Output that would normally go to the terminal goes to a file called nohup.out if it has not already been redirected.
nohup is a low-level utility simply configuring a command to ignore a signal. As seen below, nohup is very far from being a full-featured batch system solving all the problems of running programs asynchronously.
The first of the commands below starts the program
abcd in the background in such a way that the subsequent logout does not stop it.
$ nohup abcd & $ exit
Note that these methods prevent the process from being sent a 'stop' signal on logout, but if input/output is being received for these standard I/O files (stdin, stdout, or stderr), they will still hang the terminal. See Overcoming hanging, below.
nohup is often used in combination with the nice command to run processes on a lower priority.
$ nohup nice abcd &
Existing jobs, processes
Some shells (e.g. bash) provide a shell builtin that may be used to prevent SIGHUP being sent or propagated 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. Before using
disown on an active job, it should be stopped by
Ctrl-Z, and continued in the background by the
bg command. Another relevant bash option is
shopt huponexit, which automatically sends the HUP signal to jobs when the shell is exiting normally.
The AIX and Solaris versions of nohup have a
-p option that modifies a running process to ignore future SIGHUP signals. Unlike the above-described
disown builtin of bash,
nohup -p accepts process IDs.
Note that nohupping backgrounded jobs is typically used to avoid terminating them when logging off from a remote SSH session. A different issue that often arises in this situation is that ssh is refusing to log off ("hangs"), since it refuses to lose any data from/to the background job(s). This problem can also be overcome by redirecting all three I/O streams:
$ nohup ./myprogram > foo.out 2> foo.err < /dev/null &
- The screen or tmux command can also detach a process from the current shell. It allows to reattach to the process later on.
- e.g. the following call will run somescript.sh detached from SIGHUP and in the background:
$ screen -A -m -d -S somename ./somescript.sh &
- The disown command is used to remove jobs from the job table, or to mark jobs so that a SIGHUP signal is not sent
- "Re: nohup/disown and logout". Zsh.org. 2005-02-07. Retrieved 2009-06-10.
- "SSH Frequently Asked Questions". Snailbook.com. Retrieved 2009-06-10.
- "OpenSSH FAQ". Openssh.com. 2005-09-20. Retrieved 2009-06-10.
- "Bug 396 – sshd orphans processes when no pty allocated". Bugzilla.mindrot.org. Retrieved 2009-06-10.