How does Ctrl-C terminate a child process?

by

Signals by default are handled by the kernel. Old Unix systems had 15 signals; now they have more. You can check </usr/include/signal.h> (or kill -l). CTRL+C is the signal with name SIGINT.

The default action for handling each signal is defined in the kernel too, and usually it terminates the process that received the signal.

All signals (but SIGKILL) can be handled by program.

And this is what the shell does:

  • When the shell running in interactive mode, it has a special signal handling for this mode.
  • When you run a program, for example find, the shell:
    • forks itself
    • and for the child set the default signal handling
    • replace the child with the given command (e.g. with find)
    • when you press CTRL+C, parent shell handle this signal but the child will receive it – with the default action – terminate. (the child can implement signal handling too)

You can trap signals in your shell script too…

And you can set signal handling for your interactive shell too, try enter this at the top of you ~/.profile. (Ensure than you’re a already logged in and test it with another terminal – you can lock out yourself)

trap 'echo "Dont do this"' 2

Now, every time you press CTRL+C in your shell, it will print a message. Don’t forget to remove the line!

If interested, you can check the plain old /bin/sh signal handling in the source code here.

At the above there were some misinformations in the comments (now deleted), so if someone interested here is a very nice link – how the signal handling works.

More Related Contents:

  1. Are shell scripts sensitive to encoding and line endings?
  2. Difference between sh and Bash
  3. Command not found error in Bash variable assignment
  4. Pass commands as input to another command (su, ssh, sh, etc)
  5. Why would a correct shell script give a wrapped/truncated/corrupted error message? [duplicate]
  6. Why is testing “$?” to see if a command succeeded or not, an anti-pattern?
  7. What is the difference between $(command) and `command` in shell programming?
  8. Reading quoted/escaped arguments correctly from a string
  9. Quick-and-dirty way to ensure only one instance of a shell script is running at a time
  10. How can I remove the extension of a filename in a shell script?
  11. I just assigned a variable, but echo $variable shows something else
  12. How can I declare and use Boolean variables in a shell script?
  13. How do I run a shell script without using “sh” or “bash” commands?
  14. Ubuntu says “bash: ./program Permission denied” [closed]
  15. Assignment of variables with space after the (=) sign?
  16. How to echo shell commands as they are executed
  17. How to get a password from a shell script without echoing
  18. Count occurrences of a char in a string using Bash
  19. How to redirect output of an entire shell script within the script itself?
  20. Quick-and-dirty way to ensure only one instance of a shell script is running at a time
  21. Bash conditional based on exit code of command
  22. source command not found in sh shell
  23. What does it mean in shell when we put a command inside dollar sign and parentheses: $(command)
  24. How to kill all subprocesses of shell?
  25. Should I use a Shebang with Bash scripts?
  26. How to pipe multiple commands into a single command in the shell? (sh, bash, …)
  27. Is there an elegant way to store and evaluate return values in bash scripts?
  28. Using groovy, how do you pipe multiple shell commands?
  29. ^word^replacement^ on all matches in Bash?
  30. How to wait on all child (and grandchild etc) process spawned by a script

Leave a Comment