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while loop in Linux Shell Script

By: Dorris in Linux Tutorials on 2011年01月15日 [フレーム]

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ドル number"
  echo " Use to print multiplication table for
 given number"
 exit 1
 fi
 n=1ドル
 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ドル 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.

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