Linux Screen

Imagine having to execute a long running process and waiting for it to finish, most likely you are connected via ssh and you can’t do anything else but waiting for that process to finish. When the process is taking too long your connection will time out and you will be kicked out. When that happens your process will stop its execution and you will have to rerun it. This is painful and here is where screen comes into the picture.

For example, you are downloading a big file or a database using mysql dump command. This usually takes 10 to 20 minutes.

You can use the Linux screen command to push running terminal applications or processes to the background and pull them forward when you want to see them. It also supports split-screen displays and works over SSH connections, even after you disconnect and reconnect.

How does screen work?

The usual operation with screen is to create a new window with a shell in it, run a command, and then push the window to the background (called “detaching”). When you want to see how your process is doing, you can pull the window to the foreground again (“reattach”) and use it again. This is great for long processes you don’t want to accidentally terminate by closing the terminal window.

Once you’ve got a screen session running, you can create new windows and run other processes in them. You can easily hop between windows to monitor their progress. You can also split your terminal window into vertical or horizontal regions, and display your various screen windows in one window.

You can connect to a remote machine, start a screen session, and launch a process. You can disconnect from the remote host, reconnect, and your process will still be running.

You can share a screen session between two different SSH connections so two people can see the same thing, in real-time.

Intall screen

# ubuntu
sudo apt update
sudo apt install screen

# centos or fedora
sudo yum install screen

Start a screen session

screen

# start screen session with a name
screen -S loading-database

This will open a screen session, create a new window, and start a shell in that window.

Most used commands for a screen window

Ctrl+a c Create a new window (with shell).
Ctrl+a " List all windows.
Ctrl+a 0 Switch to window 0 (by number).
Ctrl+a A Rename the current window.
Ctrl+a S Split current region horizontally into two regions.
Ctrl+a | Split current region vertically into two regions.
Ctrl+a tab Switch the input focus to the next region.
Ctrl+a Ctrl+a Toggle between the current and previous windows
Ctrl+a Q Close all regions but the current one.
Ctrl+a X Close the current region.

Detach from a screen session

Ctrl+a Ctrl+d

The program running in the screen session will continue to run after you detach from the session.

Reattach to a screen session

# resume current screen session, assuming one screen session
screen -r

In case you have multiple screen sessions running on your machine, you will need to append the screen session ID after the r switch.

# list the current running screen sessions 
screen -ls

# output
There is a screen on:
        30093.pts-0.ip-172-31-8-213     (09/24/21 16:07:23)     (Attached)
1 Socket in /run/screen/S-ubuntu.

# command
screen -r 30093

Terminate a screen session

Make sure you on the screen session and then run the exit command.

exit

or run this command out of the screen session

screen -X -S screen_id kill (or quit)

or 

screen -XS <screen_id> kill (or quit)

# Example
screen -X -S 30093 kill 

or 

screen -XS 30093 quit