expr - evaluate experession
expr arg ...
x=`expr $x + 1` # Add 1 to shell variable x
Expr computes the value of its argument and writes the result on standard output. The valid operators, in order of increasing precedence, are listed below. Operators grouped by {...} have the same precedence. The operators are: |, &, {<, <=, ==, !=, >=, >}, {+, -}, *, /, %, and :. Parentheses are permitted.
sh(1), test(1).