awk or gawk (gnu awk)

Find and Replace text, database sort/validate/index.


      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.

 --assign VAR=VAL
     Assign the variable VAR the value VAL before program execution begins.

 -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 V.7 Unix version of awk.

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

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

 --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 functions.

     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')

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.

Suppose we have a file in which each line is a name followed by a phone number. Let's say the file contains the line "Audrey 5550164."
In AWK, the first field is referred to as $1, the second as $2 and so on.
So an AWK program to retrieve Audrey's phone number is:
awk '$1 == "Audrey" {print $2}' numbers.txt
which means if the first field matches Audrey, then print the second field.

In awk, $0 is the whole line of arguments.

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 samplefile containing the string "123" or "abc" or "some text":

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

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 can 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

The special patterns BEGIN and END can be used to capture control before the first input line is read and after the last. BEGIN and END do not combine with other patterns.

Variable names with special meanings:

     CONVFMT  conversion  format  used  when  converting  numbers
              (default %.6g)

     FS       regular  expression  used  to separate fields; also
              settable by option -Ffs.

     NF       number of fields in the current record

     NR       ordinal number of the current record

     FNR      ordinal number of the current record in the current

     FILENAME the name of the current input file

     RS       input record separator (default newline)

     OFS      output field separator (default blank)

     ORS      output record separator (default newline)

     OFMT     output format for numbers (default %.6g)

     SUBSEP   separates multiple subscripts (default 034)

     ARGC     argument count, assignable

     ARGV     argument  array,  assignable;  non-null members are
              taken as filenames

     ENVIRON  array  of  environment  variables;  subscripts  are

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

The AWK language was named after its authors, Al Aho, Peter Weinberger and Brian Kernighan.


From an ls - l listing, return the fifth item ($5) from each line of the output:

$ ls -l | awk '{print $5}'

Print the Row Number (NR), then a dash and space ("- ") and then the first item ($1) from each line in samplefile.txt:

$ awk '{print NR "- " $1 }' samplefile.txt

Print the first item ($1) and then the third last item $(NF-2) from each line in samplefile.txt:

$ awk '{print $1, $(NF-2) }' samplefile.txt

Print 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.txt

Comparison with grep:

Running grep Dec against the following file listing would return the 3 rows shown in bold as it matches text in different places:

-rw-r--r-- 7 simon simon 12043 Jan 31 09:36 December.pdf
-rw-r--r-- 3 simon simon 1024 Dec 01 11:59 README
-rw-r--r-- 3 simon simon 5096 Nov 14 18:22 Decision.txt

Running awk '$6 == "Dec"'against the same file listing, the relational operator $6 matches the exact field (column 6 = Month) so it will list only the December file:

$ ls -l /tmp/demo | awk '$6 == "Dec"'

Print the length of the longest input line:

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

Print seven random numbers from zero to 100, inclusive:

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

Print the total number of bytes used by FILES:

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

Print the average file size of all .PNG files within a directory:

 ls -l *.png | gawk '{sum += $5; n++;} END {print sum/n;}' 

Print a sorted list of the login names of all users:

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

Count the lines in a file:

 awk 'END { print NR }' data

Print 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

“When the Last of the Great Auks Died, It Was by the Crush of a Fisherman’s Boot” ~ Samantha Galasso (Smithsonian)

Related linux commands

AWK Help page - Brian Kernighan.
Why you should learn just a little Awk - Gregable.
Multiversity - Tutorial aimed at beginners.
AWK tutorial and introduction - Bruce Barnett.
Learn By Example - AWK text processing.
GNU Awk User Guide - Full guide with examples.
Book: The Awk Programming Language(pdf) 1988, by Aho, Kernighan, and Weinberger
awk one liners - Eric Pement.
awk one liners explained & pt2 - Peteris Krumin (
Patrick Hartigan - How to use awk.

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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