The scope of variables in shell scripts

2008-01-01 00:00:00

Just today I ran into something shiny that peeked my interest. A shell script I'd written in Bash didn't work like I expected it to, with regards to the scope of a variable. I thought the incident was interesting enough to report, although I won't go into the whole scoping story too deeply.

What is basically boils down to is that there was a difference in the way two shells handle a certain situation. A difference that I didn't expect to be there. Not that exciting, but still very educational.

Scope?

Yeah. In most programming languages variables have a certain range within your program, within which they can be used. Some variables only exist within one subroutine, while other exist across the whole program or even across multiple parts of the whole.

In shell scripting things aren't that complicated, luckily. In most cases a variable that's set in one part of the script can be used in every other part of the script. There are some notable exceptions, one of which I ran into today without realising it.

The real code

My situation:

I have a command that outputs a number of lines, some of which I need. The lines that I'm interested in consist of various fields, two of which I need as variables. Depending on the value of one of these variables, a counter needs to be incremented.

I guess that sounds kinda complicated, so here's the real code snippet:

function check_transport_paths

{

TOTAL=`scstat -W | grep "Transport path:" | wc -l`

let COUNT=0



scstat -W | grep "Transport path:" | awk '{print $3" "$6}' | while read PATH STATUS

do

if [ $STATUS == "online" ]

then

let COUNT=$COUNT+1

fi

done



if [ $COUNT -lt 1 ]

then

echo "NOK - No transport paths online."

exit $STATE_CRITICAL

elif [ $COUNT -lt $TOTAL ]

then

echo "NOK - One or more transport paths offline."

exit $STATE_WARNING

fi

}

Where it goes wrong

While testing my script, I found out that $COUNT would never retain the value it gained in the while-loop. This of course led to the script always failing the check. After some fiddling about, I found out that the problem lay in the use of the while loop: it was being used that the end of a pipe.

To illustrate, the following -does- work.

let COUNT=0

while read i

do

let COUNT=$COUNT+$i

echo $COUNT

done



echo "Total is $COUNT."

This leads to the following output.

$ ./baka.sh

1

1

2

3

3

6

4

10

^D

Total is 10.

However, if I were to create a script called neko.sh that outputs the numbers one through four on seperate lines, which is then used in baka.sh... well... it doesn't work :D Regardez!

let COUNT=0

./neko.sh | while read i

do

let COUNT=$COUNT+$i

echo $COUNT

done



echo "Total is $COUNT."

This gives the following output

1

3

6

10

Total is 0.

Conclusions

After discussing the matter with two of my colleagues (one of them as puzzled as I was, and the other knowing what was going wrong) we came to the following conclusions.

This conclusion is supported by an example in the "Advanced Bash-scripting guide" by Mendel Cooper. In the following example an additional comment is made about the scoping of variables with redirected while loops. The comment warns that older shells branch a redirected while into a sub-shell, but also tells that Bash and Ksh this properly.

I guess our version of Bash is too old :3

Work around

A word of thanks

I'd like to thank my colleagues Dennis Roos and Tom Scholten for spending a spare hour with me, hacking at this problem. And I'd like to thank Ondrej Jombik for pointing out the fact that this article didn't make my conclusions very clear in its original version.


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