How to run commands in the background in Linux

Suppose you have a long-running task (for example, compiling a large program) that you need to run, but you also want to get some other work done. Linux lets you start a task in the background and keep on doing other things from the command prompt.

We will see in this article how to send commands in the background, then return them to the foreground, and make sure that also closing the current shell or terminal the process doesn’t remains tied to the session but continue to work.

An alternative to these command is using screen, as read in a former article, but now let’s see the command bg and the special character &

Task 1 : start a process directly in background

sleep 100 &

This will start the sleep as a background task, executing it in parallel with other tasks on your system, the special character & after the command means that.

There is no output and you’ll be back at command prompt, after 100 seconds you’ll get

[1]+ Done sleep 100

Task 2 : start a process and put it in background

This is useful when you start something long, and realize you need your prompt back to give some other command.

sleep 100 ; < CTRL > Z ; bg

In this sequence we run a command (sleep, but could be anything), after that we use < ctrl > Z to put the process in Suspend State and we use the command bg to put the process in background

Example of output:

laptop:~$ sleep 100

^Z

[1]+ Stopped sleep 100

laptop:~$ bg

[1]+ sleep 100 &

laptop:~$

[1]+ Done sleep 100

Note: It’s a good idea to redirect the output of background tasks to a file, since the background task still shares the console with foreground tasks. If you don’t, the background task will splash any output it might produce all over your screen while you’re editing a file or typing another command.

Task 3 : Move a process from background to foreground

Usefull if we want to bring back on foreground a command we have sent in the bg.

sleep 100 &; sleep 100; < CTRL > Z ; bg ; jobs ; fg 2

In this sequence we put 2 process of sleep in background, after that we use the command jobs to have a list of all the processes in the background and fg 2 to bring in foreground the process number 2 .

Example of output:

laptop:~$ jobs

[1]- Running sleep 100 &

[2]+ Running sleep 100 &

Task 4 : Start a process, log off and have the process running

As a simple example, assume that you have a Linux command named myLongRunningJob.sh that you want to run, but you know if will take over three hours to run and you have to log off now, how can you do it ?

nohup myLongRunningJob.sh &

If standard input is a terminal, redirect it from /dev/null. If standard output is a terminal, append output to `nohup.out’ if possible, `$HOME/nohup.out’ otherwise. If standard error is a terminal, redirect it to standard output. To save output to FILE, use `nohup COMMAND > FILE'.

So we’ll find the output of our command in the file nohup.out.

Note:By itself, nohup does not run tasks in background, you must provide the character & at the end of the command line or use the CTRL-Z and put the tasks in background with the command bg.

Task 5 : Log off from a terminal keeping active a process we started without nohup

You have to log off from the terminal, but you are in the middle of that long task what to do ?

mylongtask.sh ; < ctrl > Z; bg; disown -h %1

After we put on background the process, disown it from the current terminal. The disown prevents a SIGHUP to the process if my terminal dies.

from the man page –

disown [-ar] [-h] [jobspec …]
Without options, each jobspec is removed from the table of active jobs. If the -h option is given, each jobspec is not removed from the table, but is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP. If no jobspec is present, and neither the -a nor the -r option is supplied, the current job is used. If no jobspec is supplied, the -a option means to remove or mark all jobs; the -r option without a jobspec argument restricts operation to running jobs. The return value is 0 unless a jobspec does not specify a valid job.

Of course, in the X Windows environment, all these unnatural gyrations are not necessary. Just start another shell window and run the other command there. You can watch both processes running in separate windows at the same time, and you don’t have to w orry about adding ampersands, piping output to files, or keeping track of foreground versus background processes.

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