212 lines
7.7 KiB
Markdown
212 lines
7.7 KiB
Markdown
# Getting started with GNU screen - Beginners Guide
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[Screen](https://www.gnu.org/software/screen/) is a terminal multiplexer and has a wide feature set. It allows you to split your terminal window into multiple windows (split screen feature), detach sessions to let commands run in the background, connect to a device via serial interface, and many more.
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Screen sessions keep running even if you disconnect, which is especially great for unreliable connections. There are more advanced use cases, but we will focus on the basics.
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# Basics <a href="#basics" id="basics">#</a>
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You can have multiple **sessions** within the screen and each session can contain multiple **windows**. When you use the split screen function, each panel would be a window called **region** in screen.
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```markdown
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screen
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│
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│
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├───── session 29324.x
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│ │
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│ ├────── window 0: name x
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│ │
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│ └────── window 1: name y
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│
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└───── session 29399.a
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│
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├───── window 0: name a
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│
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└───── window 1: name b
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```
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#### Escape combination (Prefix) <a href="#prefix" id="prefix">#</a>
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In this blog post, I'll call the **escape combination** 'prefix', but there are multiple names for it: meta key, leading key, escape combination, and some others.
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The prefix tells the terminal that the following command or shortcut will be used in the screen context. Almost every shortcut starts with it and the **default prefix** is `CTRL` + `a`. So, if you see `Prefix` in the reference section, I mean this key combination. I'll show you how to change the prefix as an example in the configuration section.
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A list of all default key bindings can be found in the [official documentaion](https://www.gnu.org/software/screen/manual/html_node/Default-Key-Bindings.html).
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## Configuration files <a href="#configuration" id="configuration">#</a>
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Screen won't create the startup configuration file by default but will look for these two files if it gets started.
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`~/.screenrc` / `/usr/local/etc/screenrc`
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**Comments** in the configuration file start with a `#`.
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The following two sections will show some simple examples of different configurations.
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#### Example 1: change prefix for screen
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Adding the following line to you your configuration file changes the prefix to `CTRL` + `f`:
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: `escape ^Ff`
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You can change it to a difference key combination, especially as the default prefix key combination is commonly used otherwise.
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#### Example 2: turn off the copyright message at the start
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```markdown
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#Do not show copyright msg at the start of a session
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startup_message off
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```
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Simply add these lines to your configuration file, and the copyright message won't appear again.
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## Logging <a href="#logging" id="logging">#</a>
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Before we start with the sessions and windows, it might be beneficial to talk about logging. For most troubleshooting sessions, it is required to save the logs. I am going to show you some ways to do it.
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#### Hardcopy
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Use `Prefix` + `h` to create an output file with the content of the current screen window you are in. It will be saved as `hardcopy.n` (*'n' for the number of the current window *) in the directory from where you have started the screen initially. If you repeat the shortcut, the initial file will be overwritten.
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If you want to **append** the output to a file, you can add `hardcopy_append on` to your configuration file.
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If you want to change the directory in which the harcopy files will be saved, simply add `hardcopydir /your/dir/` to your configuration file.
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#### Continuous logging
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Logging is disabled by default.
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You can start a logged screen session with `-L` flag + `-Logfile /path/to/logfile.txt`. If you are already in a session, you can activate it with `Prefix` + `SHIFT` + `h`. The output file will be called `screenlog.n`, where 'n' is the number of the current window.
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## Working with sessions <a href="#sessions" id="sessions">#</a>
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Show all sessions:
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: `screen -ls`
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```markdown
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kuser@pleasejustwork:~$ screen -ls
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There are screens on:
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29265.demo-session (27.01.2023 02:19:51) (Detached)
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26508.pts-8.pleasejustwork (26.01.2023 23:20:50) (Detached)
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2 Sockets in /run/screen/S-kuser.
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```
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Start a new session:
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: `screen`
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Start a new session with a specific name:
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: `screen -S nameofthissession`
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Start a new detached session and run a command:
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: `screen -d -m ping 10.10.10.10`
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Detach the current session:
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: `Prefix` + `d`
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Create new session if there is none, or re-attach the last session:
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: `screen -d -RR`
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Re-attach session in terminal:
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: `screen -r 2232` *# screen will auto complete if the prompt is unique*
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: `screen -r nameofthissession` *# either use the session number or the name*
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Kill session in terminal:
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: `screen -X -S nameofsession quit`
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: `screen -X 269 quit` *# auto-completes if unique*
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Rename session in terminal:
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: `screen -S OLDSESSIONNAME -X sessionname NEWSESSIONNAME`
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: `Prefix` + `:sessionname NEW-NAME` *# screen command to change the current session name*
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## Working with Windows <a href="#windows" id="windows">#</a>
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Show list of all windows of current session:
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: `Prefix` + `SHIFT` + `w`
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Rename the current windows:
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: `Prefix` + `SHIFT` + `a`
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Jump to the next window:
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: `Prefix` + `SPACE`
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Jump to the previous window:
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: `Prefix` + `p`
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Kill the current window:
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: `exit`
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: `Prefix` + `k`
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## Working with Regions / Split screen <a href="#split-screen" id="split-screen">#</a>
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Screen has the feature to show multiple windows in a split screen. Every window would then be a so called 'Region' in screen.
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Horizontally split window into two regions:
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: `Prefix` + `SHIFT` +`s`
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Vertically split window into two regions:
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: `Prefix` + `|`
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Jump to the next region:
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: `Prefix` + `Tab`
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Close the current region:
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: `Prefix` + `x`
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: *the window won't be terminated and just the split screen will be removed.*
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Close all but the current region:
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: `Prefix` + `q`
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Fit the regions to a resized terminal window:
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: `Prefix` + `SHIFT` +`f`
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#### Layouts
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You could create layouts, and save and reuse them later. This topic is out of the scope of this post and I am going to write about it later. You can get a reference and further information in the [official documentaion](https://www.gnu.org/software/screen/manual/screen.html#Layout).
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## Screen commands <a href="#commands" id="commands">#</a>
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It can be used to try out configurations and screen-specific commands.
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`Prefix` + `:` + config
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`Prefix` + `:logfile ~/path/to/new/logfile.txt`
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I am not too familiar with screen commands, so I won't go into detail. A list of all commands can be found in the [official documentaion](https://www.gnu.org/software/screen/manual/screen.html#Command-Summary).
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# Check if you are still in a screen session <a href="#active-session" id="active-session">#</a>
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Screen sets an environment variable `STY`. If the output is empty, you are not in a screen session.
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```markdown
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kuser@pleasejustwork:~$ echo $STY
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22829.demo
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```
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This won't work if you start up screen and SSH into a remote machine. Without further configuration, the variables stay local.
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---
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Another environment variable you could try is `TERM`.
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```markdown
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kuser@pleasejustwork:~$ screen
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kuser@pleasejustwork:~$ echo $TERM
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screen.xterm-256color
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kuser@pleasejustwork:~$ exit
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kuser@pleasejustwork:~$ echo $TERM
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xterm-256color
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```
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Screen will add the prefix `screen.` in front of it.
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This works even after connecting to a remote machine but presumes that you didn't mess with the `TERM` variable.
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---
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Another method would be to work with the screen prefix. You could simply use `Prefix` + `CTRL` + `t` to let screen tell you the time in the bottom left corner.
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---
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