The trap of Bash trap

Posted 2022-01-14 weidagang2046

Can you spot the problem with the following Bash script?

resource_created="false"

function cleanup() 
  echo "Exit code: $?"
  if [[ $resource_created == "true" ]]; then
    echo "Clean up resource"
  else
    echo "Nothing to clean up"
  fi


function main() 
  resource_created="true"
  echo "Resource created"
  exit 1


trap cleanup EXIT

main | tee /tmp/my.log

The intent is that, we use global variable resource_created to track the state of the program, and register function cleanup as exit trap which prints the exit code and cleans up the resource if created. But it didn‘t work as expected, the actual output is:

Resource created
Exit code: 0
Nothing to clean up

Why? The catch is with the pipe |. When executing a pipe, each command of the pipe is in a separate process from the Bash process. The variable modified by main is lost. The exit code of main is also lost, because the exit code of a pipe is the exit code of the last command of the pipe. It becomes clear when we print the process IDs out. Watch out the difference between $$ and $BASHPID, we should use $BASHPID in this case.

resource_created="false"

function cleanup() 
  echo "Exit code: $?"
  if [[ $resource_created == "true" ]]; then
    echo "Clean up resource"
  else
    echo "Nothing to clean up"
  fi
  echo "cleanup() PID: $BASHPID"


function main() 
  echo "main() PID: $BASHPID"
  resource_created="true"
  echo "Resource created"
  exit 1


trap cleanup EXIT

echo "Bash PID: $BASHPID"
main | tee /tmp/my.log

Output:

Bash PID: 9852
main() PID: 9853
Resource created
Exit code: 0
Nothing to clean up
cleanup() PID: 9852

Then if global variable and exit code don‘t work, how do we untangle? File!

function cleanup() 
  if [[ -f /tmp/resource_created ]]; then
    echo "Exit code: $(cat /tmp/resource_created)"
    echo "Clean up resource"
  else
    echo "Nothing to clean up"
  fi


function main() 
  echo "Resource created"
  echo 1 >/tmp/resource_created
  exit 1


trap cleanup EXIT

main | tee /tmp/my.log

Output:

Resource created
Exit code: 1
Clean up resource

Okay, it is almost the end of this short blog post. Bash programming is tricky, watch out for the traps. Thanks for reading and happy coding!