The Bash String Operators

A common task in bash programming is to manipulate portions of a string and return the result. bash provides rich support for these manipulations via string operators. The syntax is not always intuitive so I wanted to use this blog post to serve as a permanent reminder of the operators.

The string operators are signified with the ${} notation. The operations can be grouped in to a few classes. Each heading in this article describes a class of operation.

Substring Extraction


Extract from a position

${string:position}

Extraction returns a substring of string starting at position and ending at the end of string. string is treated as an array of characters starting at 0.

> string="hello world"
> echo ${string:1}
ello world
> echo ${string:6}
world

Extract from a position with a length

${string:position:length}

Adding a length returns a substring only as long as the length parameter.

> string="hello world"
> echo ${string:1:2}
el
> echo ${string:6:3}
wor

Substring Removal


Remove shortest starting match

${variable#pattern}

If variable starts with pattern, delete the shortest part that matches the pattern.

> string="hello world, hello jim"
> echo ${string#*hello}
world, hello jim

Remove longest starting match

${variable##pattern}

If variable starts with pattern, delete the longest match from variable and return the rest.

> string="hello world, hello jim"
> echo ${string##*hello}
jim

Remove shortest ending match

${variable%pattern}

If variable ends with pattern, delete the shortest match from the end of variable and return the rest.

> string="hello world, hello jim"
> echo ${string%hello*}
hello world,

Remove longest ending match

${variable%%pattern}

If variable ends with pattern, delete the longest match from the end of variable and return the rest.

> string="hello world, hello jim"
> echo ${string%%hello*}

Substring Replacement


Replace first occurrence of word

${variable/pattern/string}

Find the first occurrence of pattern in variable and replace it with string. If string is null, pattern is deleted from variable. If pattern starts with #, the match must occur at the beginning of variable. If pattern starts with %, the match must occur at the end of the variable.

> string="hello world, hello jim"
> echo ${string/hello/goodbye}
goodbye world, hello jim

Replace all occurrences of word

${variable//pattern/string}

Same as above but finds all occurrences of pattern in variable and replace them with string. If string is null, pattern is deleted from variable.

> string="hello world, hello jim"
> echo ${string//hello/goodbye}
goodbye world, goodbye jim
bash 
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