bash while loop comparison not evaluating

Question

I am trying to understand the difference between coding for if and while statements in bash. In the script below, if I use IF I get the expected result using == OR -eq etc. However with the while loop each while seems to evaluate as true for all tests. While loops 1 & 3 should fail, correct?

A != B, 
A == B,
A == C

I have tried different combinations of [ "" "" ] or [[ "" "" ]] etc.

What is the correct syntax for while

thx Art

#!/bin/sh
set -x
myVarA="abc"
myVarB="abc"
myVarC="def"

while [[ ${myVarA} != ${myVarB} ]]
    echo "A does not equal B"
    i=$(($i+1))
    do sleep 1s
    break
done

while [[ ${myVarA} == ${myVarB} ]]
    echo "A does equal B"
    i=$(($i+1))
    do sleep 1s
    break
done

while [[ ${myVarA} == ${myVarC} ]]
    echo "A does equal C"
    i=$(($i+1))
    do sleep 1s
    break
done

renders the following,

+ myVarA=abc
+ myVarB=abc
+ myVarC=def
+ [[ abc != abc ]]
+ echo 'A does not equal B'
A does not equal B
+ i=1
+ sleep 1s
+ break
+ [[ abc == abc ]]
+ echo 'A does equal B'
A does equal B
+ i=2
+ sleep 1s
+ break
+ [[ abc == def ]]
+ echo 'A does equal C'
A does equal C
+ i=3
+ sleep 1s
+ break

The do part of your while loops are a few lines down the block. Possibly you should move them up to the beginning, i.e. while [[ condition ]]; do — Tom Fenech
– Tom Fenech, Commented Mar 4, 2015 at 0:16
@Tom, thanks for the lead, the answer below explained the wile condition extends until the :do and that I did not understand. thanks Art — art vanderlay
– art vanderlay, Commented Mar 4, 2015 at 0:39

Etan Reisner · Accepted Answer · 2015-03-04 00:34:54Z

1

Your do keywords are wildly misplaced.

Everything between while and do is the condition list.

So your condition list in the first block is

[[ ${myVarA} != ${myVarB} ]]; echo "A does not equal B"; i=$(($i+1))

which, as you might imagine, evaluates as true since i=$(($i+1)) evaluates as true.

Similarly for the other blocks.

Move the do to the end of the while lines

while [[ ${myVarA} != ${myVarB} ]]; do
    : do something
done

edited Mar 4, 2015 at 0:34

answered Mar 4, 2015 at 0:28

Etan Reisner

81.7k8 gold badges120 silver badges154 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

bash while loop comparison not evaluating

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related