Linux Signals

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External

Internal

Overview

Signals are numeric messages sent to running applications by the operating system, other applications, or the user. The signals are sent asynchronously, and allows the kernel to communicate asynchronously with processes. A signal is managed in a cascading manner. It's sent to the application or script, then if the application doesn't have a specific handler, it's pushed back to the shell or operating system. Some signals can't be managed within individual apps: for example SIGKILL is caught by the operating system and immediately kills the running application. For the rest of the signals, processes may or may not intercept the signals.

To send a specific signal to a process:

kill -<signal-value> <pid>
kill -2 3345

Signals

SIGHUP (1)

POSIX signal. Hangup.

Hangup is the signal that is sent to the process when the terminal closes on a foreground process. The default action the process should respond with is "terminate". Programs like nohup chose to intercept, then ignore this signal, protecting their children from termination when the TTY is disconnected.

Also see:

nohup
httpd Restart Now

SIGINT (2)

Sends the process an interrupt. Guaranteed to be present on all systems. The default action a process should respond with is "terminate".

bash sends SIGINT to the process running in foreground when Ctrl-C is pressed.

SIGQUIT (3)

The default action is "core dump". When sent into a Java Virtual Machine, Will trigger the JVM to generate a thread dump.

SIGILL (4)

The SIGILL signal is sent to a process when it attempts to execute an illegal, malformed, unknown, or privileged instruction.

SIGTRAP (5)

SIGFPE (8)

SIGKILL (9)

POSIX. Kill the process. The signal cannot be caught or ignored. Guaranteed to be present on all systems.

SIGUSR1 (10)

Also see:

httpd Graceful Restart

SIGSEGV (11)

SIGUSR2 (12)

SIGPIPE (13)

The default action is "terminate". The kernel sends SIGPIPE to any process which tries to write to a pipe with no readers.

SIGALRM (14)

SIGTERM (15)

Also see:

httpd Stop Now

SIGSTKFLT (16)

SIGCHLD (17)

The default action is "ignore".

When a process dies or changes state, the kernel sends a SIGCHLD to the parent process. The session leader (shell) keeps track of its jobs using this signal.

SIGCONT (18)

POSIX. Default action is "wake up". Continue executing, if stopped. This is the signal sent explicitly by the shell to a stopped application when the user invokes the fg command.

Also see:

Suspending JVM Execution

SIGSTOP (19)

POSIX. The default action is "suspend". It means stop executing. The signal cannot be caught or ignored, so it will unconditionally suspend the recipient.

Also see:

Suspending JVM Execution

SIGTSTP (20)

The default action is "suspend". Suspends a process executing in foreground. bash sends SIGTSTP to the process running in foreground when Ctrl-Z is pressed.

SIGTTIN (21)

SIGTTOU (22)

SIGURG (23)

SIGXCPU (24)

SIGXFSZ (25)

SIGVTALRM (26)

SIGPROF (27)

SIGWINCH (28)

The TTY device keeps track of the terminal size. When this information is updated, the TTY device sends SIGWINCH to the foreground job. Well-behaved interactive applications, such as editors, should react upon this, fetch the new terminal size from the TTY device and re-render themselves.

Also see:

httpd Graceful Stop

SIGIO (29)

SIGPWR (30)

SIGSYS (31)

SIGRTMIN (34)

Signals in Go

Signals in Go

Signals and bash

Handling Signals in bash