while loop in Linux Shell Script

By: Dorris in Linux Tutorials on 2011-01-15

Syntax:

           while [ condition ]
           do
                 command1
                 command2
                 command3
                 ..
                 ....
            done

Loop is executed as long as given condition is true. For e.g..the for loop program can be written using while loop as:

$cat > nt1
        #!/bin/sh
        #
        #Script to test while statement
        #
        #
        if [ $# -eq 0 ]
        then
           echo "Error - Number missing form command line
        argument"
           echo "Syntax : $0 number"
           echo " Use to print multiplication table for
        given number"
        exit 1
        fi
        n=$1
        i=1
        while [ $i -le 10 ]
        do
          echo "$n * $i = `expr $i \* $n`"
          i=`expr $i + 1`
        done

Save it and try as

$ chmod 755 nt1
$./nt1 7

Above loop can be explained as follows:

n=$1	Set the value of command line argument to variable n. (Here it's set to 7 )
i=1	Set variable i to 1
while [ $i -le 10 ]	This is our loop condition, here if value of i is less than 10 then, shell execute all statements between do and done
do	Start loop
echo "$n * $i = `expr $i \* $n`"	Print multiplication table as 7 * 1 = 7 7 * 2 = 14 .... 7 * 10 = 70, Here each time value of variable n is multiply be i.
i=`expr $i + 1`	Increment i by 1 and store result to i. ( i.e. i=i+1) Caution: If you ignore (remove) this statement than our loop become infinite loop because value of variable i always remain less than 10 and program will only output 7 * 1 = 7 ... ... E (infinite times)
done	Loop stops here if i is not less than 10 i.e. condition of loop is not true. Hence loop is terminated.