Getting out of while loop on console

Question

I ran a command:

while true ; do ..... ....... ; done

on the Linux console/terminal, and now this while loop is continuously running.

I have two questions here:

1. How to get out of this while loop?
2. If my PuTTY session to Linux gets disconnected, how can I go back and find this loop and kill it?

Answer 1

You can cancel the loop with: CTRL + C

Your session would have ended when your putty session closed so the loop will no longer be running

• unless it was running in a screen session or similar in which case you would have to reattach it using the method of that particular program.

Answer 2

Sometimes hitting control-c doesn't work: the loop body gets the control-c and the loop keeps iterating. (Although I can't reproduce this behaviour right now with sleep 1 as the loop condition or the body, I remember seeing it.)

control-z can stop this, and the loop won't resume after fg unless the loop was inside a subshell like (while true;do : ;done). (With bash on GNU/Linux).

Or control-z / fg is useful if you want the currently-running iteration of the loop to finish cleanly, but not start another iteration.

(Control-z stops a job, getting the kernel to deliver SIGTSTP exactly like how control-c delivers SIGINT, on a TTY that's not in raw mode. The default action for this signal is to stop the job. fg and bg are job-control commands, see the bash built-in "help" for them and the jobs command.)

Answer 3

Given what @PeterCordes correctly points out, that Ctrl-C doesn't always work, a solution I sometimes use is to avoid making the loop infinite. Instead, I often write:

while sleep 1; do ...; done

If that one-second delay is too much of a slowdown, such as when you want to loop as quickly as possible, then condition the while loop on the presence of a semaphore file:

touch /tmp/ENABLE-LOOP-FILE
while [ -f '/tmp/ENABLE-LOOP-FILE' ]; do ...; done

When you need to kill the loop, go to another terminal session and remove the file:

rm /tmp/ENABLE-LOOP-FILE