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 dis‐

position (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 com‐

mand, every arithmetic for command, and before the first command

executes in a shell function (see SHELL GRAMMAR above).

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.

If a sigspec is ERR, the command arg is executed whenever a a pipe‐

line (which may consist of a single simple command), a list, or a

compound command returns 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

command executed in a && or || list except the command following

the final && or ||, any command in a pipeline but the last, or if

the command’s return value is being inverted using !. These are

the same conditions obeyed by the errexit (-e) option.

Signals ignored upon entry to the shell cannot be trapped or reset.

Trapped signals that are not being ignored are reset to their orig‐

inal values in a subshell or subshell environment when one is cre‐

ated. The return status is false if any sigspec is invalid; other‐

wise trap returns true.