On Unix-like operating systems, the trap command is a function of the shell that responds to hardware signals and other events.

This page covers the bash built-in version of trap.

Description

trap defines and activates handlers to run when the shell receives signals or other special conditions.

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ARG is a command to be read and executed when the shell receives the signal(s) SIGNAL_SPEC. If ARG is absent (and a single SIGNAL_SPEC is supplied) or ARG is a dash ("-"), each specified signal is reset to its original value. If ARG is the null string, each SIGNAL_SPEC is ignored by the shell and by the commands it invokes.

If a SIGNAL_SPEC is EXIT (0), ARG is executed upon exit from the shell.

If a SIGNAL_SPEC is DEBUG, ARG is executed before every simple command.

If a SIGNAL_SPEC is RETURN, ARG is executed each time a shell function or a script run by the “.” or source built-in commands finishes executing.

A SIGNAL_SPEC of ERR means to execute ARG each time a command’s failure would cause the shell to exit when the -e option is enabled.

If no arguments are supplied, trap prints the list of commands associated with each signal.

Syntax

trap [-lp] [[ARG] SIGNAL_SPEC…]

Options

Examples

trap -l

Display a list of signal names and their corresponding numbers. The list resembles the following:

  1. SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP
  2. SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1
  3. SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
  4. SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
  5. SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ
  6. SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR
  7. SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3
  8. SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8
  9. SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
  10. SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
  11. SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7
  12. SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2
  13. SIGRTMAX-1 64) SIGRTMAX

trap

Display a list of the currently-set signal traps.

trap ‘rm -f /tmp/xyz$$; exit’ ERR EXIT

Set a trap which, on shell error or shell exit, deletes a temporary file xyz$$.

csh — The C shell command interpreter.exit — Exit the command shell.ksh — The Korn shell command interpreter.sh — The Bourne shell command interpreter.