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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