Shell Scripting - Shell Signals Values

Prerequisites: Processes, Bash Scripting, Shell Function Library

Signals help the operating system to communicate with processes and vice-versa. Signals are also generated when processes don't function properly or try to access prohibited memory. Different signals are mapped to different numbers which are referred to as signal values. The Linux OS implements about 30 standard signals.

We can send signals to particular processes by commands like kill and wait. The syntax generally is

$ kill -signalNumber processID(or %JOBSPEC) 
$ kill -signalName   processID(or %JOBSPEC)

Here is a list of the most commonly used signals and their meanings :

List of other Shell Signals

To get the list of all the signals and their corresponding signal values, use the command kill -l.

For demonstration, we will use a program named Gedit to which we will send signals from our bash script and see the effect of different signal values on the program and how the program responds to it. Gedit is a GUI text editor for Linux.

Install gedit if it is not present in your system.

$ sudo apt-get install gedit

Creating Shell Script

The name of the script will be shellSignals.sh

$ touch shellSignals.sh

Make it an executable file.

$ chmod +x shellSignals.sh

Open the file and add the following Script.

#!/bin/bash

# enabling job control in the script
set -m

# starting the program in background
$1 &
echo "$1 process started in background..."

# storing the Process ID(PID)
PID=`pgrep $1`

while [[ true ]]
do
  # print the job status
  echo "$(jobs -l)"

  # terminate the script
  echo -n "Do you want to continue?(y/n) :"
  read
  if [[ $REPLY == "n" ]]
  then
    # send SIGTERM signal to the process
    echo "Sending SIGTERM signal to the process."
    kill -SIGTERM $PID
    sleep 1;
    echo "$(jobs -l)"
    break
  fi

  # restart the process if it has been terminated
  # checks whether PID exists or not. If not, it 
  # creates new instance of the process
  if ! ps -p $PID > /dev/null ; then
    echo -e "\nRestarting $1 process which was terminated earlier"
    $1 &
    sleep 1;
    echo "$(jobs -l)"
    PID=`pgrep $1`
  fi

  # sending signal to the process via kill command
  echo -e "\nEnter Signal Number"
  read NUM 
  kill -$NUM $PID;
  sleep 1;
done

exit 0

The various shell signals can be sent via this shell script to any process. It is a user-interactive script that prompts whether to exit the script or not and prompts what shell signal values we want to send to the process. It also automatically restarts a process if we terminated it earlier.

Run the Script

$ ./shellSignals.sh gedit

Sending SIGSTOP(19), SIGCONT(18), SIGINT(2) and SIGTERM(15) signals.