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Signals in Linux

Overview

Signals are a way of sending simple messages to processes. Most of these messages are already defined and can be found in <linux/signal.h>. However, signals can only be processed when the process is in user mode. If a signal has been sent to a process that is in kernel mode, it is dealt with immediately on returning to user mode.

Every signal has a unique signal name, an abbreviation that begins with SIG (SIGINT for interrupt signal, for example). Each signal name is a macro which stands for a positive integer - the signal number for that kind of signal. Your programs should never make assumptions about the numeric code for a particular kind of signal, but rather refer to them always by the names defined. This is because the number for a given kind of signal can vary from system to system, but the meanings of the names are standardized and fairly uniform.

Signals can be generated by the process itself, or they can be sent from one process to another. A variety of signals can be generated or delivered, and they have many uses for programmers. (To see a complete list of signals in the Linux environment, uses the command kill -l.)

  • There are total 64 signals in Linux, the list of all the signal can be sen by
    $ kill –l
$ kill -l
1) SIGHUP       2) SIGINT       3) SIGQUIT      4) SIGILL       5) SIGTRAP
6) SIGABRT      7) SIGBUS       8) SIGFPE       9) SIGKILL     10) SIGUSR1
11) SIGSEGV     12) SIGUSR2     13) SIGPIPE     14) SIGALRM     15) SIGTERM
16) SIGSTKFLT   17) SIGCHLD     18) SIGCONT     19) SIGSTOP     20) SIGTSTP
21) SIGTTIN     22) SIGTTOU     23) SIGURG      24) SIGXCPU     25) SIGXFSZ
26) SIGVTALRM   27) SIGPROF     28) SIGWINCH    29) SIGIO       30) SIGPWR
31) SIGSYS      34) SIGRTMIN    35) SIGRTMIN+1  36) SIGRTMIN+2  37) SIGRTMIN+3
38) SIGRTMIN+4  39) SIGRTMIN+5  40) SIGRTMIN+6  41) SIGRTMIN+7  42) SIGRTMIN+8
43) SIGRTMIN+9  44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9  56) SIGRTMAX-8  57) SIGRTMAX-7
58) SIGRTMAX-6  59) SIGRTMAX-5  60) SIGRTMAX-4  61) SIGRTMAX-3  62) SIGRTMAX-2
63) SIGRTMAX-1  64) SIGRTMAX

Few Important Signals with its descriptions:

Signal Value Action Comment
SIGHUP 1 Term Hangup detected on controlling terminal or death of controlling process
sIGINT 2 Term Interrupt from keyboard
SIGQUIT 3 Core Quit form keyboard
SIGILL 4 Core Illegal Instruction
SIGABRT 6 Core Abort signal form Abort(3)
SIGFPE 8 Core Floating point exception
SIGKILL 9 Term Kill signal
SIGSEGV 11 Core Invalid memory reference
SIGPIPE 13 Term Broken pipe: write to pipe with no readers
SIGALRM 14 Term Timer signal form alarm(2)
SIGTERM 15 Term Termination signal
SIGUSR1 30,10,16 Term User-defined signal-1
SIGUSR2 31,12,17 Term User-defined signal-2
SIGCHLD 20,17,18 Ign child stopped or terminated
SIGCONT 19,18,25 cont continue if stopped
SIGSTOP 17,19,23 Stop stop process
SIGTSTP 18,20,24 Stop stop typed at tty
SIGTTIN 21,21,26 Stop tty input for background process
SIGTTOU 22,22,27 Stop tty output for background process

Using most common signals

value usage
1 Reloading process
9 Killing process
15 Terminating process
20 Stopping process

Working with signals

  • To kill the process completely
    • First find out the process running in the system
      $ ps –u <user name>
      $ kill <signal no> <process id>
$ ps -u
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
krishna+    60  0.9  0.0  15104  3628 tty1     S    15:22   0:00 -bash
krishna+    75  4.0  0.0  24152  5348 tty1     T    15:22   0:00 vi newfile
krishna+    76  0.0  0.0  15664  1848 tty1     R    15:22   0:00 ps -u

[1]+  Stopped                 vi newfile

$ kill -9 75
[1]+  Killed                  vi newfile
  • To Terminate the process
$ ps -u
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
krishna+    60  0.1  0.0  15104  3644 tty1     S    15:22   0:00 -bash
krishna+    78  1.0  0.0  12196   788 tty1     T    15:24   0:00 cat
krishna+    79  0.0  0.0  15664  1844 tty1     R    15:24   0:00 ps -u

[1]+  Stopped                 cat
$ kill -15 78
$ ps -u
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
krishna+    60  0.1  0.0  15104  3644 tty1     S    15:22   0:00 -bash
krishna+    80  0.0  0.0  15664  1848 tty1     R    15:25   0:00 ps -u
[1]+  Terminated              cat

  • To stop the process

    • First login as a normal user and start a process
      su krishnaprasadkv
      cat >testing.txt

    • open one more terminal and change to root

    • Check its pid and kill it by using 20.
      ps -u krishnaprasad 

    root@DESKTOP-JDIP4H0:~# ps -u krishnaprasadkv
PID TTY          TIME CMD
8 tty1     00:00:02 bash
301 tty1     00:00:00 crontab
302 tty1     00:00:00 sh
303 tty1     00:00:00 sensible-editor
311 tty1     00:00:00 select-editor
408 tty1     00:00:00 crontab
409 tty1     00:00:00 sh
410 tty1     00:00:00 sensible-editor
418 tty1     00:00:00 editor
1170 tty1     00:00:00 bash
1181 tty1     00:00:00 cat
1183 tty2     00:00:00 bash
root@DESKTOP-JDIP4H0:~#

    root@DESKTOP-JDIP4H0:~# kill -20 1181

    • Change to user console and check its effect or not
    cat > hello
[1]+  Stopped                 cat > hello
    • Restart the process continue working type the below command in user console
      fg 1
    cat > hello

Setting up the priority of a process

Linux can run a lot of processes at a time, which can slow down the speed of some high priority processes and result in poor performance.

To avoid this, you can tell your machine to prioritize processes as per your requirements.

This priority is called Niceness in Linux, and it has a value between -20 to 19. The lower the Niceness index, the higher would be a priority given to that task.

The default value of all the processes is 0.

Working with renice

  • To schedule a priority of a process before starting it

    • To set a priority to a process before starting it, the syntax is
      $ nice –n <nice value range (-20 to 19)> <command>
      $ nice –n 5 cat > file.txt

    • Log in to other terminal and check the nice value for the above command/ process.
      $ ps –elf

  • To change the nice value of any process while it is running

    • To reschedule the nice value of existing process, first check the PID of that process by running $ps –elf command.
    • As from previous task we know the PID of cat command i.e. 1277
    • Use the following command to renice the value of a cat command which is still running.
      $ renice <nice value (-20 to 19)> <PID>
      $ renice 2 1277
    krishnaprasadkv@DESKTOP-JDIP4H0:~/test$ renice 2 1277
1277 (process ID) old priority 0, new priority 2