Arithmetic in Shell Scripts

The Borne shell does not have any built-in ability to evaluate simple arithmetic statements. This is all right because the Unix command expr performs simple mathematics as well as numerical comparisons from the command line:

% expr 3 + 2   	Add 3 and 2.   
5   	The shell's answer.   

These numerical comparisons are only for integers, with mandatory spaces separating the integers and operators. Operators that are special shell characters, like * or |, must be preceded with an backslash \ or surrounded by quotes. The permitted operations are:

---

expr OPERATIONS

x + y

Add two numbers.

x - y

Subtraction.

x * y

Multiplication.

x / y

Division.

x % y

Remainder.

x < y

Returns 1 if true 0, if false.

x <= y

Less than or equal to.

x = y

Test for equal to.

x != y

Test for not equal to.

x >= y

Greater than or equal to.

x > y

Greater than.

x | y

x if , else y.

x & y

x if both x and y , else 0.

x : y

String compare. See manual.

From within a shell script you use expr to increment a variable by assigning the output of expr to the variable:

 
N=`expr $N + 1`   	Increment N by one.   

We use this idea in the example below in which we run the user program bigmat with input files given on the command line and with the output stored in a different file for each input file name. The output file names are output.N where N is used as a counter:

 
#!/bin/sh   	   
N="1"   	Be sure to initialize N.   
for file   	Loop over the arguments.   
do   	    
  outfile="output.$N"   	Create output file name.   
  echo "Running bigmat - input: $file output: $outfile."   	   
  bigmat < $file > $outfile   	   
  N=`expr $N + 1`   	Increment N.   
done   	   
exit