Linux Signals and Traps

Links: 104 Linux Index

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Signals

• 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
  • 

We can kill processes using process names instead of pid by using pkill.

All the processes with the name will be killed.

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
• 

• Difference between SIGKILL and SIGTERM

  • 
  • Notice that SIGTERM kills all the child processes also.

• Killing a process group using SIGTERM: kill -15 -<pid>

  • 
• Both foreground and background processes are removed when we send a SIGTERM to the process group.
  • 

Traps

• 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.

#!/bin/bash

# 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:

  #!/bin/bash
  function my_fun() {
      echo "inside trapped function"
      whoami
  }
  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.

Examples

• 
• 
• Different Trap functions for different signals.
  • 

References

• signals and traps in shell scripting | trap command | ctrl + C on a shell script | trap handlers - YouTube
• Bash trap Command Explained {With Examples} (phoenixnap.com)
• What is SIGTERM? What's the difference between SIGKILL & SIGTERM? (linuxhandbook.com)

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Last updated: 2023-01-22