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20.10. Quick Reference: awk

Up to this point, we've shown you tools to do basic batch editing of text files. These tools, although powerful, have limitations. Although you can script ex commands, the range of text manipulation is quite limited. If you need more powerful and flexible batch editing tools, you need to look at programming languages that are designed for text manipulation. One of the earliest Unix languages to do this is awk, created by Al Aho, Peter Weinberger, and Brian Kernighan. Even if you've never programmed before, there are some simple but powerful ways that you can use awk. Whenever you have a text file that's arranged in columns from which you need to extract data, awk should come to mind.

For example, every Red Hat Linux system stores its version number in /etc/redhat-release. On my system, it looks like this:

Red Hat Linux release 7.1 (Seawolf)

When applying new RPM files to your system, it is often helpful to know which Red Hat version you're using. On the command line, you can retrieve just that number with:

awk '{print $5}' /etc/redhat-release

What's going on here? By default, awk splits each line read from standard input on whitespace, as is explained below. In effect, it's like you are looking at one row of a spreadsheet. In spreadsheets, columns are usually named with letters. In awk, columns are numbered and you only can see one row (that is, one line of input) at a time. The Red Hat version number is in the fifth column. Similar to the way shells use $ for variable interpolation, the values of columns in awk are retrieved using variables that start with $ and are followed by an integer.

As you can guess, this is a fairly simple demostration of awk, which includes support for regular expressions, branching and looping, and subroutines. For a more complete reference on using awk, see Effective awk Programming or sed & awk Pocket Reference, both published by O'Reilly.

Since there are many flavor of awk, such as nawk and gawk (Section 18.11), this article tries to provide a usable reference for the most common elements of the language. Dialect differences, when they occur, are noted. With the exception of array subscripts, values in [ brackets] are optional; don't type the [ or ].

20.10.1. Command-Line Syntax

awk can be invoked in one of two ways:

awk [options] 'script' [var=value] [file(s)]
awk [options] -f scriptfile [var=value] [file(s)]

You can specify a script directly on the command line, or you can store a script in a scriptfile and specify it with -f. In most versions, the -f option can be used multiple times. The variable var can be assigned a value on the command line. The value can be a literal, a shell variable ($name), or a command substitution ('cmd'), but the value is available only after a line of input is read (i.e., after the BEGIN statement). awk operates on one or more file(s). If none are specified (or if - is specified), awk reads from the standard input (Section 43.1).

The other recognized options are:

Set the field separator to character c. This is the same as setting the system variable FS. nawk allows c to be a regular expression (Section 32.4). Each record (by default, one input line) is divided into fields by whitespace (blanks or tabs) or by some other user-definable field separator. Fields are referred to by the variables $1, $2, . . . $n. $0 refers to the entire record. For example, to print the first three (colon-separated) fields on separate lines:

    % awk -F: '{print $1; print $2; print $3}' /etc/passwd
-v var=value
Assign a value to variable var. This allows assignment before the script begins execution. (Available in nawk only.)

20.10.2. Patterns and Procedures

awk scripts consist of patterns and procedures:

pattern {procedure}

Both are optional. If pattern is missing, {procedure} is applied to all records. If {procedure} is missing, the matched record is written to the standard output. Patterns

pattern can be any of the following:

/regular expression/
relational expression
pattern-matching expression

Except for BEGIN and END, patterns can be combined with the Boolean operators || ( OR), && (AND), and ! (NOT). A range of lines can also be specified using comma-separated patterns:

pattern,pattern Simple pattern-procedure examples

  • Print the first field of each line:

    { print $1 }
  • Print all lines that contain pattern:

  • Print first field of lines that contain pattern:

    /pattern/{ print $1 }
  • Print records containing more than two fields:

    NF > 2
  • Interpret input records as a group of lines up to a blank line:

    BEGIN { FS = "\n"; RS = "" }
    { ...process records... }
  • Print fields 2 and 3 in switched order, but only on lines whose first field matches the string URGENT:

    $1 ~ /URGENT/ { print $3, $2 }
  • Count and print the number of pattern found:

    /pattern/ { ++x }
    END { print x }
  • Add numbers in second column and print total:

    {total += $2 };
    END { print "column total is", total}
  • Print lines that contain fewer than 20 characters:

    length($0) < 20
  • Print each line that begins with Name: and that contains exactly seven fields:

    NF == 7 && /^Name:/

20.10.7. Alphabetical Summary of Commands

The following alphabetical list of statements and functions includes all that are available in awk, nawk, or gawk. Unless otherwise mentioned, the statement or function is found in all versions. New statements and functions introduced with nawk are also found in gawk.


Returns the arctangent of y/x in radians. (nawk)

Exit from a while, for, or do loop.

close(filename-expr) close(command-expr)

In some implementations of awk, you can have only ten files open simultaneously and one pipe; modern versions allow more than one pipe open. Therefore, nawk provides a close statement that allows you to close a file or a pipe. close takes as an argument the same expression that opened the pipe or file. (nawk)

Begin next iteration of while, for, or do loop immediately.


Return cosine of x (in radians). (nawk)

delete array[element]

Delete element of array. (nawk)

do body while (expr)

Looping statement. Execute statements in body, then evaluate expr. If expr is true, execute body again. More than one command must be put inside braces ({}). (nawk)


Do not execute remaining instructions and do not read new input. END procedure, if any, will be executed. The expr, if any, becomes awk's exit status (Section 34.12).


Return the natural exponent of arg.

for ([init-expr]; [test-expr]; [incr-expr]) command

C-language-style looping construct. Typically, init-expr assigns the initial value of a counter variable. test-expr is a relational expression that is evaluated each time before executing the command. When test-expr is false, the loop is exited. incr-expr is used to increment the counter variable after each pass. A series of commands must be put within braces ({}). For example:

for (i = 1; i <= 10; i++)
     printf "Element %d is %s.\n", i, array[i]
for (item in array) command

For each item in an associative array, do command. More than one command must be put inside braces ({}). Refer to each element of the array as array[item].

getline [var][<file] or command | getline [var]

Read next line of input. Original awk does not support the syntax to open multiple input streams. The first form reads input from file, and the second form reads the standard output of a Unix command. Both forms read one line at a time, and each time the statement is executed, it gets the next line of input. The line of input is assigned to $0, and it is parsed into fields, setting NF, NR, and FNR. If var is specified, the result is assigned to var and the $0 is not changed. Thus, if the result is assigned to a variable, the current line does not change. getline is actually a function, and it returns 1 if it reads a record successfully, 0 if end-of-file is encountered, and -1 if for some reason it is otherwise unsuccessful. (nawk)


Globally substitute s for each match of the regular expression r in the string t. Return the number of substitutions. If t is not supplied, defaults to $0. (nawk)

if (condition) command [else command]

If condition is true, do command(s), otherwise do command(s) in else clause (if any). condition can be an expression that uses any of the relational operators <, <=, ==, != , >=, or >, as well as the pattern-matching operators ~ or !~ (e.g., if ($1 ~ /[Aa].*[Zz]/)). A series of commands must be put within braces ({}).


Return position of first substring substr in string str or 0 if not found.


Return integer value of arg.


Return the length of arg.


Return the natural logarithm of arg.


Function that matches the pattern, specified by the regular expression r, in the string s and returns either the position in s where the match begins or 0 if no occurrences are found. Sets the values of RSTART and RLENGTH. (nawk)

Read next input line and start new cycle through pattern/procedures statements.

print [args] [destination]

Print args on output, followed by a newline. args is usually one or more fields, but it may also be one or more of the predefined variables -- or arbitrary expressions. If no args are given, prints $0 (the current input record). Literal strings must be quoted. Fields are printed in the order they are listed. If separated by commas (,) in the argument list, they are separated in the output by the OFS character. If separated by spaces, they are concatenated in the output. destination is a Unix redirection or pipe expression (e.g., > file) that redirects the default standard output.

printf format [, expression(s)] [destination]

Formatted print statement. Fields or variables can be formatted according to instructions in the format argument. The number of expressions must correspond to the number specified in the format sections. format follows the conventions of the C-language printf statement. Here are a few of the most common formats:

A string.

A decimal number.

A floating-point number, where n is the total number of digits and m is the number of digits after the decimal point.

n specifies minimum field length for format type c, while - left-justifies value in field; otherwise value is right-justified.

format can also contain embedded escape sequences: \n (newline) or \t (tab) are the most common. destination is a Unix redirection or pipe expression (e.g., > file) that redirects the default standard output.

For example, using the following script:

{printf "The sum on line %s is %d.\n", NR, $1+$2}

and the following input line:

5   5

produces this output, followed by a newline:

The sum on line 1 is 10.
rand( )

Generate a random number between 0 and 1. This function returns the same series of numbers each time the script is executed, unless the random number generator is seeded using the srand( ) function. (nawk)

return [expr]

Used at end of user-defined functions to exit the function, returning value of expression expr, if any. (nawk)


Return sine of x (in radians). (nawk)


Split string into elements of array array[1], . . . ,array[n]. string is split at each occurrence of separator sep. (In nawk, the separator may be a regular expression.) If sep is not specified, FS is used. The number of array elements created is returned.

sprintf (format [, expression(s)])

Return the value of expression(s), using the specified format (see printf). Data is formatted but not printed.


Return square root of arg.


Use expr to set a new seed for random number generator. Default is time of day. Returns the old seed. (nawk)


Substitute s for first match of the regular expression r in the string t. Return 1 if successful; 0 otherwise. If t is not supplied, defaults to $0. (nawk)


Return substring of string, beginning at character position m and consisting of the next n characters. If n is omitted, include all characters to the end of string.


Function that executes the specified Unix command and returns its status (Section 34.12). The status of the command that is executed typically indicates its success (0) or failure (nonzero). The output of the command is not available for processing within the nawk script. Use command | getline to read the output of the command into the script. (nawk)


Translate all uppercase characters in str to lowercase and return the new string. (nawk)


Translate all lowercase characters in str to uppercase and return the new string. (nawk)

while (condition) command

Do command while condition is true (see if for a description of allowable conditions). A series of commands must be put within braces ({}).

-- DG

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