1

I tried it with some example but it is not clear to me.

FIRST EXAMPLE

#!/bin/bash
i=0
while [ $i -lt 10 ]
do
    echo "$i"
    i=`echo "$i + 1" | bc`
done

output ===> 0,1,2,3,4,5,6,7,8,9

now when i executed while "while [ true ] " it gives the output as infinite loop //I agree//
but when i executed while "while [ false ] " again it gives the output as infinite loop. /I dont agree/

Can you please explain the second while loop? and how it is working actually?

while [ false ]    #give the infinite loop
while [ true ]     #give the infinite loop
while [ 0 ]        #give the infinite loop
while [ 1 ]        #give the infinite loop
while [ `ls` ]
while [ `echo 1` ] #give the infinite loop
while((0))         #loop not execute
while((1))         #give the infinite loop
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6
  • 1
    First, do you know the difference between [] and (()) ? [] is a test operator. (()) arithmetic evaluator. Commented Feb 10, 2016 at 17:46
  • what is return statement of "[ $i -lt 10 ]" if i=5 @Vikyboss Commented Feb 10, 2016 at 17:50
  • 1
    When you evaluate a expression inside test operator [], it should return an exit code of 0 for the expression evaluating to true and non-zero if false. [ $i -lt 10 ] is saying i(5) is less than 10, so it returns an exit code of 0. Commented Feb 10, 2016 at 17:52
  • 1
    I would suggest you to read man page of 'test' and 'bash'. Commented Feb 10, 2016 at 17:53
  • if we write the script like while 1 do echo "hello" done while 0 do echo "hello" done Commented Feb 10, 2016 at 18:04

1 Answer 1

Reset to default
2

In while [ false ] the false is neither a command nor a boolean value. The while expects a command but [ ... ] with no operators just checks for any non-empty string. Thus [ false ] is true. It is in this sense the same as [ faaaalseeee ].

What you meant is:

while true; do ...

while false; do ...
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9
  • then how it execute...?? like c language "while(0)" --> it will never execute Commented Feb 10, 2016 at 17:40
  • 2
    @Tiger I don't understand the question. Commented Feb 10, 2016 at 17:41
  • then what about this statement " echo [ $i -lt 10 ] " Commented Feb 10, 2016 at 17:43
  • simply i want when while loop is execute and what is return statement of "[ $i -lt 10 ]" if i=5 Commented Feb 10, 2016 at 17:47
  • @Tiger Your first example does show that the exit code of [ 5 -lt 10 ] is zero (i.e. true). echo [ $i -lt 10 ] is just an echo statement, the [ $i -lt 10 ] part has no meaning other than a string (or rather: several strings). echo always exists with zero no matter what the output string is. Commented Feb 10, 2016 at 17:56

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