Variables value gets lost in subshell

Question

This bash script concatenates the names for jar files to a classpath (variable CP), in the while loop the value is correct but is lost in the subshell as descibed in this re

Answer 1

Here is another solution, which avoid the spawning of a subshell altogether. The key is to capture the loop input into a variable (in my example called "JARS") and then redirect this variable into the loop using << EOF:

JARS=$(ls -1)

while read JAR
do
        if [ ! -z "$CP" ]; then
                CP=${CP}':'
        fi
        CP=${CP}${JAR}
done <<EOF
$JARS
EOF

echo $CP

Answer 2

Implicit subshells are confusing; always make them explicit by using parentheses. To solve your problem, just move the echo inside the subshell.

#!/bin/bash
CP="AAA"
func() {
  ls -1 | (
    while read JAR
    do
      if [ ! -z "$CP" ]; then
        CP=${CP}':'
      fi
      CP=${CP}${JAR}
    done
    echo $CP
  )
}
func

Answer 3

You might find using find a bit more versatile.

For example:

export CP=$( find /home/depesz/q/ -maxdepth 1 -type f -name '*.jar' -printf ':%p' | cut -b 2- )

Of course set of find options is dependant on what you need/want.

This one is closer to what you had previously:

export CP=$( find . -maxdepth 1 -type f -name '*.jar' -printf ':%f' | cut -b 2- )

Answer 4

The issue here is that using while in a pipeline creates a subshell, and a subshell cannot affect its parent. You can get around this in a few ways. For what you are doing now, this will suffice:

for JAR in *; do
    # Your stuff
done

Another thing to note is that you shouldn't rely on parsing ls

This also shows you ways to avoid the subshell.

Answer 5

None of these answers seem to get into actually returning the value properly, echoing out the answer is fine with a very simple routine, but say you want output of the script in action, this is useless. I don't have the best answer but I don't have much time to figure it out either, so I'm just going to suggest to output exactly what you want to a temp file and read that (surprised it hasn't been mentioned to date) :

#!/bin/bash
CP="AAA"
func() {
  ls -1 | (
    while read JAR
    do
      if [ ! -z "$CP" ]; then
        CP=${CP}':'
      fi
      CP=${CP}${JAR}
    done
    echo "$CP" > /tmp/cp-value-file
  )
}

func
CP=$(cat /tmp/cp-value-file)
echo $CP

Downside: will in some cases need to write to the disk every iteration of the loop.