A terminal multiplexer mainly facilitates three things:
- Create persistent terminal sessions that remain intact even when you log out of a machine--as long as the machine itself remains running. For example:
- SSH into a remote machine
- Start a screen session
- Start a process
- Log out of the machine
- Log back in later, attach to the screen session, and continue working with that process.
- Allow you to work in multiple terminal sessions at the same time without having to launch as individual connections. For example:
- Open a terminal session and start a multiplexer
- Create multiple shell sessions in "windows" you can switch between without needing to open additional terminal instance.
- Split the terminal screen into multiple "panes" which have independent shells.
- Edit a file in one pane and test running it in the other pane without having to switch between terminal sessions or create a 2nd ssh connection.
- Increase productivity by creating work environments (with the above features) which you can detatch from and re-attach to without having to re-create them.
The two most popular terminal multiplexers are screen and tmux. Screen is older and more basic. Tmux is newer and has some snazzier features.
NOTE: As with terminal-based text editors, you can use features of the multiplexer by dropping to a dedicated command line but will almost always use keyboard shortcuts. To access the keyboard shortcuts you preceed them by typing a metacharacter or "prefix." For screen this is CTRL-a. For tmux it is CTRL-b. However you can customize it through a configuration file.
Screen has been around for a long time and I think is installed by default in many Linux distributions. In any case, you can easily install it with the package manager of your choice.
Let's run through some exercises to learn the basics.
Start a named screen session Open a terminal shell session and type
screen -S test
You will now be in a terminal session and may not see anything different at all. However, type CTRL-a and then t and you will see a little information strip showing that you are in a screen session. You can customize this.
Enter a command
Type your favorite shell command like ls or echo hello.
Detacth
Type CTRL-a and then d. If your terminal screen looks all messed up it's because some terminals don't clear the screen when you detatch. So just enter the command clear.
List Screen Sessions Type
screen -ls
You should see something like
(base) balter@spectre:~$ screen -ls
There is a screen on:
7981.test (05/19/22 15:18:34) (Detached)
1 Socket in /home/balter/.screen.
Re-attach type
screen -r test
You will now be back in the screen session.
Kill a Screen session
The easiest way is to type exit in the screen session. You can also kill it remotely with screen -XS <bame> quit.
There are many ways you can customize your screen session with custom status bars and other niceties.
One of the most useful features of screen (or tmux) is that you can log out of an SSH connection and keep a running shell you can get back to.
SSH to a machine For example:
(base) balter@spectre:~$ gcloud beta compute ssh balter-host --zone us-east1-b --project psjh-eacri-data --tunnel-through-iap
Welcome to Ubuntu 20.04.4 LTS (GNU/Linux 5.13.0-1025-gcp x86_64)
Last login: Thu May 19 19:22:06 2022 from 35.235.240.81
(base) balter@balter-host:~$
Start a screen session
screen -S test
Start top
Type top
Detatch
CTRL-a then d
Logout
exit (or whatever)
SSH again
gcloud beta compute ssh balter-host --zone us-east1-b --project psjh-eacri-data --tunnel-through-iap
Check Screen Sessions
screen -ls
Re-attach
screen -r test
Top is still running!!!
It's pretty common to want to maintain two simultanous SSH connections to a single machine so we can do two things, say monitor a process while doing something else. Or edit a file and run it without having to constantly close the editor.
"Windows" make this possible.
Start a screen session Just like before
Type a command
You can use ls so that you will have something on the screen to come back to, or maybe top again.
Start a new window
CTRL-a then c
Do something in that window
Switch back to the other window Windows are numbered in the order they are created. So you can type:
CTRL-a then 0 for the first window or
CTRL-a then 1 for the 2nd window.
Also, you can type CTRL-a then n for the "next" window and CTRL-a then p for the "previous" window.
Also, you can type CTRL-a then " to get a menu of windows you can navigate through.
I believe there are ways to name windows if you really want to.
With Panes you can turn a single screen session into something almost like an IDE. You can also mix panes and windows. For instance, you can have two windows, create a 2nd pane in one window, then attach the other window to the 2nd pane.
Create Two Windows As above.
Create a second pane
For a vertical divider, type CTRL-a then |. For a horizontal divider, type CTRL-a then %.
Jump to the other pane
CTRL-a then TAB
Your cursor will now be in the other pane, but it can't do anything because there is no terminal session there.
Attach the other window
Type CTRL-a then t to see which window you are in. The highlighted number is the one you are in.
If the other window is #0, then type
CTRL-a then 0
And window #0 will now be in the 2nd pane.
Switch back and forth between panes
CTRL-a then TAB
Do Stuff For example, create a file in one pane
test.py
a = 10
b = 20
print("a*b = ", a*B)
Switch to the other pane
python -i test.py
Etc.
Try doing this in the other order. Create two panes. Jump to the other pane. Then create a new window there CTRL-a then c