awk: Find and Replace text

On August 28, 2009, in awk, find, replace, Text, by Dipin Krishna
Syntax

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

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

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

Examples

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
 

2 Responses to “awk: Find and Replace text”

  1. Damir Colak says:

    Find and replace text and there is no REPLACE. Bravó!

  2. Matthew says:

    So where’s the replace portion of the post? I don’t see any sub()s or the like …

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