$ Linux Command

Processes

trap

Execute commands when signals or events occur.

shellsignalhandlercleanupbuiltin

Additional Notes

trap is a shell builtin that registers commands to run when the shell receives specific signals or when specified events occur, such as when a command exits, a function returns, or the shell exits. It is most commonly used in scripts to clean up temporary files, reset terminal settings, or ensure critical operations complete on interruption.

The trap builtin can catch most POSIX signals (SIGINT, SIGTERM, SIGHUP, SIGEXIT, SIGERR, SIGDEBUG, SIGRETURN). SIGKILL and SIGSTOP cannot be caught. The special pseudo-signals EXIT, ERR, RETURN, and DEBUG are bash extensions that trigger on shell events rather than OS signals.

Syntax

trap [-lp] [command] [signal_spec...]

Parameters

  • command: Shell command(s) to execute when the signal is received. An empty string "" resets the signal to be ignored. Omitting the command removes the handler (restores default behavior).
  • signal_spec: Signal name (e.g. INT, TERM, EXIT) or signal number.

Common Options

  • -l: List all signal names and their numbers.
  • -p: Print the current trap handlers for all signals.

Examples

trap 'rm -f /tmp/tmpfile' EXIT

Remove a temporary file when the script exits.

trap 'echo "Interrupted!"; exit 1' INT

Print a message and exit on Ctrl+C.

cleanup() {
    rm -f "$TMPDIR"/*
    rmdir "$TMPDIR"
}
trap cleanup EXIT

Use a named function for cleanup on exit.

trap '' INT

Ignore Ctrl+C (SIGINT) in a script.

trap -p

Print all current trap handlers.

trap -l

List all signal names.

trap 'echo "Command failed: $BASH_COMMAND"' ERR

Print the command that failed when any command returns non-zero.

Practical Notes

  • Always use EXIT instead of 0 for compatibility and clarity.
  • Multiple signal specs can be listed: trap handler INT TERM HUP.
  • Inside a trap handler, $? is the exit code of the command that triggered the trap (useful with ERR).
  • Reset a trap back to the default with trap - SIGNAL.
  • The ERR trap is not inherited by shell functions unless set -o errtrace is enabled.
  • Traps are reset when a function or subshell is entered unless inherit_errexit or funcext settings change the behavior.