Remote administration with SSH is something every sysadmin is
familiar with. Likewise, doing multiple things at the same time, including lengthy
compiles or long tasks is also something many of us are familiar with. Freeing
up a console usually involves opening a new terminal and initiating a new SSH connection
to get further work done, or—if observation of a task isn’t critical—you can
send the task to the background to free up the terminal.

Another solution exists in the form of the screen tool, a
program that is included with every Linux distribution. Screen creates virtual terminals
that you can control and interact with via one terminal. What’s even better, a
screen session can be disconnected without killing a running task. Imagine
starting a long compile on a remote server and the connection dies; when the
connection dies, so does your task. Screen works around this by allowing you to
detach from a running session, log out, and resume it later, even from a
different location.

To get started, make sure the screen package is installed
using your distribution’s package manager and then type:

$ screen

This will start screen
and open a new session. To disconnect from a session, type CTRL-A then d. You will
return to the prompt from which you issued screen, but everything you have done
in screen is still available. If only one screen session is running you can
reconnect to it using:

$ screen -R

If there are multiple screen sessions running, this won’t work;
but you can view a list of running screen sessions by using:

$ screen -list
There are screens on:
        13995.pts-0.host        (Detached)
        14529.pts-0.host        (Attached)
2 Sockets in /home/joe/tmp.

Here, you can see there are two sessions running. To connect
to the detached session from a different location, you would use (after SSH-ing
to that machine, of course):

$ screen -r 13995

where 13995 is the process ID of the screen session you wish
to attach to.

There is a lot of help available for screen, and a lot of
things you can do with it. You can view the screen
manpage, the output of screen –help,
and within a screen session, type CTRL-A
then ? to get a list of commands you
can use when in command mode (invoked by CTRL-A).

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