expr
expr command is used to perform shell arithmetic and evaluate expressions
Syntax/Usage
expr expression
- Variables defined in Linux bash shell stores values in the form of string.
- Integers are also stored as strings
- So to perform arithmetic operation on such variables ‘expr’ command is used.
Examples:- a=7
b=10
expr $a + $b // output: 17
c=`expr $a – 4` //result 3 is stored in variable ‘c’
echo `expr $b % $a` //remainder 3 is printed
Note: Notice space on either side of the ‘+’ operator .
- To access the value of value of ‘a’ and ‘b’ variable use ‘$’ symbol as prefix to variables.
- Use back tick ( ` ) for command substitution i.e using one command in the other. (Back tick is found above the tab key on your PC keyboard)
Figure 1
- Supported arithmetic operators are +, - , * , /, %
-
The operator ‘*’ must be preceded by a ‘\’backslash character else it will be considered as a wildcard character.
expr $a \* $b - Using brackets ( ) in expr command gives an error.
Alternative Way of arithmetic expression
- Another way to calculate the arithmetic expression is to enclose the expression in $((…))
- This way of evaluation let us make use of brackets as in normal mathematics.
Examples:-
c=$((a+b))
c=`expr $a + $((b*b*b))`
c=$((a+(b*b*b)))
Note:-
- Here there is no space on either side of the operator
- To access the value of variables no need of using ‘$’ sign separately again and again for each variable
- ‘*’ operator does not need to be preceded with backslash
- Double brackets preceded with ‘$’ symbol are must i.e $((…)) .