Difference between nohup, disown and &

Question

What are the differences between

$ nohup foo

and

$ foo &

and

$ foo & 
$ disown

Answer 1

Let's first look at what happens if a program is started from an interactive shell (connected to a terminal) without & (and without any redirection). So let's assume you've just typed foo:

• The process running foo is created.
• The process inherits stdin, stdout, and stderr from the shell. Therefore it is also connected to the same terminal.
• If the shell receives a SIGHUP, it also sends a SIGHUP to the process (which normally causes the process to terminate).
• Otherwise the shell waits (is blocked) until the process terminates or gets stopped.

Now, let's look what happens if you put the process in the background, that is, type foo &:

• The process running foo is created.
• The process inherits stdout/stderr from the shell (so it still writes to the terminal).
• The process in principle also inherits stdin, but as soon as it tries to read from stdin, it is halted.
• It is put into the list of background jobs the shell manages, which means especially:
  • It is listed with jobs and can be accessed using %n (where n is the job number).
  • It can be turned into a foreground job using fg, in which case it continues as if you would not have used & on it (and if it was stopped due to trying to read from standard input, it now can proceed to read from the terminal).
  • If the shell received a SIGHUP, it also sends a SIGHUP to the process. Depending on the shell and possibly on options set for the shell, when terminating the shell it will also send a SIGHUP to the process.

Now disown removes the job from the shell's job list, so all the subpoints above don't apply any more (including the process being sent a SIGHUP by the shell). However note that it still is connected to the terminal, so if the terminal is destroyed (which can happen if it was a pty, like those created by xterm or ssh, and the controlling program is terminated, by closing the xterm or terminating the SSH connection), the program will fail as soon as it tries to read from standard input or write to standard output.

What nohup does, on the other hand, is to effectively separate the process from the terminal:

• It closes standard input (the program will not be able to read any input, even if it is run in the foreground. it is not halted, but will receive an error code or EOF).
• It redirects standard output and standard error to the file nohup.out, so the program won't fail for writing to standard output if the terminal fails, so whatever the process writes is not lost.
• It prevents the process from receiving a SIGHUP (thus the name).

Note that nohup does not remove the process from the shell's job control and also doesn't put it in the background (but since a foreground nohup job is more or less useless, you'd generally put it into the background using &). For example, unlike with disown, the shell will still tell you when the nohup job has completed (unless the shell is terminated before, of course).

So to summarize:

• & puts the job in the background, that is, makes it block on attempting to read input, and makes the shell not wait for its completion.
• disown removes the process from the shell's job control, but it still leaves it connected to the terminal. One of the results is that the shell won't send it a SIGHUP. Obviously, it can only be applied to background jobs, because you cannot enter it when a foreground job is running.
• nohup disconnects the process from the terminal, redirects its output to nohup.out and shields it from SIGHUP. One of the effects (the naming one) is that the process won't receive any sent SIGHUP. It is completely independent from job control and could in principle be used also for foreground jobs (although that's not very useful).

Answer 2

Using & causes the program to run in the background, so you'll get a new shell prompt instead of blocking until the program ends. nohup and disown are largely unrelated; they suppress SIGHUP (hangup) signals so the program isn't automatically killed when the controlling terminal is closed. nohup does this when the job first begins. If you don't nohup a job when it begins, you can use disown to modify a running job; with no arguments it modifies the current job, which is the one that was just backgrounded

Answer 3

Here is my experience trying to run soffice in the background, following a non-terminating command (e.g. tail). For this example I will use sleep 100.

In all the cases below I execute like this:

./scriptfile
<Ctl-C>

&

#!/bin/bash
/opt/libreoffice4.4/program/soffice -invisible -nofirststartwizard &
sleep 100

I see soffice logs / by pressing Ctrl-C soffice stops

nohup .. &

#!/bin/bash
nohup /opt/libreoffice4.4/program/soffice -invisible -nofirststartwizard &
sleep 100

I don't see soffice logs / by pressing Ctrl-C soffice stops

& disown

#!/bin/bash
/opt/libreoffice4.4/program/soffice -invisible -nofirststartwizard & disown
sleep 100

I see soffice logs / by pressing Ctrl-C soffice stops

setsid .. &

#!/bin/bash
setsid /opt/libreoffice4.4/program/soffice -invisible -nofirststartwizard &
sleep 100

I see soffice logs / by pressing Ctrl-C soffice DOES NOT STOP

To save space:
nohup setsid .. : does not show logs / soffice DOES NOT STOP on Ctrl-C
nohup with & disown at the end : does not show logs / soffice stops on Ctrl-C

Answer 4

Short answer:

• use & when you want your command to run in the background, so you can run the next one without waiting for it to finish
• use nohup if you want your command to ignore the SIGHUP signal, so when you close the terminal or log out from ssh session the process keeps running
• use disown if you forgot to run the command with nohup and want to log out without killing the process (it will disown all processes in the background). To put a process in the background and disown it:
  1. Ctrl+Z to pause process
  2. bg to put stopped process to the background
  3. disown to make process ignore terminal termination

Answer 5

See also the daemonize(1) utility, which handles all of the chores about running a "true background" process. As of its docs:

daemonize runs a command as a Unix daemon. As defined in W. Richard Stevens' 1990 book, Unix Network Programming (Addison-Wesley, 1990), a daemon is “a process that executes `in the background' (i.e., without an associated terminal or login shell) either waiting for some event to occur, or waiting to perform some specified task on a periodic basis.” Upon startup, a typical daemon program will:

• Close all open file descriptors (especially standard input, standard output and standard error)
• Change its working directory to the root filesystem, to ensure that it doesn't tie up another filesystem and prevent it from being unmounted
• Reset its umask value
• Run in the background (i.e., fork)
• Disassociate from its process group (usually a shell), to insulate itself from signals (such as HUP) sent to the process group
• Ignore all terminal I/O signals
• Disassociate from the control terminal (and take steps not to reacquire one)
• Handle any SIGCLD signals

Most programs that are designed to be run as daemons do that work for themselves. However, you'll occasionally run across one that does not. When you must run a daemon program that does not properly make itself into a true Unix daemon, you can use daemonize to force it to run as a true daemon.

It superseeds all of the &/nohup/disown mess.

Answer 6

& puts the job in the background, i.e. makes it block input from the shell, and makes the shell not wait for its completion.

nohup and disown both can be said to suppress stdin from the terminal and SIGHUP, but in different ways. nohup works when the job is initialized, disown works after the fact.

nohup also redirects stderr to stdout and stdout to $HOME/nohup.out. It does not block other signals being sent indirectly from the original shell.

disown does not redirect stdout or stderr. It simply removes the process from the shells job table, so no signals will be indirectly sent to the process from the original shell.