trap Manual Page
Usage:
trap [-lp] [ [arg] sigspec ...]
The command arg is to be read and 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.
The -l option causes the shell to print a list of signal names and their corresponding numbers.
Each sigspec is either a signal name defined in <signal.h>, or a signal number.
Signal names are case insensitive and the SIG prefix is optional.
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 above).
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. The
return status is
false if any sigspec is invalid; otherwise trap returns true (Return code: 0)