Execute a command when the shell receives a signal.

      trap [-lp] [[arg] sigspec ...]

   -p   Display signal commands (or trap commands for sigspec)

   -l   Print a list of signal names and their corresponding numbers.

   arg  The command to be executed when the shell receives signal(s) sigspec.

If arg is absent (and there is a single sigspec) or -, each specified signal is reset to its original disposition (the value it had upon entrance to the shell).

If arg is the null string the signal specified by each sigspec is ignored by the shell and by the commands it invokes.

If arg is not present and -p has been supplied, then the trap commands associated with each sigspec are displayed. If no arguments are supplied or if only -p is given, trap prints the list of commands associated with each signal.

Each sigspec is either a signal name, or a signal number. Signal names are case insensitive and the SIG prefix is optional.

Number SIG      Meaning
0     0        On exit from shell
1     SIGHUP   Clean tidyup
2     SIGINT   Interrupt (CTRL-C)
3     SIGQUIT  Quit
6     SIGABRT  Cancel
9     SIGKILL  Die Now (cannot be trap'ped)
14    SIGALRM  Alarm Clock
15    SIGTERM  Terminate

If a sigspec is EXIT (0) the command arg is executed on exit from the shell.
If a sigspec is DEBUG, the command arg is executed before every simple command, for command, case command, select command, every arithmetic for command, and before the first command executes in a shell function (see SHELL GRAMMAR). Refer to the description of the extdebug option to the shopt builtin for details of its effect on the DEBUG trap.

If a sigspec is ERR, the command arg is executed whenever a simple command has a non-zero exit status, subject to the following conditions. The ERR trap is not executed if the failed command is part of the command list immediately following a while or until keyword, part of the test in an if statement, part of a && or || list, or if the command’s return value is being inverted via !.
These are the same conditions obeyed by the errexit option.

If a sigspec is RETURN, the command arg is executed each time a shell function or a script executed with the . or source builtins finishes executing.

Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals that are not being ignored are reset to their original values in a child process when it is created.

When Bash receives a signal for which a trap has been set while waiting for a command to complete, the trap will not be executed until the command completes. When Bash is waiting for an asynchronous command via the wait built-in, the reception of a signal for which a trap has been set will cause the wait built-in to return immediately with an exit status greater than 128, immediately after which the trap is executed.

The return status is False if any sigspec is invalid; otherwise trap returns True.

This is a BASH shell builtin, to display your local syntax from the bash prompt type: help trap


A shell script routine that will create a temporary file and then remove it if the script exits with a non-zero signal:


$ SCRATCH=$(mktemp -t tmp.XXXXXXXXXX)

trap cleanup 1 2 3 6

  echo "Cleaning up temp files:"
  rm -rf "$SCRATCH"
  exit 1

### continue main script below

“Meetings are a great trap. Soon you find yourself trying to get agreement and then the people who disagree come to think they have a right to be persuaded. However, they are indispensable when you don’t want to do anything” ~ John Kenneth Galbraith

Related macOS commands

exec - Execute a command.

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