Expr

expr is a command line utility on Unix and Unix-like operating systems which evaluates an expression and outputs the corresponding value. It first appeared in Unix v7. The command has also been ported to the IBM i operating system.
Overview.
"expr" evaluates integer or string expressions, including pattern matching regular expressions. Each symbol (operator, value, etc.) in the "expression" must be given as a separate parameter. Most of the challenge posed in writing expressions is preventing the invoking command line shell from acting on characters intended for "expr" to process.
Syntax.
Syntax: 
The operators available
Example.
The following is an example involving boolean expressions:
 expr length "abcdef" "<" 5 "|" 15 - 4 ">" 8
This example outputs "1". This is because length "abcdef" is 6, which is not less than 5 (so the left side of the | returns zero). But 15 minus 4 is 11 and is greater than 8, so the right side is true, which makes the "or" true, so 1 is the result. The program exit status is zero for this example.
For pure arithmetic, it is often more convenient to use bc. For example:
 echo "3*4+14/2" | bc
since it accepts the expression as a single argument.
For portable shell programming use of the length and substr commands is not recommended.