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Bash substring syntax

Bash supports a powerful substring syntax using variable expansion. With the right indizes it's possible to extract any part of a string. The syntax has 2 forms (example of a variable $str):

  1. ${str:position}
  2. ${str:position:length}

The behavior depends on the syntax used and whether position (pos) and length (len) are positive or negative:

pos len Extraction start Extraction end Example with str='0123456789'
< 0 string end - pos string end ${str: -5} => 56789
>= 0 pos string end ${str:3} => 3456789
< 0 < 0 string end - pos string end - len ${str: -5:-2} => 567
< 0 > 0 string end - pos string end - pos + len ${str: -5:3} => 567
>= 0 < 0 pos string end - len ${str:3:-3} => 3456
>= 0 > 0 pos pos + len ${str:3:3} => 345

It can be helpful to see the extraction limiters like markers which are either a certain number of characters
in the string or a certain amount of characters from the string end. It's as if initially the marker was before the first character respectively after the last one in case of backward search. Example: ${str:3:-3} for str='0123456789':

  • pos is 3, ^ is the marker => "012^3456789"
  • len is -3, ^ is the marker => "0123456^789"
  • the extracted string is 3456

The examples in the table below shows commands where a space was inserted when position is negative. This is required to avoid a syntax collision with bash's "default value fallback" syntax ${var:-default} (returns default if $var not set). When a variable is used, it's not required:

Code Result
1 str='0123456789'
2 minus=-5
3 echo "\$str: $str - \$minus: $minus" $str: 0123456789 - $minus: -5
4 echo "\${str:-5}: ${str:-5}" ${str:-5}: 0123456789
5 echo "\${str: -5}: ${str: -5}" ${str: -5}: 56789
6 echo "\${str:\$minus}: ${str:$minus}" ${str:$minus}: 56789
7 echo "\${str:3}: ${str:3}" ${str:3}: 3456789
8 echo "\${str:-5:-2}: ${str:-5:-2}" ${str:-5:-2}: 0123456789
9 echo "\${str: -5:-2}: ${str: -5:-2}" ${str: -5:-2}: 567
10 echo "\${str:\$minus:-2}: ${str:$minus:-2}" ${str:$minus:-2}: 567
11 echo "\${str:-5:3}: ${str:-5:3}" ${str:-5:3}: 0123456789
12 echo "\${str: -5:3}: ${str: -5:3}" ${str: -5:3}: 567
13 echo "\${str:\$minus:3}: ${str:$minus:3}" ${str:$minus:3}: 567
14 echo "\${str:3:-3}: ${str:3:-3}" ${str:3:-3}: 3456
15 echo "\${str:3:3}: ${str:3:3}" ${str:3:3}: 345