FOSSwire

I recently posted about using nohup to run a command, particularly on a remote machine, that keeps running even when you close the terminal or connection that started it.

Several people in the comments there also suggested GNU Screen for a similar purpose.

So, what is Screen? It describes itself as:

... a full-screen window manager that multiplexes a physical terminal between several processes, typically interactive shells.

Basically, among other things, it can create multiple 'virtual terminals' that run inside a single physical terminal or connection, and offers you additional features, such as resuming sessions later and basic copy and paste.

What we're interested in in the context of my other post is running commands in the background on remote machines, so I can start a command running, disconnect from SSH, but the command will stay running.

Screen, unlike Nohup, will allow me to come back later and interact directly with the terminal that I started, not just dump the results of a command to a file.

On your remote machine, start the program:

$ screen

You'll get a brief copyright notice and such, just press Space as directed. You are now running Screen (although it won't look any different to a normal terminal session by default).

Now, feel free to go off and start that important task. Once it's up and running, press Ctrl+A, then Ctrl+D. Screen sends you back to your shell and you can now disconnect.

Later, when you want to come back, run:

$ screen -r

Your old session is restored! Anything you started should still be running.

Screen is a lot more powerful than just offering this feature, however, but we'll save the rest for another day.

Finally, when you are actually done with a Screen session for good, quit it by pressing Ctrl+A, then Ctrl+\ or you can simply type exit into the terminal as normal.

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Oftentimes you'll be in a situation where you want to run a command on a remote machine that will take a long time to complete, but you want to be able to issue the command and then log off and have that command run in the background.

There are many ways you could achieve this, perhaps by using cron or at to schedule the command to run right away. However, there is a better way.

There is a command called nohup built into both the GNU toolset, and most shells, which allows you to run a command in this way. It is so called because the command being run is executed ignoring 'hang up' signals, which are given when you close the terminal you started the program from.

To use this, simply prefix your command with nohup, for example:

nohup wget bigfile

This will still run in the foreground, however, meaning that you will lose the ability to use that terminal while the command is executing. In most cases, you'll want to use the ampersand (&) to run the command in the background.

nohup wget bigfile &

Now you can log off your remote machine, or close your terminal and the command will continue running in the background.

The output and errors from the command you run with nohup are stored in a file called nohup.out in the directory where you started the command, or your home directory if for some reason that's not possible (e.g. permissions).

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