Unix Signals

A signal is a message that can be sent to a running process.

Programs, users, or administrators can initiate signals.

For example, the proper method of telling the Internet Daemon (inetd) to re-read its configuration file is to send it a SIGHUP signal.

For example, if the current process ID (PID) of inetd is 4140, type:

kill -SIGHUP 4140

Another common use of signals is to stop a running process. To stop the inetd process completely, use this command:

kill 4140

By default, the kill command sends the SIGTERM signal. If SIGTERM fails, escalate to using the SIGKILL signal to stop the process:

kill -9 4140

Because SIGKILL cannot be handled, stopping a process with SIGKILL is generally considered a bad idea. Using SIGKILL prevents a process from cleaning up after itself and exiting gracefully.

Handling Signals

Each Unix signal has a default set of effects on a Unix program. Programmers can code their applications to respond in customized ways to most signals. These custom code pieces are called signal handlers.

A signal handler is unable to redefine two signals. SIGKILL always stops a process and SIGSTOP always moves a process from the foreground to the background. A signal handler cannot “catch” these two signals.

FreeBSD Signals

Solaris Signals