How is return handled in a function while loop?

Question

I have a function, and inside that function is a while loop.

When I try to set a non local variable inside the while loop with an IF statement, then exit the entire function, suddenly the variable is no longer set?

function EXAMPLE {
  cat test.txt | while read LINE; do
    if [ "$LINE" = "FAIL" ]; then
      echo "Detected FAIL in file! Setting RETURN=fail and exiting function."
      RETURN="fail"
      return
    fi
  done
}

### START SCRIPT ###
EXAMPLE (Call example function)
echo "$RETURN"

For some reason, RETURN is empty. I have done this many many many times in the past though. Something about the while loop is preventing RETURN from being passed out of the function. Is "return" causing the script to break the loop and not the function?

Thanks

something | otherthing : in bash, otherthing runs in a subshell. When it ends, the subshell exits. Any variable creater or modified in the subshell is not available in the parent shell, which still have their former values. — Olivier Dulac
– Olivier Dulac, Commented Jan 3, 2014 at 18:17
instead, use "return 1" when it finds "FAIL", and "return 0" if it ends the do..while without finding it. Then call it and use it's return value to decide what happened. — Olivier Dulac
– Olivier Dulac, Commented Jan 3, 2014 at 18:19

chepner · Accepted Answer · 2017-05-27 15:07:42Z

5

The easiest solution is to avoid the subshell in the first place, by using input redirection instead of a pipeline.

function EXAMPLE {
  while IFS= read -r line; do
    if [ "$line" = "FAIL" ]; then
      echo "Detected FAIL in file! Setting RETURN=fail and exiting function."
      RETURN="fail"
      return
    fi
  done < test.txt
}

In cases where the pipeline is unavoidable, bash 4.2 introduced the lastpipe option, which when enabled allows the last command in a pipeline to run in the current shell, not a subshell. This way, the value assigned to RETURN would be retained after the pipeline completes.

Even better, use the standard mechanism for signaling an error. Instead of setting the value of a custom parameter, just return a non-zero value:

function EXAMPLE {
  while IFS= read -r line; do
    if [ "$line" = "FAIL" ]; then
      echo "Detected FAIL in file! Exiting function with status 1."
      return 1
    fi
  done < test.txt
}

edited May 27, 2017 at 15:07

answered Jan 3, 2014 at 18:20

chepner

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3 Comments

Atomiklan Over a year ago

OMG FINALLY THANKYOU!!!!!!!!!!!!!!!!!!!!!!! Been stuck on this for two days now! You are a life saver. The official solution is rather complex. This was just an example question, but your solution is perfect conceptually. THANK YOU!

Daniel Griscom Over a year ago

I think the "while read LINE" should actually be "while read -r LINE"; "while" without the "-r" can give you strange behavior with uncontrolled input.

chepner Over a year ago

Yes, I wasn't as careful as I should have been with read in the past.

glenn jackman · Accepted Answer · 2014-01-03 18:18:32Z

1

When you use a pipeline in bash, all the commands within the pipeline are executed in subshells. That means variables cannot survive when the subshell exits. In this cate it's easy to remove the pipe

function EXAMPLE {
  while read LINE; do
    if [ "$LINE" = "FAIL" ]; then
      echo "Detected FAIL in file! Setting RETURN=fail and exiting function."
      RETURN="fail"
      return
    fi
  done < text.txt
}

I'd recommend not using ALL CAPS so often: one day you'll use a variable named PATH and then have to figure out why commands stop working.

answered Jan 3, 2014 at 18:18

glenn jackman

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2 Comments

Olivier Dulac Over a year ago

I'd do a "return 1" instead of a "return" [which returns 0, which usually means "ok", in many contexts]. That way you differentiate the return 1 when you find "FAIL" from the (induced) return 0 that will happen if the while...done ends without finding "FAIL"

Atomiklan Over a year ago

Thank you, this was correct too, but someone beat you too it.

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