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Linux Signals and Traps

Links: 104 Linux Index


  • Signals are software interrupts sent to a program to indicate that an important event has occurred.
  • It is limited form of inter process communication since we cannot transfer data using it.
  • Signals are delivered to the process by the kernel either directly or indirectly.

    • Signals can originate from any where but in the end it is the kernel that delivers them.
  • Listing all the signals: kill -l

  • Every signal has a value. For example SIGINT has a value of 2.
All the signals have a value greater than 0.

Success signal has a value of 0.

  • SIGINT: Interrupt signal.
    • We can do this using CTRL + C
  • SIGTSTP: Stopping the current process
    • We can do this using CTRL + Z for foreground processes
    • For background processes we use kill -SIGTSTP <pid>
    • If SIGTSTP doesn't work we can stop a
  • SIGKILL: Kill the current process
    • We can do this using kill -9 <process-pid>
    • We use -9 since SIGKILL has a value of 9.
      • This means kill -9 <process-pid> is same as kill SIGKILL <process-pid>
  • SIGTERM: Terminate a process gracefully
    • We can do this using kill -15 <process-pid>
    • This is the default signal for kill.
  • SIGHUP: A SIGHUP signal is sent to a process when its controlling terminal is closed.
    • nohup prevents the process from getting a SIGHUP signal.
    • nohup sleep 1000
  • Some other signals
    • attachments/Pasted image 20230122115232.jpg
We can kill processes using process names instead of pid by using pkill.

All the processes with the name will be killed. attachments/Pasted image 20230122171446.jpg

killing a parent process DOES NOT kill the children processes.
  • So interrupting a process using signals won't interrupt the subprocesses spawned by that process.
  • For example if there is a sleep infinity command in the bash script and we send a SIGTERM to the bash script process then the script will be killed but not the sleep command.
  • Though SIGKILL will kill the parent as well as the child processes.
Then why does CTRL + C kill parent and the child processes?
  • Ctrl-C at your terminal typically sends SIGINT to all processes in the foreground process group.
  • Both your parent and your child process are in this process group.
  • This also means that if some process is in the background then it won't be stopped by CTRL-C
  • attachments/Pasted image 20230122165947.jpg
  • Difference between SIGKILL and SIGTERM

    • attachments/Pasted image 20230122162208.jpg
    • Notice that SIGTERM kills all the child processes also.
  • Killing a process group using SIGTERM: kill -15 -<pid>

    • attachments/Pasted image 20230122165148.jpg
  • Both foreground and background processes are removed when we send a SIGTERM to the process group.
    • attachments/Pasted image 20230122170326.jpg


  • Signals can interrupt the execution of our script.
  • Trap commands allow us to do some task (like clean up) whenever our script receives a signal.
  • Trap statements are always placed at the beginning of the script.

# specify the command to be executed on sigint
trap "echo interrupt signal detected" SIGINT

echo "Testing trap"
sleep 100

# ------
# Output
# Testing trap
# ^Cinterrupt signal detected
Trap CANNOT catch SIGKILL and SIGSTOP command.
  • We can catch multiple signals:

    • trap "commands" SIGINT 0
    • This is particularly useful in scenarios where you are writing to a file temporarily and would want to remove it incase the script is interrupted or script successfully completes execution.
      • trap "rm -f $file; exit" 0 2 15
      • Clean up example.
  • We can use functions if we want to provide a lot of commands:

    function my_fun() {
        echo "inside trapped function"
    trap my_fun 15 2

  • trap "some commands" EXIT gets called in case program exits successfully or unsuccessfully.

  • You can also use trap to ensure the user cannot interrupt the script execution.

    • This feature is important when executing sensitive commands whose interruption may permanently damage the system.
    • The syntax for disabling a signal is trap "" SIGINT. Double quotation marks mean that no command will be executed.
    • Remember SIGKILL cannot be trapped so this won't work in case user passed SIGKILL.
  • Suppose a script is doing something and it is waiting for a specific process to complete and it receives as SIGTERM then the trap won't be executed until and unless that process finishes.


  • attachments/Pasted image 20230122144250.jpg
  • attachments/Pasted image 20230122144430.jpg
  • Different Trap functions for different signals.
    • attachments/Pasted image 20230122162411.jpg


Last updated: 2023-01-22