The trap builtin command

trap [-lp] [[ARGUMENT] SIGNAL]

The trap command is used to "trap" signals and other events. In this context, "trapping" means to install handler code.

The shell code ARGUMENT is to be read and executed whenever the shell receives a signal or another event SIGNAL. The given SIGNAL specification can be

  • the name of a signal with the SIG prefix, e.g. SIGTERM
  • the name of a signal without the SIG prefix, e.g. TERM
  • the number of a signal (see trap -l), e.g. 15
  • the name or number of a special event (see table below), e.g. EXIT

Without any options or operands, trap prints a list of installed traps in a reusable format (equivalent to the -p option).

Special ARGUMENTs

  • if ARGUMENT is absent or - (dash), the signal/event handler is reset to its original value
  • if ARGUMENT is the null string, the signal/event is ignored

Special events

Name Code Description
EXIT 0 executed on shell exit
DEBUG executed before every simple command
RETURN executed when a shell function or a sourced code finishes executing
ERR executed each time a command's failure would cause the shell to exit when the ''-e'' option (''errexit'') is enabled

Options

Option Description
-l print a list of signal names and their corresponding numbers
-p display the trap commands associated with each signal specification in a reusable format

Return status

Status Reason
0 no error/success
!=0 invalid option
!=0 invalid signal specification

List installed traps

trap

Ignore terminal interrupt (Ctrl-C, SIGINT)

trap '' INT

  • trap is specified by POSIX(R) without the -l and -p options
  • in POSIX(R), beside signals, only EXIT (0) is valid as an event
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