KevinWMatthews / bash-signal_handler

Introduction to Linux signals and trapping them in a bash script.

Signals in Bash

Introduction to Linux systems and catching them in Bash.

Signals

Signals can signify errors, external events, and explicit requests.

SIGKILL and SIGSTOP always kill or stop a process. Applications can choose to handle or ignore all other signals.

TODO synchronous vs asynchronous

Default Bash signals

Bash will catch SIGINT. Bash wil ignore SIGTERM and SIGQUIT. Bash will exit a shell with SIGHUP.

List of signals

Signal	Value	Key	Name	Description
SIGHUP	1		Hangup	Terminate - terminal was disconnected
SIGINT	2	Ctrl+C	Interrupt	Terminate the program gracefully.
SIGQUIT	3	Ctrl+\	Quit	Terminate the program gracefully, with core dump. Used for debugging.
SIGABRT	6		Abort	Program detected an error and aborted itself.
SIGBUS	7		Bus error	Dereference invalid pointer. Attempt to access invalid address.
SIGKILL	9		Kill	Immediately terminate the program. Can not be handled, ignored or blocked; always fatal.
SIGUSR1	10		User-defined 1
SIGSEGV	11		Segfault	Segmentation violation. Invalid access to valid memory.
SIGUSR2	12		User-defined 2
SIGPIPE	13		Broken pipe
SIGTERM	15		Terminate	Terminate, gracefully or not. Chance for cleanup.
SIGCHLD	17		Child	Child process terminated. Sent to parent process.
SIGCONT	18		Continue	Resume program that was paused with SIGSTOP.
SIGSTOP	19		Stop	Stop (pause) the program. Resume with SIGCONT? Can not be handled or ignored.
SIGTSTP	20	Ctrl+Z	Suspend	Interactive stop. Can be handled or ignored, unlike SIGSTOP.
	20	Ctrl+Y	Delayed suspend
SIGIO	29		IO	File descriptor is ready for IO. Typically terminals and sockets only, but can be set with fcntl.
SIGSYS	31		Bad system call
SIGRTMIN	34
SIGRTMAX	64

Links

• Linux docs: man 7 signal
• List of signals: kill -l
• GNU.org
• Bash signal traps