nohup
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nohup is a Unix command that is used to run another command while suppressing the action of the HUP (hangup) signal, enabling the command to keep running after the user who issues the command has logged out. It is most often used to run commands in the background as daemons. Output that would normally go to the terminal goes to a file called nohup.out if it has not already been redirected.
nohup is part of the GNU Coreutils.
The first of the commands below starts the program abcd in the background in such a way that the subsequent log out does not stop it.
$ nohup abcd &
$ exit
Below example shows how to redirect output to a file other than nohup.out.
$ nohup abcd > out.file &
Note that these methods prevent the process from being sent a 'stop' signal on logout, but if input/output is being received for these files, they will still hang the terminal [1]. See below for a fix (redirect them away).
[edit] Existing jobs
Some shells (e.g. bash) provide a shell builtin that may be used to prevent SIGHUP being sent to existing jobs, even if they were not started with nohup. In bash, this can be obtained by using disown -h job; using the same builtin without arguments removes the job from the job table, which also implies that the job will not receive the signal.
Nohuping backgrounded jobs is for example useful when logged in via SSH, since backgrounded jobs can cause the shell to hang on logout due to a race condition [2]. This problem can also be overcome by redirecting all three I/O streams:
myprogram > foo.out 2> foo.err < /dev/null &
[edit] Alternatives
There are other ways to accomplish the "keep program running while one logs out" ability. For example, you can run the program inside a GNU Screen-style screen multiplexer, and then detach the screen. The screen runs independently of the user's sessions, and can be reattached by any session of the user. This has the advantage of being able to continue to interact with the program once reattached (as opposed to nohup, which cannot reattach nohup'ed programs).
Another alternative would be to use 'setsid' which will run a program in a new session.
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