Bash – Interrupting a for-loop in bash, but only after an iteration is completed

bashforshellshell-script

Is there a way to interrupt/stop a for-loop in bash, but only after the current iteration is completed – ie. "between" done and do?

I'm running a for-loop on several directories, with a command that could be destructive unless it's allowed to complete – so I'd like to stop it when the command is finished with one directory, but before it starts on the next.

(Yes, I know I could've added a sleep or a read command before done – but I didn't, and the loop is already running…)

Best Answer

For already running: assuming the command is a program and thus runs as a child process under the shell running the script (or part thereof), and the script didn't give it a heredoc or herestring for input or it has read or at least buffered that input, you can stop after the process finishes by:

kill -STOP the shell (so it won't do anything after the current command)
wait until the child process finishes and goes zombie
kill -KILL the shell

(All as the same user or root, from another shell, of course.) If the body of the do ... done does two or more things on each iteration, this only catches it after a program that takes long enough real time you can get the -STOP in, which may or may not be the last program in the iteration.

In some (many?) cases it will work to simply kill -KILL the shell up front, which prevents the shell cleaning up its children as normally does, and let the child run and finally get reaped by init (or equivalent on your system). However, this might result in the child seeing some I/O errors or environment differences or errors or variations with any grandchildren it has, and this might still trigger the 'destructive' results you are trying to avoid.

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Bash – timeout without killing process in bash

With ksh/bash/zsh:

{
  (./slowprocess.sh >&3 3>&-; echo "$?") |
    if read -t 3 status; then
      echo "Cool it completed with status $status, do stuff..."
    else
      echo "It didn't complete, do something else..."
    fi
} 3>&1

We duplicate the original stdout onto fd 3 (3>&1) so we can restore it for slowprocess.sh (>&3), while stdout for the rest of the (...) subshell goes to the pipe to read -t 3.

Alternatively, if you want to use timeout (here assuming GNU timeout):

timeout --foreground 3 sh -c './slowprocess.sh;exit'

would avoid slowprocess.sh being killed (the ;exit is necessary for sh implementations that optimise by executing the last command in the shell process).

Shell Script – Prevent SIGINT from Interrupting Function Calls and Child Processes

There are several ways you can cut off the effect of Ctrl+C:

Change the terminal setting so that it doesn't generate a signal.
Block the signal so that it is saved for later delivery, when the signal becomes unblocked.
Ignore the signal, or set a handler for it.
Run subprocesses in a background process group.

Since you want to detect that Ctrl+C has been pressed, ignoring the signal is out. You could change the terminal settings, but then you would need to write custom key processing code. Shells don't provide access to signal blocking.

You can however isolate subprocesses from receiving the signal automatically by running them in a separate process group. Interactive shells run background commands in a separate process group by default, but non-interactive shells run them in the same process group, and all processes in the foreground process group receive a signal from terminal events. To tell the shell to run background jobs in a separate process group, run set -m. Running setsid ping … is another way of forcing ping to run in a separate process group.

set -m
interrupted=
trap 'echo Interrupted, but ping may still be running' INT
set -m
ping … &
while wait; [ $? -ge 128 ]; do echo "Waiting for background jobs"; done
echo ping has finished

If you want Ctrl+Z to suspend a background process group, you'll need to propagate the signal from the shell.

Controlling signals finely is a bit of a stretch for a shell script, and shells other than ATT ksh tend to be a little buggy when you reach the corner cases, so consider a language that gives you more control such as Perl, Python or Ruby.

Best Answer

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Bash – timeout without killing process in bash

Shell Script – Prevent SIGINT from Interrupting Function Calls and Child Processes

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