Why can't your kill command list the signal table

calendar 2023-11-15 · 8 min read · Linux  ·
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Sometimes you want to use the kill command to send special signals to certain programs. For a while, you don’t know the signal name and number or are afraid of spelling mistakes. At this time, the kill -l or kill -L command that comes with the kill command may can help you.

Some readers may encounter this situation:

1➜ ~ kill -l
2HUP INT QUIT ILL TRAP IOT BUS FPE KILL USR1 SEGV USR2 PIPE ALRM TERM STKFLT CHLD CONT STOP TSTP TTIN TTOU URG XCPU XFSZ VTALRM PROF WINCH POLL PWR SYS
3➜ ~ kill -L
4kill: unknown signal: SIGL
5kill: type kill -l for a list of signals

Look at the output above:

  • kill -l also only displays a simple list without numbers;

  • kill -L actually reports an error;

All this is most likely because you used zsh

1➜ ~ echo $SHELL
2/usr/bin/zsh

If you switch to bash, the familiar interface is back.

 1mephisto@pc:~$ kill -l
 2  1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP
 3  6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1
 411) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
 516) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
 621) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ
 726) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR
 831) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3
 938) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8
1043) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
1148) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
1253) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7
1358) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2
1463) SIGRTMAX-1 64) SIGRTMAX
15mephisto@pc:~$ kill -L
16  1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP
17  6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1
1811) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
1916) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
2021) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ
2126) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR
2231) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3
2338) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8
2443) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
2548) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
2653) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7
2758) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2
2863) SIGRTMAX-1 64) SIGRTMAX

Why is this so? Please see the output below (zsh environment).

1➜ ~ type -a kill
2kill is a shell builtin
3kill is /usr/bin/kill
4kill is /bin/kill
5➜ ~ md5sum /usr/bin/kill /bin/kill
6bec5c7ac875a475f103603d8170dbc31 /usr/bin/kill
7bec5c7ac875a475f103603d8170dbc31 /bin/kill
8➜ ~ echo $SHELL
9/usr/bin/zsh

Currently, both bash and zsh have builtin kill commands (high priority, hit first). The relevant documentation states:

When you use the kill command in bash and zsh, the kill command that comes with the shell will be hit first. The output difference between the two leads to the above difference.

The independent kill command can be called using /bin/kill (on my machine, /usr/bin/kill is the same program and the md5 code is the same):

1➜ ~ /bin/kill -l
2HUP INT QUIT ILL TRAP ABRT BUS FPE KILL USR1 SEGV USR2 PIPE ALRM TERM STKFLT
3CHLD CONT STOP TSTP TTIN TTOU URG XCPU XFSZ VTALRM PROF WINCH POLL PWR SYS
4➜ ~ /bin/kill -L
5  1 HUP 2 INT 3 QUIT 4 ILL 5 TRAP 6 ABRT 7 BUS
6  8 FPE 9 KILL 10 USR1 11 SEGV 12 USR2 13 PIPE 14 ALRM
715 TERM 16 STKFLT 17 CHLD 18 CONT 19 STOP 20 TSTP 21 TTIN
822 TTOU 23 URG 24 XCPU 25 XFSZ 26 VTALRM 27 PROF 28 WINCH
929 POLL 30 PWR 31 SYS

The output is normal, with fewer numbers than bash's built-in number (less than 31 or more).

The original signal is defined in these two header files /usr/include/asm/signal.h and /usr/include/asm/siginfo.h

 1➜ ~ cat /usr/include/linux/signal.h
 2/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
 3#ifndef _LINUX_SIGNAL_H
 4#define _LINUX_SIGNAL_H
 5
 6#include <asm/signal.h>
 7#include <asm/siginfo.h>
 8
 9#define SS_ONSTACK 1
10#define SS_DISABLE 2
11
12/* bit-flags */
13#define SS_AUTODISARM (1U << 31) /* disable sas during sighandling */
14/* mask for all SS_xxx flags */
15#define SS_FLAG_BITS SS_AUTODISARM
16
17#endif /* _LINUX_SIGNAL_H */

Take a look at the header file:

 1➜ ~ head -n 62 /usr/include/asm/signal.h
 2/_ SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note _/
 3#ifndef \_ASM_X86_SIGNAL_H
 4#define \_ASM_X86_SIGNAL_H
 5
 6#ifndef **ASSEMBLY**
 7#include <linux/types.h>
 8#include <linux/time.h>
 9
10/_ Avoid too many header ordering problems. _/
11struct siginfo;
12
13/_ Here we must cater to libcs that poke about in kernel headers. \_/
14
15#define NSIG 32
16typedef unsigned long sigset_t;
17
18#endif /_ **ASSEMBLY** _/
19
20#define SIGHUP 1
21#define SIGINT 2
22#define SIGQUIT 3
23#define SIGILL 4
24#define SIGTRAP 5
25#define SIGABRT 6
26#define SIGIOT 6
27#define SIGBUS 7
28#define SIGFPE 8
29#define SIGKILL 9
30#define SIGUSR1 10
31#define SIGSEGV 11
32#define SIGUSR2 12
33#define SIGPIPE 13
34#define SIGALRM 14
35#define SIGTERM 15
36#define SIGSTKFLT 16
37#define SIGCHLD 17
38#define SIGCONT 18
39#define SIGSTOP 19
40#define SIGTSTP 20
41#define SIGTTIN 21
42#define SIGTTOU 22
43#define SIGURG 23
44#define SIGXCPU 24
45#define SIGXFSZ 25
46#define SIGVTALRM 26
47#define SIGPROF 27
48#define SIGWINCH 28
49#define SIGIO 29
50#define SIGPOLL SIGIO
51/_
52#define SIGLOST 29
53_/
54#define SIGPWR 30
55#define SIGSYS 31
56#define SIGUNUSED 31
57
58/_ These should not be considered constants from userland. _/
59#define SIGRTMIN 32
60#define SIGRTMAX \_NSIG

For detailed explanations of various signals, please see the Help Document. The explanations are quite in place, including the introduction of real-time signals.

Commonly used signals:

  • HUP 1 terminal hang up
  • INT 2 interrupt (same as Ctrl + C)
  • QUIT 3 to exit (same as Ctrl + )
  • KILL 9 forced termination
  • TERM 15 Termination (Default action is to terminate the process)
  • CONT 18 continue (opposite of STOP, fg/bg command)
  • STOP 19 pause (same as Ctrl + Z)

KILL Number 9 is like having no choice but to use the ultimate move in the game.

CONT/STOP Readers can simply test and control the output of yes, tail and other commands, which is quite interesting.

The TERM signal is the default. Usually after the program receives this signal, it will trigger the relevant code, do some finishing actions, and then exit, which is the so-called graceful exit.

For example k8s:

Typically, with this graceful termination of the pod, kubelet makes requests to the container runtime to attempt to stop the containers in the pod by first sending a TERM (aka. SIGTERM) signal, with a grace period timeout, to the main process in each container. The requests to stop the containers are processed by the container runtime asynchronously. There is no guarantee to the order of processing for these requests. Many container runtimes respect the STOPSIGNAL value defined in the container image and, if different, send the container image configured STOPSIGNAL instead of TERM. Once the grace period has expired, the KILL signal is sent to any remaining processes, and the Pod is then deleted from the API Server. If the kubelet or the container runtime's management service is restarted while waiting for processes to terminate, the cluster retries from the start including the full original grace period.

Each software handles signals inconsistently, but Nginx is different. Its graceful shutdown is to send SIGQUIT 😅:

1nginx -s <SIGNAL>
2
3where <SIGNAL> can be one of the following:
4
5     quit – Shut down gracefully (the SIGQUIT signal)
6     reload – Reload the configuration file (the SIGHUP signal)
7     reopen – Reopen log files (the SIGUSR1 signal)
8     stop – Shut down immediately (or fast shutdown, the SIGTERM singal)

Another example is gunicorn, which conforms to the routine and plays cards regularly:

 1
 2Master process
 3
 4     QUIT, INT: Quick shutdown
 5     TERM: Graceful shutdown. Waits for workers to finish their current requests up to the graceful_timeout.
 6     HUP: Reload the configuration, start the new worker processes with a new configuration and gracefully shutdown older workers. If the application is not preloaded (using the preload_app option), Gunicorn will also load the new version of it.
 7     TTIN: Increment the number of processes by one
 8     TTOU: Decrement the number of processes by one
 9     USR1: Reopen the log files
10     USR2: Upgrade Gunicorn on the fly. A separate TERM signal should be used to kill the old master process. This signal can also be used to use the new versions of pre-loaded applications. See Upgrading to a new binary on the fly for more information.
11     WINCH: Gracefully shutdown the worker processes when Gunicorn is daemonized.

This reminds me of -v, -V, --verson, -version used to display software versions. I feel emotional every time and can only say that the world is rich and colorful.

Consoling words from great men:

"My life has a limit, but my knowledge has no limit." -- Zhuangzi

Lastmod： Wednesday, November 15, 2023

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