some notes on Linux string manipulation

linux bash shell script string manipulation

Bash String Manipulation Examples – Length, Substring, Find and Replace

1. Identify String Length inside Bash Shell Script

${#string}

The above format is used to get the length of the given bash variable.

$ cat len.sh#! /bin/bashvar="Welcome to the geekstuff"echo ${#var}$ ./len.sh24

To understand more about bash variables, read 6 Practical Bash Global and Local Variable Examples.

2. Extract a Substring from a Variable inside Bash Shell Script

Bash provides a way to extract a substring from a string. The following example expains how to parse n characters starting from a particular position.

${string:position}

Extract substring from $string at $position

${string:position:length}

Extract $length of characters substring from $string starting from $position. In the below example, first echo statement returns the substring starting from 15th position. Second echo statement returns the 4 characters starting from 15th position. Length must be the number greater than or equal to zero.

$ cat substr.sh#! /bin/bashvar="Welcome to the geekstuff"echo ${var:15}echo ${var:15:4}$ ./substr.shgeekstuffgeek

Also, refer to our earlier article to understand more about $*, $@, $#, $$, $!, $?, $-, $_ bash special parameters.

3. Shortest Substring Match

Following syntax deletes the shortest match of $substring from front of $string

${string#substring}

Following syntax deletes the shortest match of $substring from back of $string

${string%substring}

Following sample shell script explains the above two shortest substring match concepts.

$ cat shortest.sh#! /bin/bashfilename="bash.string.txt"echo ${filename#*.}echo ${filename%.*}$ ./shortest.shAfter deletion of shortest match from front: string.txtAfter deletion of shortest match from back: bash.string

In the first echo statement substring ‘*.’ matches the characters and a dot, and # strips from the front of the string, so it strips the substring “bash.” from the variable called filename. In second echo statement substring ‘.*’ matches the substring starts with dot, and % strips from back of the string, so it deletes the substring ‘.txt’

4. Longest Substring Match

Following syntax deletes the longest match of $substring from front of $string

${string##substring}

Following syntax deletes the longest match of $substring from back of $string

${string%%substring}

Following sample shell script explains the above two longest substring match concepts.

$ cat longest.sh#! /bin/bashfilename="bash.string.txt"echo "After deletion of longest match from front:" ${filename##*.}echo "After deletion of longest match from back:" ${filename%%.*}$ ./longest.shAfter deletion of longest match from front: txtAfter deletion of longest match from back: bash

In the above example, ##*. strips longest match for ‘*.’ which matches “bash.string.” so after striping this, it prints the remaining txt. And %%.* strips the longest match for .* from back which matches “.string.txt”, after striping  it returns “bash”.

5. Find and Replace String Values inside Bash Shell Script

Replace only first match

${string/pattern/replacement}

It matches the pattern in the variable $string, and replace only the first match of the pattern with the replacement.

$ cat firstmatch.sh#! /bin/bashfilename="bash.string.txt"echo "After Replacement:" ${filename/str*./operations.}$ ./firstmatch.shAfter Replacement: bash.operations.txt

Replace all the matches

${string//pattern/replacement}

It replaces all the matches of pattern with replacement.

$ cat allmatch.sh#! /bin/bashfilename="Path of the bash is /bin/bash"echo "After Replacement:" ${filename//bash/sh}$ ./allmatch.shAfter Replacement: Path of the sh is /bin/sh

Taking about find and replace, refer to our earlier articles – sed substitute examples and Vim find and replace.

Replace beginning and end

${string/#pattern/replacement

Following syntax replaces with the replacement string, only when the pattern matches beginning of the $string.

${string/%pattern/replacement

Following syntax replaces with the replacement string, only when the pattern matches at the end of the given $string.

$ cat posmatch.sh#! /bin/bashfilename="/root/admin/monitoring/process.sh"echo "Replaced at the beginning:" ${filename/#\/root/\/tmp}echo "Replaced at the end": ${filename/%.*/.ksh}$ ./posmatch.shReplaced at the beginning: /tmp/admin/monitoring/process.shReplaced at the end: /root/admin/monitoring/process.ksh