Bash – Killing background process in bash script when exiting the script

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.

I would disconnect the command from its standard input/output and error flows:

nohup python3 -u <script> </dev/null >/dev/null 2>&1 &  

ssh needs an indicator that doesn't have any more output and that it does not require any more input. Having something else be the input and redirecting the output means ssh can safely exit, as input/output is not coming from or going to the terminal. This means the input has to come from somewhere else, and the output (both STDOUT and STDERR) should go somewhere else.

The </dev/null part specifies /dev/null as the input for <script>. Why that is useful here:

Redirecting /dev/null to stdin will give an immediate EOF to any read call from that process. This is typically useful to detach a process from a tty (such a process is called a daemon). For example, when starting a background process remotely over ssh, you must redirect stdin to prevent the process waiting for local input. https://stackoverflow.com/questions/19955260/what-is-dev-null-in-bash/19955475#19955475

Alternatively, redirecting from another input source should be relatively safe as long as the current ssh session doesn't need to be kept open.

With the >/dev/null part the shell redirects the standard output into /dev/null essentially discarding it. >/path/to/file will also work.

The last part 2>&1 is redirecting STDERR to STDOUT.

There are three standard sources of input and output for a program. Standard input usually comes from the keyboard if it’s an interactive program, or from another program if it’s processing the other program’s output. The program usually prints to standard output, and sometimes prints to standard error. These three file descriptors (you can think of them as “data pipes”) are often called STDIN, STDOUT, and STDERR.

Sometimes they’re not named, they’re numbered! The built-in numberings for them are 0, 1, and 2, in that order. By default, if you don’t name or number one explicitly, you’re talking about STDOUT.

Given that context, you can see the command above is redirecting standard output into /dev/null, which is a place you can dump anything you don’t want (often called the bit-bucket), then redirecting standard error into standard output (you have to put an & in front of the destination when you do this).

The short explanation, therefore, is “all output from this command should be shoved into a black hole.” That’s one good way to make a program be really quiet!
What does > /dev/null 2>&1 mean? | Xaprb