Bash not equal string comparison

Shell Programming and Scripting
1

Why does this cause an infinite loop?

#!/bin/bash


while [[ "$(ipcs | awk '{print $2}')" != "Semaphore" ]]; do
    echo "$(ipcs | awk '{print $2}')"

done
       
echo 
exit 0

I have verified I eventually get Semaphore so it should break out of the while loop.

$ echo $(ipcs | awk '{print $2}')
Shared shmid 262145 294914 2326531 Semaphore semid Message msqid
2

Hi cokedude,
Could you please try following.

while [[ "$(ipcs | awk '{print $2}')" !~ "Semaphore" ]]; do
    echo "$(ipcs | awk '{print $2}')" 
done 
echo 
exit 0

Thanks,
R. Singh

3

For me what he tried looks right, but unfortunately he is comparing contents are not equal to a fixed string, so caught inside infinite loop.

---------- Post updated at 09:06 PM ---------- Previous update was at 09:04 PM ----------

Something like this should work I guess

#!/bin/bash


while [[ "$(ipcs | awk '{print $2}')" != *"Semaphore"* ]]; do
    echo "$(ipcs | awk '{print $2}')"

done
       
echo 
exit 0

I am bit confused with the contents

I tested like this, as soon as count reaches 500 it creates file and breaks while loop

#!/bin/bash

i=0
while [[ "$( ls )" != *"akshay"* ]]; do
	echo $i
	[ $i -eq 500 ] && touch unix.com_akshay_unix.com
i=$((i+1))
done

echo "released"
4

No need to use awk:-

#!/bin/bash

ipcs | while read v1 v2 skip
do
        [ "$v2" != "Semaphore" ] && echo "$v2" || break;
done

exit 0
5

This won't work :(.

$ bash ./whilee1
./whilee1: line 2: conditional binary operator expected
./whilee1: line 2: syntax error near `!~'
./whilee1: line 2: `while [[ "$(ipcs | awk '{print $2}')" !~ "Semaphore" ]]; do'

Is there a trick to get this to work?

$ while [[ "$(ipcs | awk '{print $2}')" != *"Semaphore"* ]]; do
>     echo "$(ipcs | awk '{print $2}')"
> 
> done


$ bash ./whilee1
6

The "trick" is to find out what the string is, not just what it looks like, so you can make a comparison which actually works.

I don't suppose you tried the while loop yet? It would be easiest to use that to check with, you could just print the value.

#!/bin/bash

ipcs | while read v1 v2 skip
do
        printf "Got [%s]\n" "$v2"

        [ "$v2" != "Semaphore" ] && echo "$v2" || break;
done

exit 0
7

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