(coreutils.info)Examples of expr

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16.4.4 Examples of using `expr'
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Here are a few examples, including quoting for shell metacharacters.

   To add 1 to the shell variable `foo', in Bourne-compatible shells:

     foo=`expr $foo + 1`

   To print the non-directory part of the file name stored in `$fname',
which need not contain a `/':

     expr $fname : '.*/\(.*\)' '|' $fname

   An example showing that `\+' is an operator:

     expr aaa : 'a\+'
     => 3

     expr abc : 'a\(.\)c'
     => b
     expr index abcdef cz
     => 3
     expr index index a
     error--> expr: syntax error
     expr index + index a
     => 0