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16.4.4 Examples of using expr

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