A quick reference to Screen aka GNU Screen
Created on: 2020-03-17
Tag: cheat_sheet, living_in_the_shell
to install in Ubuntu 18.04:
sudo apt install screen -y
to start a new session:
screen
We will be greeted with the welcome screen with information related to screen. We can turn it off by default by editing the ~/.screenrc and adding the following:
startup_message off
If we type screen then screen will autmaticalley designate a name for the session that follows this scheme:
11547.pts-5.gun_screen
^ ^ ^
| | |
PID of | hostname
the session |
psudo terminal
number
Session name is used while reattaching to a session, for example we will reattach to the session in the example with screen -r pts-5.gun_screen. Though in bash tab completion for session name is supported but this long complex naming structure can be problematic when frequently detaching and reattaching. So we can assign name to a session like this with the -S flag:
screen -S screen_test
By default each session is created with one window numbered as 0. To create a new window:
Ctrl + a then press c
The Ctrl + a is called the escape command and used to invoke screen. The c is the command which screen gets after invocation.
Note
The escape command can be changed, which we will see later.
Note
The commands after the escape command is case sensitive.
to switch between window, we can go to next window:
Ctrl + a then press n
or go to previous window:
Ctrl + a then press p
another quick way to switch between window is to use the window numbers like 0 or 1 but remember that this only works for window 0 to 9 like this:
Ctrl + a then press 5
to use two digit numbered window do the following:
Ctrl + a then press '
There should be a prompt at the left bottom saying Switch to window:. Now type the window number:
12
This obviously works with single digit numbered window.
to list all the window:
Ctrl + a then press ". [" is Shift + ']
This shows a list of windows with 3 column: Num, Name and Flags. We can use the ↑ and ↓ to select a window and press Enter to switch to that window.
to see all the windows of a session:
Ctrl + a then press w
This will show the number and name of all the windows at the bottom of the screen like this:
0-$ bash 1*$ bash 2$ bash
The * after the 1 means that we are currently on window 1.
we have already seen that we can list and see window with window name but by default all of the are name are bash as that is the programming running in them. To change the name of a window:
Ctrl + a then press A
We will see a prompt at the left bottom saying Set window's title to: bash. We need to press Backspace until it reaches : now we can type the window title and press Enter to rename it.
to kill a window:
Ctrl + a then press K
We should get a prompt at the usual left bottom corner saying Really kill this window [y/n] type y and press Enter to kill the window.
to detach from a session:
Ctrl + a then press d
In the terminal we should see something similar to [detached from 23537.screen_test].
to list all session:
screen -ls
If we have multiple session we would see something like this:
There are screens on:
23537.screen_test (03/18/2020 05:10:48 PM) (Detached)
23547.screen_test (03/18/2020 05:10:48 PM) (Attached)
23557.screen_test1 (03/18/2020 05:10:48 PM) (Detached)
3 Sockets in /run/screen/S-$USER.
Yes, we can assign same name for two different session but that is not that useful.
From our previous output, we can see that we have 3 session and 2 with same name. To attach to a detached session we normal use the -r flag like this:
screen -r screen_test1
As we can see for unique name we can use the session name but for sessions with same name we have to specify the PID as well, like this:
screen -r 23537.screen_test
We can also use tab completion to get our window without typing.
to exit from a session, first we need to attach to that session:
screen -r 23547.screen_test
Now we will Ctrl + a to invoke screen then type :quit and press Enter. Back in the terminal we should see:
[screen is terminating]
This is one of few odd things that a new user face while using screen. While inside the screen the default scroll don't work but we still can scroll through the history. Moreover, we can copy, paste and log that history. To start scrolling back:
Ctrl + a. next press [
This will activate the Copy mode [just like vim 😉]. Now we can use the ↑, ↓, or Page up, Page down, Home, End or vim's j k buttons to navigate through the history. We can also copy the history. To copy history, we have to move our courser to the position where we want to start copying and press Enter then move the courser to the place where we want the copy to end and Enter again. To paste the copied text do:
Ctrl + a. next press [
to log output of a screen window we need to press:
Ctrl + a. next press H
This will save the output to a file name screenlog.$NUM. The NUM variable starts from 0. If we do Ctrl + a. next press w to see the windows title we will see something like this:
0*$(L) bash 1-$(L) bash 2$ bash
The * means we are currently in that window and the (L) means we are logging those windows.
We can split the screen in horizontal and vertical half. To split the screen in horizontal half:
Ctrl + a. next press S
to split the screen in vertical half:
Ctrl + a. next press |
By default the screen neither switch to the splitted half nor starts a new window. We need to do both manually. First, to switch between the half use:
Ctrl + a. next press Tab
Now we can either create a new window with Ctrl + a. next press c or place a previously created window with Ctrl + a. next press $WINDOW_NUMBER.
to exit from splitted screen, go to the window you want to use and do:
Ctrl + a. next press Q
Note
till this most of the things are from Softpedia News: GNU Screen Tutorial: How to use the screen's most important features. article because this was one of the article I got that was very easy for me to understand the besics and start using it.
Sometimes while invocing screen with Ctrl + a we misplace our finger and press Ctrl + s. This will freeze our screen session. To unfreeze:
Ctrl + Q
to kill a screen session:
screen -X -S $SESSION_YOU_WANT_TO_KILL quit