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