Intro to Linux Screen

Background

The linux screen utility allows you to run commands or programs in the background easily. This is great for when you need to multi-task or simply run programs without being attached to them. screen is pre-installed on most linux systems but can be easily installed if not. For Debian based systems, run the below.

sudo apt install screen

For others, use their respective package manager (ex: yum for centOS)

Using screen

To start a session, run screen with the name of the session

screen -S web_server

Now you are in the new session and can run any command or program. We will run a simple sleep and echo to emulate some long running task.

sleep 30 && echo "Hi"

Detaching from the screen

Now that the 30 second “program” is running, you can detach from it so that it can run in the background while you work on something else.

To detach from the session, hit CTRL a d. That is hold the CTRL key, then a and d

Listing your screens

To list all the current sessions, run

screen -ls

Reattaching to the screen

After 30 seconds has passed, re-attach to the screen to see the program output. To re-attach run

screen -r web_server

You should see it echo-ed "Hi"

Starting screens detached from the start (programmatically)

Creating a screen and then running a command works fine as a user but does not work well in scripts (programmatically). For this, you can run a screen in detached mode from the start. To executed screen in detached mode run

screen -d -m [command] -S [session_name]

• -d means to run in daemon mode (detached from start).
• -m is the command to run.
• -S is the name of the session as we have learned before.

Sending commands to the program running in screen

To send a command to the program running inside screen, as opposed to a program for screen to run, use the -X stuff. Assuming you had a screen running a backup job that asks for a remote location when run, you could “stuff” the url in.

screen -S backup_job -X stuff s3://my_bucket/backup^M

This would pass s3://my_bucket/backup to your backup job and the escape sequence for the enter key.

Ending screens

screen -S [session_name] -X quit #Kill a specific screen
killall screen #Kill all screens

Configuring your screen

Screen windows can be configured per user using a ~/.screenrc file or system wide using a /etc/screenrc file.

Custom screen configuration can also be passed per screen invocation using the -c flag.

A sample screenrc is linked here.