coreutils: Examples of expr

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1 16.4.4 Examples of using ‘expr’
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1 Here are a few examples, including quoting for shell metacharacters.
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1    To add 1 to the shell variable ‘foo’, in Bourne-compatible shells:
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1      foo=$(expr $foo + 1)
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1    To print the non-directory part of the file name stored in ‘$fname’,
1 which need not contain a ‘/’:
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1      expr $fname : '.*/\(.*\)' '|' $fname
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1    An example showing that ‘\+’ is an operator:
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1      expr aaa : 'a\+'
1      ⇒ 3
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1      expr abc : 'a\(.\)c'
1      ⇒ b
1      expr index abcdef cz
1      ⇒ 3
1      expr index index a
1      error→ expr: syntax error
1      expr index + index a
1      ⇒ 0
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