Unix signals: a crash course



  1. Learn to send and handle common Unix signals
  2. Be able to handle sigterm and sigint while cleaning up temporary processes

List of Unix signals

We’re going to focus on four signals:

Signal Name Effect
1 SIGHUP Reload configuration (hack!)
2 SIGINT Terminate process, allow for cleanup
9 SIGKILL Terminate immediately - no cleanup
15 SIGTERM Terminate process, allow for cleanup

The manual (man signal) contains a longer list.

Signal Name Default Action Description
1 SIGHUP terminate process terminal line hangup
2 SIGINT terminate process interrupt program
3 SIGQUIT create core image quit program
4 SIGILL create core image illegal instruction
5 SIGTRAP create core image trace trap
6 SIGABRT create core image abort program (formerly SIGIOT)
7 SIGEMT create core image emulate instruction executed
8 SIGFPE create core image floating-point exception
9 SIGKILL terminate process kill program
10 SIGBUS create core image bus error
11 SIGSEGV create core image segmentation violation
12 SIGSYS create core image non-existent system call invoked
13 SIGPIPE terminate process write on a pipe with no reader
14 SIGALRM terminate process real-time timer expired
15 SIGTERM terminate process software termination signal
16 SIGURG discard signal urgent condition present on socket
17 SIGSTOP stop process stop (cannot be caught or ignored)
18 SIGTSTP stop process stop signal generated from keyboard
19 SIGCONT discard signal continue after stop
20 SIGCHLD discard signal child status has changed
21 SIGTTIN stop process background read attempted from control terminal
22 SIGTTOU stop process background write attempted to control terminal
23 SIGIO discard signal I/O is possible on a descriptor (see fcntl(2))
24 SIGXCPU terminate process cpu time limit exceeded (see setrlimit(2))
25 SIGXFSZ terminate process file size limit exceeded (see setrlimit(2))
26 SIGVTALRM terminate process virtual time alarm (see setitimer(2))
27 SIGPROF terminate process profiling timer alarm (see setitimer(2))
28 SIGWINCH discard signal Window size change
29 SIGINFO discard signal status request from keyboard
30 SIGUSR1 terminate process User defined signal 1
31 SIGUSR2 terminate process User defined signal 2

How to list running processes

ps Show some running processes
ps aux Show lots of running processes

ps and ps aux pair great with fuzzy-finders — such as grep and fzf. To find a running process interactively, try ps | fzf or ps aux | fzf.

But what can we do with a process? Well, we can kill it. And … I must reluctantly admit I don’t know anything else we can do with it.

How do I kill a process?

First, we need something to kill. ☠️

# Leave this running in a terminal
sleep 9999

To kill a process, we can use kill or pkill. Kill requires a process ID (number).

# Find the process ID with `ps` and `grep`
ps | grep sleep
33312 ttys003    0:00.00 sleep 9999999

33312 is our ID.

# Now kill it!
kill 33312

With pkill, we can give a pattern - or just the full process name.

# Start a long running process in another terminal
sleep 9999999
# Then kill it off.
pkill sleep

How do I kill a process with a specific signal?

kill and pkill accept a first argument as signal code. To kill with SIGINT, use kill -2 PID or pkill -2 PNAME.

# Again ..
sleep 9999999

# Now, try SIGHUP, SIGINT, SIGKILL or SIGKILL and see what happens.
pkill -1 sleep
pkill -2 sleep
pkill -9 sleep
pkill -15 sleep
# Or with kill if you want.

# Does the sleep process stop? Does it print anything?

How to handle Unix signals with Python


How to handle Unix signals with other languages


Interactive tools to work with running processes

VERY Primitive TUI for process management
Primitive TUI for process management
Interactive process editor for Emacs