Shell Scripting – How to send Signal to a Processes
Signals allow the operating system to communicate with programs(or processes). We will create an interactive bash script to send signals to a process and observe the status of the process. For demonstration, we will use a program named xlogo to which we will send signals from our bash script. It’s a program that displays the X Window System Logo.
Install xlogo if it is not present in your system
$ sudo apt-get install x11-apps
Creating Shell Script
The name of the script will be signals.sh.
$ touch signals.sh
Make it an executable file.
$ chmod +x signals.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 "Do you want to continue?(y/n)" read CHOICE if [[ $CHOICE == "n" ]] then # send SIGHUP(hangup signal to process) kill -1 $PID echo "$(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" $1 & echo "$(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
It takes the name of a valid process as an argument in the terminal. The input is the signal number. Signals are sent via kill command and the jobs -l print the status of the process. While running the script if we terminate the process, there is an option to restart the process.
Run the Script
$ ./signals.sh xlogo
You would see a small window pops up showing the X logo on your screen. By dragging it and resizing the window, the logo accordingly changes shape which means the program is running properly. The program is running in the background.
Press y to continue or n to exit the script.
Signal number 19 corresponds to a signal called SIGSTOP. It kind of pauses or freezes a program. It is used when we want to stop a process but do not want to close it. The shell prompt returns. The terminal shows the current status of the process.
We can see that the X logo stops resizing when the window is dragged.
Sending SIGSTOP(19), SIGCONT(18), SIGINT(2) and SIGHUP(1) signals.
List of common signals
|1||SIGHUP||If a process is being run from a terminal and that terminal itself is closed/terminated then the process receives this signal and consequently terminates.|
|2||SIGINT||It politely tells the program to terminate. Performs the same function as Ctrl+C. It’s up to the process whether it will listen to it or not.|
|9||SIGKILL||Unlike other signals, the SIGKILL signal is never sent to the process. Instead, the terminal immediately kills the program and the program doesn’t get the time to save its data or clean up its work. Only use this as a last resort.|
|15||SIGTERM||This is the default signal of the kill command|
|18||SIGCONT||This will restore a process that was paused by a SIGSTOP or SIGTSTP signal|
|19||SIGSTOP||This signal pauses/freezes the process. The process cannot choose to ignore it.|
|20||SIGTSTP||It is similar to SIGSTOP but the process receiving this signal is under no obligation to listen to it. The process may choose to ignore it.|