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When Bash is interactive, in the absence of any traps, it ignores
SIGTERM (so that ‘kill 0’ does not kill an interactive shell),
and SIGINT
is caught and handled (so that the wait builtin is interruptible).
When Bash receives a SIGINT, it breaks out of any executing loops.
In all cases, Bash ignores SIGQUIT.
If job control is in effect (see Job Control), Bash
ignores SIGTTIN, SIGTTOU, and SIGTSTP.
Non-builtin commands started by Bash have signal handlers set to the
values inherited by the shell from its parent.
When job control is not in effect, asynchronous commands
ignore SIGINT and SIGQUIT in addition to these inherited
handlers.
Commands run as a result of
command substitution ignore the keyboard-generated job control signals
SIGTTIN, SIGTTOU, and SIGTSTP.
The shell exits by default upon receipt of a SIGHUP.
Before exiting, an interactive shell resends the SIGHUP to
all jobs, running or stopped.
Stopped jobs are sent SIGCONT to ensure that they receive
the SIGHUP.
To prevent the shell from sending the SIGHUP signal to a
particular job, it should be removed
from the jobs table with the disown
builtin (see Job Control Builtins) or marked
to not receive SIGHUP using disown -h.
If the huponexit shell option has been set with shopt
(see The Shopt Builtin), Bash sends a SIGHUP to all jobs when
an interactive login shell exits.
If Bash is waiting for a command to complete and receives a signal
for which a trap has been set, the trap will not be executed until
the command completes.
When Bash is waiting for an asynchronous
command via the wait builtin, the reception of a signal for
which a trap has been set will cause the wait builtin to return
immediately with an exit status greater than 128, immediately after
which the trap is executed.
When job control is not enabled, and Bash is waiting for a foreground
command to complete, the shell receives keyboard-generated signals
such as SIGINT (usually generated by ‘^C’) that users
commonly intend to send to that command.
This happens because the shell and the command are in the same process
group as the terminal, and ‘^C’ sends SIGINT to all processes
in that process group.
See Job Control, for a more in-depth discussion of process groups.
When Bash is running without job control enabled and receives SIGINT
while waiting for a foreground command, it waits until that foreground
command terminates and then decides what to do about the SIGINT:
SIGINT, Bash concludes
that the user meant to end the entire script, and acts on the
SIGINT (e.g., by running a SIGINT trap or exiting itself);
SIGINT, the program
handled the SIGINT itself and did not treat it as a fatal signal.
In that case, Bash does not treat SIGINT as a fatal signal,
either, instead assuming that the SIGINT was used as part of the
program’s normal operation (e.g., emacs uses it to abort editing
commands) or deliberately discarded. However, Bash will run any
trap set on SIGINT, as it does with any other trapped signal it
receives while it is waiting for the foreground command to
complete, for compatibility.
Previous: Exit Status, Up: Executing Commands [Contents][Index]