11

I'm a little confused with my script regarding functions, variable scope, and possibly subshells. I saw in another post that pipes spawn a subshell and the parent shell can't access variables from the subshell. Is this the same case with cmds run in backticks too?

To not bore people, I've shortened my 100+ line script but I tried to remember to leave in the important elements (i.e. backticks, pipes etc). Hopefully I didn't leave anything out.

global1=0
global2=0
start_read=true

function testfunc {
   global1=9999
   global2=1111
   echo "in testfunc"
   echo $global1
   echo $global2
}

file1=whocares
file2=whocares2

for line in `cat $file1`
do
   for i in `grep -P "\w+ stream" $file2 | grep "$line"`   # possible but unlikely problem spot
   do
         end=$(echo $i | cut -d ' ' -f 1-4 | cut -d ',' -f 1)   # possible but unlikely spot
         duration=`testfunc $end`       # more likely problem spot
   done
done

echo "global1 = $global1"
echo "global2 = $global2"

So when I run my script, the last line says global1 = 0. However, in my function testfunc, global1 gets set to 9999 and the debug msgs print out that within the function at least, it is 9999.

Two questions here:

  1. Do the backticks spawn a subshell and thus making my script not work?
  2. How do I work around this issue?
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4
  • 1
    1. Yes they do 2. Quick fix: remove the backticks and move the assignment in the function definition. Alternatively write global1= ` testfunc $end ` Commented Jan 8, 2014 at 21:21
  • As an aside, those nested for loops look really twisted and inefficient. I guess you are trying to say grep -P "\w+ stream" "$file2" | grep -f "$file1" | while read i; do... Commented Jan 8, 2014 at 21:38
  • @damienfrancois, thx for your reply. testfunc modifies several global vars. I guess that's one thing I forgot to include. Commented Jan 8, 2014 at 22:11
  • ok, then you can try replacing duration= ` testfunc $end ` with simply testfunc $end and adding duration=... in the body of the function. You can really think of bash functions as macro that are simply expanded inline. Commented Jan 8, 2014 at 22:15

2 Answers 2

Reset to default
3

You can try something like

global1=0
global2=0
start_read=true

function testfunc {
   global1=9999
   global2=1111
   echo "in testfunc"
   echo $global1
   echo $global2
   duration=something
}

file1=whocares
file2=whocares2

for line in `cat $file1`
do
   for i in `grep -P "\w+ stream" $file2 | grep "$line"`   # possible but unlikely problem spot
   do
         end=$(echo $i | cut -d ' ' -f 1-4 | cut -d ',' -f 1)   # possible but unlikely spot
         testfunc $end       # more likely problem spot
   done
done

echo "global1 = $global1"
echo "global2 = $global2"
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1 Comment

Difference to the script of the OP: (1) duration=something has been added to the function body; (2) duration=`testfunc $end` in the loop has been changed to testfunc $end
3

Do the backticks spawn a subshell and thus making my script not work?:

Yes they do and any changes made in variable in a subshell are not visible in parent shell.

How do I work around this issue?

You can probably try this loop that avoid spawning a subshell:

while read line
do
   while read i
   do
      end=$(echo $i | cut -d ' ' -f 1-4 | cut -d ',' -f 1)
      duration=$(testfunc "$end")
   done < <(grep -P "\w+ stream" "$file2" | grep "$line")
done < "$file1"

PS: But testfunc will still be called in sub process.

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