(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