How do I detach a process from Terminal, entirely?

Question

I use Tilda (drop-down terminal) on Ubuntu as my "command central" - pretty much the way others might use GNOME Do, Quicksilver or Launchy.

However, I'm struggling with how to completely detach a process (e.g. Firefox) from the terminal it's been launched from - i.e. prevent that such a (non-)child process

• is terminated when closing the originating terminal
• "pollutes" the originating terminal via STDOUT/STDERR

For example, in order to start Vim in a "proper" terminal window, I have tried a simple script like the following:

exec gnome-terminal -e "vim $@" &> /dev/null &

However, that still causes pollution (also, passing a file name doesn't seem to work).

Answer 1

First of all; once you've started a process, you can background it by first stopping it (hit Ctrl-Z) and then typing bg to let it resume in the background. It's now a "job", and its stdout/stderr/stdin are still connected to your terminal.

You can start a process as backgrounded immediately by appending a "&" to the end of it:

firefox &

To run it in the background silenced, use this:

firefox </dev/null &>/dev/null &

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Some additional info:

nohup is a program you can use to run your application with such that its stdout/stderr can be sent to a file instead and such that closing the parent script won't SIGHUP the child. However, you need to have had the foresight to have used it before you started the application. Because of the way nohup works, you can't just apply it to a running process.

disown is a bash builtin that removes a shell job from the shell's job list. What this basically means is that you can't use fg, bg on it anymore, but more importantly, when you close your shell it won't hang or send a SIGHUP to that child anymore. Unlike nohup, disown is used after the process has been launched and backgrounded.

What you can't do, is change the stdout/stderr/stdin of a process after having launched it. At least not from the shell. If you launch your process and tell it that its stdout is your terminal (which is what you do by default), then that process is configured to output to your terminal. Your shell has no business with the processes' FD setup, that's purely something the process itself manages. The process itself can decide whether to close its stdout/stderr/stdin or not, but you can't use your shell to force it to do so.

To manage a background process' output, you have plenty of options from scripts, "nohup" probably being the first to come to mind. But for interactive processes you start but forgot to silence (firefox < /dev/null &>/dev/null &) you can't do much, really.

I recommend you get GNU screen. With screen you can just close your running shell when the process' output becomes a bother and open a new one (^Ac).

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Oh, and by the way, don't use "$@" where you're using it.

$@ means, $1, $2, $3 ..., which would turn your command into:

gnome-terminal -e "vim $1" "$2" "$3" ...

That's probably not what you want because -e only takes one argument. Use $1 to show that your script can only handle one argument.

It's really difficult to get multiple arguments working properly in the scenario that you gave (with the gnome-terminal -e) because -e takes only one argument, which is a shell command string. You'd have to encode your arguments into one. The best and most robust, but rather cludgy, way is like so:

gnome-terminal -e "vim $(printf "%q " "$@")"

Answer 2

nohup cmd &

nohup detaches the process completely (daemonizes it)

Answer 3

If you are using bash, try disown [jobspec]; see bash(1).

Another approach you can try is at now. If you're not superuser, your permission to use at may be restricted.

Answer 4

Reading these answers, I was under the initial impression that issuing nohup <command> & would be sufficient. Running zsh in gnome-terminal, I found that nohup <command> & did not prevent my shell from killing child processes on exit. Although nohup is useful, especially with non-interactive shells, it only guarantees this behavior if the child process does not reset its handler for the SIGHUP signal.

In my case, nohup should have prevented hangup signals from reaching the application, but the child application (VMWare Player in this case) was resetting its SIGHUP handler. As a result when the terminal emulator exits, it could still kill your subprocesses. This can only be resolved, to my knowledge, by ensuring that the process is removed from the shell's jobs table. If nohup is overridden with a shell builtin, as is sometimes the case, this may be sufficient, however, in the event that it is not...

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disown is a shell builtin in bash, zsh, and ksh93,

<command> &
disown

or

<command> &; disown

if you prefer one-liners. This has the generally desirable effect of removing the subprocess from the jobs table. This allows you to exit the terminal emulator without accidentally signaling the child process at all. No matter what the SIGHUP handler looks like, this should not kill your child process.

After the disown, the process is still a child of your terminal emulator (play with pstree if you want to watch this in action), but after the terminal emulator exits, you should see it attached to the init process. In other words, everything is as it should be, and as you presumably want it to be.

What to do if your shell does not support disown? I'd strongly advocate switching to one that does, but in the absence of that option, you have a few choices.

1. screen and tmux can solve this problem, but they are much heavier weight solutions, and I dislike having to run them for such a simple task. They are much more suitable for situations in which you want to maintain a tty, typically on a remote machine.
2. For many users, it may be desirable to see if your shell supports a capability like zsh's setopt nohup. This can be used to specify that SIGHUP should not be sent to the jobs in the jobs table when the shell exits. You can either apply this just before exiting the shell, or add it to shell configuration like ~/.zshrc if you always want it on.
3. Find a way to edit the jobs table. I couldn't find a way to do this in tcsh or csh, which is somewhat disturbing.
4. Write a small C program to fork off and exec(). This is a very poor solution, but the source should only consist of a couple dozen lines. You can then pass commands as commandline arguments to the C program, and thus avoid a process specific entry in the jobs table.

Answer 5

1. nohup $COMMAND &
2. $COMMAND & disown
3. setsid command

I've been using number 2 for a very long time, but number 3 works just as well. Also, disown has a nohup flag of -h, can disown all processes with -a, and can disown all running processes with -ar.

Silencing is accomplished by $COMMAND &>/dev/null.

Hope this helps!

Answer 6

Most simple and only correct answer for bash:

command & disown

_{You dont have to detach the process from terminal, but from the shell.}

Answer 7

in tcsh (and maybe in other shells as well), you can use parentheses to detach the process.

Compare this:

> jobs # shows nothing
> firefox &
> jobs
[1]  + Running                       firefox

To this:

> jobs # shows nothing
> (firefox &)
> jobs # still shows nothing
>

This removes firefox from the jobs listing, but it is still tied to the terminal; if you logged in to this node via 'ssh', trying to log out will still hang the ssh process.

Answer 8

I think screen might solve your problem

Answer 9

To disassociate tty shell run command through sub-shell for e.g.

(command)&

When exit used terminal closed but process is still alive.

check -

(sleep 100) & exit

Open other terminal

ps aux | grep sleep

Process is still alive.

Answer 10

Backgrounding and foregrounding a job is probably one of the very first things every Unix sys-admin should know.

Here is how it is done with bash:

./script.sh
# suspend process
{ctrl-Z}
# background process
bg
# list all backgrounded jobs
jobs
# bring it back to foreground
fg

Answer 11

You can run your command using the nohup command, this detach your process and redirects outputs to a given file... but I am not sure that is exactly what you need ..

Answer 12

Just add this in your bashrc/zshrc:

detach() {
  "$@" 1>/dev/null 2>/dev/null &; disown
}

Then you can run disowned commands like this:

detach gedit ~/.zshrc

Answer 13

Try daemon -- should be available from your friendly package manager and comprehensively take care of every way of disassociating itself from the terminal.

Answer 14

In my .bashrc, I have these functions for precisely that purpose:

function run_disowned() {
    "$@" & disown
}

function dos() {
    # run_disowned and silenced

    run_disowned "$@" 1>/dev/null 2>/dev/null
}

Prefix a command with dos to run it detached from the terminal.

The function is written to work with bash and zsh.

Answer 15

sudo apt install ucommon-utils

pdetach command

There you go :)

Answer 16

I have found on Mac OS X that I need to use both nohup AND disown to ensure that the child process is not torn down with the terminal.

Answer 17

I use the following script to do this. It stops the process printing to the terminal, detaches with nohup, and exits with the return status if the command finishes within the TIMEOUT.

#!/bin/bash

TIMEOUT=0.1

CMD=( "$@" )
#Could have some shortcuts here, e.g. replace "somefile.c" with "gedit somefile.c"

#use nohup to run the command, suppressing its output and allowing the terminal to be closed
#also send nohup's output to /dev/null, supressing nohup.out
#run nohup in the background so this script doesn't block
#print the command for debugging and to see bash variable expansion
printf "%q " "${CMD[@]}"
echo
nohup "${CMD[@]}" >/dev/null 2>&1 &
NOHUP_PID=$!

#kill this script after a short time, exiting with success status - command is still running
#this is needed as there is no timeout argument for `wait` below
MY_PID=$$
trap "exit 0" SIGINT SIGTERM
sleep $TIMEOUT && kill $MY_PID 2>/dev/null & #ignore "No such process" error if this exits normally

#if the command finishes before the above timeout, everything may be just fine or there could have been an error
wait $NOHUP_PID
NOHUP_STATUS=$?
#print an error if there was any. most commonly, there was a typo in the command
[ $NOHUP_STATUS != 0 ] && echo "Error: $CMD"
#return the exit status of nohup, whatever it was
exit $NOHUP_STATUS

Usage example:

>>> run false
false
Error: false
>>> echo $?
1
>>> run true
true
>>> run sleep 10
sleep 10
>>>

Answer 18

Many answers suggested using nohup. I would rather suggest using pm2. Using pm2 over nohup has many advantages, like keeping the application alive, maintain log files for application and lot more other features. For more detail check this out.

To install pm2 you need to download npm. For Debian based system

sudo apt-get install npm

and for Redhat

sudo yum install npm

Or you can follow these instruction. After installing npm use it to install pm2

npm install pm2@latest -g

Once its done you can start your application by

$ pm2 start app.js              # Start, Daemonize and auto-restart application (Node)
$ pm2 start app.py              # Start, Daemonize and auto-restart application (Python)

For process monitoring use following commands:

$ pm2 list                      # List all processes started with PM2
$ pm2 monit                     # Display memory and cpu usage of each app
$ pm2 show [app-name]           # Show all informations about application

Manage processes using either app name or process id or manage all processes together:

$ pm2 stop     <app_name|id|'all'|json_conf>
$ pm2 restart  <app_name|id|'all'|json_conf>
$ pm2 delete   <app_name|id|'all'|json_conf>

Log files can be found in

$HOME/.pm2/logs #contain all applications logs

Answer 19

If your goal is to simply launch a command line application without keeping the terminal window around, then you might try running the application after launching the terminal with alt-F2.