Command:   expr - evaluate experession
Syntax:    expr arg ...
Flags:     (none)
Example:   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.