Fedora 30 : A general intro to linux signals with python.

UNIX/Linux systems offer special mechanisms to communicate between each individual process with signals. Let's see these signals:

[mythcat@desk ~]$ 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

Each signal is represented by an integer value, and the list of signals that are available. This simple python script create a process and print one message. Each process named PID comes with a number.

[mythcat@desk ~]$ cat python3 signal_001.py 
cat: python3: No such file or directory
import os
import sys
import time
print('PID number is:', os.getpid())
while True:
    print('Waiting...')
    time.sleep(6)

[mythcat@desk ~]$ python3 signal_001.py 
PID number is: 2566
Waiting...
Waiting...
Waiting...
Waiting...
Waiting...
Hangup

The PID process can be kill with this command:

[mythcat@desk ~]$ kill -SIGTERM 2566

Let's see another example that receives the signal we have sent to the process.

import os
import signal
import time

def receiveSignal(signalNumber, frame):
    print('Received:', signalNumber)
    raise SystemExit('Exiting')
    return

if __name__ == '__main__':
    # register the signals to be caught
    signal.signal(signal.SIGHUP, receiveSignal)
    signal.signal(signal.SIGINT, receiveSignal)
    signal.signal(signal.SIGQUIT, receiveSignal)
    signal.signal(signal.SIGILL, receiveSignal)
    signal.signal(signal.SIGTRAP, receiveSignal)
    signal.signal(signal.SIGABRT, receiveSignal)
    signal.signal(signal.SIGBUS, receiveSignal)
    signal.signal(signal.SIGFPE, receiveSignal)
    #signal.signal(signal.SIGKILL, receiveSignal)
    signal.signal(signal.SIGUSR1, receiveSignal)
    signal.signal(signal.SIGSEGV, receiveSignal)
    signal.signal(signal.SIGUSR2, receiveSignal)
    signal.signal(signal.SIGPIPE, receiveSignal)
    signal.signal(signal.SIGALRM, receiveSignal)
    signal.signal(signal.SIGTERM, receiveSignal)
    # register the signal to be caught
    signal.signal(signal.SIGUSR1, receiveSignal)

    # register the signal to be ignored
    signal.signal(signal.SIGINT, signal.SIG_IGN)

    # output current process id
    print('My PID is:', os.getpid())

    signal.pause()

Let't run it and see what happend when send a signal to the PID:

[mythcat@desk ~]$ python3 py_pid.py
My PID is: 2698
Received: 10
Exiting

You can see the kill command will send this output: Received: 10 Exiting .

[mythcat@desk ~]$ kill -SIGUSR1 2698