Bash – Why are background jobs started by the shell living longer than the parent shell and controlling terminals

bashnohupshellterminal

It is commonly said that "&" puts the process into background execution, but that process would be killed when terminal is closed. But from what I see, everything I sent into background was continuing to be active long after I close the terminal. For example:

$ tail -f /var/log/messages/ &

That process would continue to run, even if there are no other process under the user that launched it, and all terminals are long gone. Why is that?

Best Answer

Just a guess (this is bash specific): the documentation for the huponexit shell option says

If set, Bash will send SIGHUP to all jobs when an interactive login shell exits

On my system, it does not seem to be set by default. You can check with

shopt -p huponexit

If the output includes -u, it is unset.

Related Solutions

Bash Shell Script – How to Terminate Running in Background

By default, ps will not show the parameters the program was called with. The options -f and -l both will show the full call.

ps -fu username

will result in output that looks like:

username 23464 66.7  0.0  11400   628 pts/5    R    15:28   1:40 bash script.sh

Bash – Prevent a shell fork from living longer than its initiator

This kills the background process before the script exits:

trap '[ "$pid" ] && kill "$pid"' EXIT

function repeat {
    while :; do
        echo repeating; sleep 1
    done
}
repeat &
pid=$!
echo running once

How it works

trap '[ "$pid" ] && kill "$pid"' EXIT

This creates a trap. Whenever the script is about to exit, the commands in single-quotes will be run. That command checks to see if the shell variable pid has been assigned a non-empty value. If it has, then the process associated with pid is killed.
pid=$!

This saves the process id of the preceding background command (repeat &) in the shell variable pid.

Improvement

As Patrick points out in the comments, there is a chance that the script could be killed after the background process starts but before the pid variable is set. We can handle that case with this code:

my_exit() {
    [ "$racing" ] && pid=$!
    [ "$pid" ] && kill "$pid"
}
trap my_exit EXIT

function repeat {
    while :; do
        echo repeating; sleep 1
    done
}

racing=Y
repeat &
pid=$!
racing=

echo running once