1 }' /etc/passwd | sort

This program counts lines in a file.

 awk 'END { print NR }' data

This program prints the even numbered lines in the data file. If you were to use the expression `NR % 2 == 1' instead, it would print the odd numbered lines.

 awk 'NR % 2 == 0' data

"Justice is such a fine thing that we cannot pay too dearly for it - Alain-Rene Lesage


GNU Awk User Guide - more examples
`awk', `oawk', and `nawk' - Alternative, older and newer versions of awk
egrep - egrep foo FILES essentially the same as awk '/foo/' FILES ...
expr - Evaluate expressions
eval - Evaluate several commands/arguments
for - Expand words, and execute commands
grep - search file(s) for lines that match a given pattern
m4 - Macro processor
tr - Translate, squeeze, and/or delete characters
Equivalent Windows command: FOR - Conditionally perform a command several times.

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Simon Sheppard