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Screen summary

The screen is a terminal multiplexer that allows you to create multiple virtual terminals within a single physical terminal. screen is a very useful tool for managing long-running processes and tasks in an SSH session, especially when there's a risk of the session being terminated by network issues, server policies, or other disruptions. When you start a screen session, the processes running within that session are not tied to the SSH session. If an SSH session is terminated for any reason, any screen sessions running within that SSH session will automatically detach. The processes running inside the screen session will continue to run in the background. You can later reattach to the screen session and resume your work where you left off.

After reconnecting via SSH, you can reattach to the detached screen session using:

screen -r [session_id_or_name]

Table of contents

Here’s a brief example of how you might use screen in this scenario:

Start a Screen Session

screen -S mysession

Run Your Long-Running Process

Start your task or process within the screen session.

Detach from Screen

If you need to, you can detach from the screen session by pressing Ctrl-a d. The processes running within the screen session will continue even if you are detached.

Reattach to Screen

If your SSH session is terminated, you can SSH back into the server and reattach to the screen session with:

screen -r mysession

This way, screen can indeed help prevent data loss due to terminated SSH sessions by allowing processes to run uninterrupted and preserving your command-line environment between connections.

Here are some basic usage modes:

Starting a new screen session

screen

or you can name (Substitue) the session (it has already a name [pid.tty.host]):

screen -S session_name

Listing sessions

screen -ls

or

screen -list

Attaching (Resume) to an existing session

screen -r [pid.tty.host]/[session_name]

or simply (if there is only one)

screen -r

Detaching from a session

To detach from the current session, use the Ctrl-a d keyboard shortcut.

Closing a screen session

After detaching from a session, you can close the session with the following command (eXecute):

screen -X -S [session_id] quit

Managing Virtual Windows

  • Create a new window: Ctrl-a c
  • Switch between windows: Ctrl-a n for the Next window, and Ctrl-a p for the Previous window
  • Switch by window number: Ctrl-a [number]
  • Naming a window: Ctrl-a A

Scroll mode / Scrolling

Activate scroll mode with Ctrl-a [ and use the arrow keys to scroll. To exit scroll mode, press Enter or Esc.

Displaying help

If you need more commands, press Ctrl-a ? to access the help.

Exiting a screen session

To exit and close the session, type exit in the prompt.

Killing all windows and terminating screen

To kill all windows and terminate screen, press Ctrl-a \.

Or

  1. Press Ctrl-a to enter command mode.
  2. Then type :quit and press Enter.

This will quit the screen session by closing all windows at once. Please note that this will immediately terminate all processes running in each window, so make sure to save any unsaved work before using this command.

Configuration file

The configuration file for screen is ~/.screenrc. Here, you can customize the settings for screen.

Example:

startup_message off
defscrollback 100000
shell -${SHELL}
#https://lizdenys.com/journal/articles/understanding-gnu-screens-captions.html
#https://www.gnu.org/software/screen/manual/html_node/String-Escapes.html
caption always '%{= kw}[ %{y}%H%{-} ][ %= %-Lw%{+b M}%n%f* %t%{-}%+LW %= ][ %{r}%l%{-} ][ %{c}%c%{-} ]'
screen -t "bash" 0 bash
screen -t "man" 1 bash
screen -t "nvim" 2 nvim
screen -t "mc" 3 mc
select 0

These are the most important commands and functions to use screen effectively.

Troubleshooting

Attach to not detached session

SSH session terminated by server remotely. When you log on again, you get this:

$ screen -ls
There is a screen on:
        40383.pts-36.host001      (Attached)
1 Socket in /tmp/uscreens/S-noname.

Given that the screen session is still marked as "Attached", you won’t be able to directly reattach to it. However, you can forcefully reattach the session using the following command:

$ screen -d -r 40383.pts-36.host001

In this command:

  • The -d option tells screen to detach the session if it is still attached.
  • The -r option is used to reattach to a session. You will need to specify the session identifier after -r, which in this case is 40383.pts-36.host001.

After running this command, you should be reconnected to your screen session, and any processes or tasks that were running in it will continue to do so.

Screen session is garbled or disturbed

If the display of your screen session is garbled or disturbed, you can try a couple of things to redraw or refresh the interface:

  1. Redraw Window: Press Ctrl-a followed by Ctrl-l. This command sends a redraw signal to the terminal, and it should clean up the display.

  2. Redraw Using Reset: Type the reset command in the terminal and press Enter. This command will reset and reinitialize the terminal, which can often clear up display issues.

  3. Resize Window: Sometimes resizing the terminal window can force it to redraw and correct any display issues.

Remember that if you are using any text-based applications with a user interface (like text editors), they may have their own redraw or refresh commands. For instance, in vim, you can press Ctrl-l to redraw the interface.