Does Linux Kill Background Processes If We Close the Terminal from Which It Has Started

Does linux kill background processes if we close the terminal from which it has started?

Who should kill jobs?

Normally, foreground and background jobs are killed by SIGHUP sent by kernel or shell in different circumstances.

When does kernel send `SIGHUP`?

Kernel sends SIGHUP to controlling process:

for real (hardware) terminal: when disconnect is detected in a terminal driver, e.g. on hang-up on modem line;
for pseudoterminal (pty): when last descriptor referencing master side of pty is closed, e.g. when you close terminal window.

Kernel sends SIGHUP to other process groups:

to foreground process group, when controlling process terminates;
to orphaned process group, when it becomes orphaned and it has stopped members.

Controlling process is the session leader that established the connection to the controlling terminal.

Typically, the controlling process is your shell. So, to sum up:

kernel sends SIGHUP to the shell when real or pseudoterminal is disconnected/closed;
kernel sends SIGHUP to foreground process group when the shell terminates;
kernel sends SIGHUP to orphaned process group if it contains stopped processes.

Note that kernel does not send SIGHUP to background process group if it contains no stopped processes.

When does `bash` send `SIGHUP`?

Bash sends SIGHUP to all jobs (foreground and background):

when it receives SIGHUP, and it is an interactive shell (and job control support is enabled at compile-time);
when it exits, it is an interactive login shell, and huponexit option is set (and job control support is enabled at compile-time).

See more details here.

Notes:

bash does not send SIGHUP to jobs removed from job list using disown;
processes started using nohup ignore SIGHUP.

More details here.

What about other shells?

Usually, shells propagate SIGHUP. Generating SIGHUP at normal exit is less common.

Telnet or SSH

Under telnet or SSH, the following should happen when connection is closed (e.g. when you close telnet window on PC):

client is killed;
server detects that client connection is closed;
server closes master side of pty;
kernel detects that master pty is closed and sends SIGHUP to bash;
bash receives SIGHUP, sends SIGHUP to all jobs and terminates;
each job receives SIGHUP and terminates.

Problem

I can reproduce your issue using bash and telnetd from busybox or dropbear SSH server: sometimes, background job doesn't receive SIGHUP (and doesn't terminate) when client connection is closed.

It seems that a race condition occurs when server (telnetd or dropbear) closes master side of pty:

normally, bash receives SIGHUP and immediately kills background jobs (as expected) and terminates;
but sometimes, bash detects EOF on slave side of pty before handling SIGHUP.

When bash detects EOF, it by default terminates immediately without sending SIGHUP. And background job remains running!

Solution

It is possible to configure bash to send SIGHUP on normal exit (including EOF) too:

Ensure that bash is started as login shell. The huponexit works only for login shells, AFAIK.
Login shell is enabled by -l option or leading hyphen in argv[0]. You can configure telnetd to run /bin/bash -l or better /bin/login which invokes /bin/sh in login shell mode.
E.g.:
```
telnetd -l /bin/login
```
Enable huponexit option.
E.g.:
```
shopt -s huponexit
```
Type this in bash session every time or add it to .bashrc or /etc/profile.

Why does the race occur?

bash unblocks signals only when it's safe, and blocks them when some code section can't be safely interrupted by a signal handler.

Such critical sections invoke interruption points from time to time, and if signal is received when a critical section is executed, it's handler is delayed until next interruption point happens or critical section is exited.

You can start digging from quit.h in the source code.

Thus, it seems that in our case bash sometimes receives SIGHUP when it's in a critical section. SIGHUP handler execution is delayed, and bash reads EOF and terminates before exiting critical section or calling next interruption point.

Reference

"Job Control" section in official Glibc manual.
Chapter 34 "Process Groups, Sessions, and Job Control" of "The Linux Programming Interface" book.

How do I kill background processes / jobs when my shell script exits?

To clean up some mess, trap can be used. It can provide a list of stuff executed when a specific signal arrives:

trap "echo hello" SIGINT

but can also be used to execute something if the shell exits:

trap "killall background" EXIT

It's a builtin, so help trap will give you information (works with bash). If you only want to kill background jobs, you can do

trap 'kill $(jobs -p)' EXIT

Watch out to use single ', to prevent the shell from substituting the $() immediately.

Start a process in background, do a task, then kill the process in the background

You can probably save the PID of the process in a variable, then use the kill command to kill it.

pushd .
nohup java -jar test/selenium-server.jar > /dev/null 2>&1 &
serverPID=$!
cd web/code/protected/tests/
phpunit functional/
kill $serverPID
popd

I haven't tested it myself, I'd like to write it on a comment, but not enough reputation yet :)

linux: kill background task

There's a special variable for this in bash:

kill $!

$! expands to the PID of the last process executed in the background.

How to prevent a background process from being stopped after closing SSH client in Linux

Check out the "nohup" program.

What killed my process and why?

If the user or sysadmin did not kill the program the kernel may have. The kernel would only kill a process under exceptional circumstances such as extreme resource starvation (think mem+swap exhaustion).

Does Linux Kill Background Processes If We Close the Terminal from Which It Has Started