# GNU Screen

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GNU Screen is a wrapper that allows separation between the text program and the shell from which it was launched. This allows the user to, for example, start a text program in a terminal in X, kill X, and continue to interact with the program. Here are a couple of tips and tricks you may be interested in.

## Installation

GNU Screen can be installed using the screen package found in the official repositories.

## Usage

Commands are entered pressing `ctrl+a` and then the key binding.

### Common Commands

- `ctrl+a` `?` Displays commands and its defaults (VERY important)
- `ctrl+a` `:` Enter to the command prompt of screen
- `ctrl+a` `"` Window list
- `ctrl+a` `0` opens window 0
- `ctrl+a` `A` Rename the current window
- `ctrl+a` `a` Sends `ctrl+a` to the current window
- `ctrl+a` `c` Create a new window (with shell)
- `ctrl+a` `S` Split current region into two regions
- `ctrl+a` `tab` Switch the input focus to the next region
- `ctrl+a` `ctrl+a` Toggle between current and previous region
- `ctrl+a` `Esc` Enter Copy Mode (use enter to select a range of text)
- `ctrl+a` `]` Paste text
- `ctrl+a` `Q` Close all regions but the current one
- `ctrl+a` `X` Close the current region
- `ctrl+a` `d` Detach from the current screen session, and leave it running. Use `screen -r` to resume

### Command Prompt Commands

- `ctrl+a` `:quit` Closes all windows and closes screen session
- `ctrl+a` `:source ~/.screenrc` Reloads screenrc configuration file (can alternatively use /etc/screenrc)

### Named sessions

To create a named session, run screen with the following command:

    $ screen -S session_name

To prints a list of pid.tty.host strings identifying your screen sessions:

    $ screen -list

To attach to a named screen session, run this command:

    $ screen -x session_name

### Autostart with systemd

This service autostarts screen for the specified user (e.g. `systemctl enable screen@florian`).

    /etc/systemd/system/screen@.service

    [Unit]
    Description=screen
    After=network.target

    [Service]
    Type=forking
    User=%i
    ExecStart=/usr/bin/screen -dmS autoscreen
    ExecStop=/usr/bin/screen -S autoscreen -X quit

    [Install]
    WantedBy=multi-user.target

## Tips and tricks

### Change the escape key

The escape key can be changed with the `escape` option in `~/.screenrc`. For example:

    escape ``

sets the escape key to ``` and

    escape ^^^

sets it to `ctrl+^`.

### Start at window 1

By default, the first screen window is 0. If you'd rather never have a window 0 and start instead with 1, add the following lines on your configuration:

    ~/.screenrc

    bind c screen 1
    bind ^c screen 1
    bind 0 select 10
    screen 1

### Nested Screen Sessions

It is possible to get stuck in a nested screen session. A common scenario: you start an ssh session from within a screen session. Within the ssh session, you start screen. By default, the outer screen session that was launched first responds to `ctrl+a` commands. To send a command to the inner screen session, use `ctrl+a` `a`, followed by your command. For example:

- `ctrl+a` `a` `d` Detaches the inner screen session.
- `ctrl+a` `a` `K` Kills the inner screen session.

### Use 256 colors

By default, screen uses an 8-color terminal emulator. Use the following line to enable more colors, which is useful if you are using a more-capable terminal emulator:

    ~/.screenrc

    term screen-256color

If this fails to render 256 colors in xterm, try the following instead:

    ~/.screenrc

    attrcolor b ".I"    # allow bold colors - necessary for some reason
    termcapinfo xterm 'Co#256:AB=\E[48;5;%dm:AF=\E[38;5;%dm'   # tell screen how to set colors. AB = background, AF=foreground
    defbce on    # use current bg color for erased chars

### Use 256 Colors with Rxvt-Unicode (urxvt)

If you are using rxvt-unicode from the official repositories, you may need to add this line in your `~/.screenrc` to enable 256 colors while in screen.

    ~/.screenrc

    terminfo rxvt-unicode 'Co#256:AB=\E[48;5;%dm:AF=\E[38;5;%dm'

### Informative statusbar

The default statusbar may be a little lacking. You may find this one more helpful:

    ~/.screenrc

    hardstatus off
    hardstatus alwayslastline
    hardstatus string '%{= kG}[ %{G}%H %{g}][%= %{= kw}%?%-Lw%?%{r}(%{W}%n*%f%t%?(%u)%?%{r})%{w}%?%+Lw%?%?%= %{g}][%{B} %m-%d %{W} %c %{g}]'

### Turn welcome message off

    ~/.screenrc

    startup_message off

### Turn your hardstatus line into a dynamic urxvt|xterm|aterm window title

This one is pretty simple; just switch your current `hardstatus` line into a `caption` line with notification, and edit accordingly:

    ~/.screenrc

    backtick 1 5 5 true
    termcapinfo rxvt* 'hs:ts=\E]2;:fs=\007:ds=\E]2;\007'
    hardstatus string "screen (%n: %t)"
    caption string "%{= kw}%Y-%m-%d;%c %{= kw}%-Lw%{= kG}%{+b}[%n %t]%{-b}%{= kw}%+Lw%1`"
    caption always

This will give you something like `screen (0 bash)` in the title of your terminal emulator. The caption supplies the date, current time, and colorizes your screen window collection.

### Use X scrolling mechanism

The scroll buffer of GNU Screen can be accessed with `ctrl+a` `[`. However, this is very inconvenient. To use the scroll bar of e.g. xterm or konsole, add the following line:

    ~/.screenrc

    termcapinfo xterm* ti@:te@

### Attach an existing running program to screen

If you started a program outside screen, but now you would like it to be inside, you can use reptyr to reparent the process from it's current tty to one inside screen.

Install the package reptyr from the official repositories.

Get the PID of the process (you can use `ps ax` for that). Now just enter the PID as argument to reptyr inside a screen window.

    $ reptyr <pid>

### Add a GRUB entry to boot into Screen

If you mostly use X but occasionally want to run a Screen-as-window-manager session, here is one way to do it by adding a GRUB entry for Screen on a virtual console (text terminal).

GRUB allows you to designate what runlevel you want so we will use runlevel 4 for this purpose. Clone an appropriate GRUB entry and add a `4` to the kernel boot parameters list, like so:

    # (0) Arch Linux
    title  Arch Linux Screen
    root   (hd0,2)
    kernel /vmlinuz-linux root=/dev/disk/your_disk ro acpi_no_auto_ssdt irqpoll 4
    initrd /initramfs-linux.img

Add some entries to `/etc/inittab` to indicate what should happen on runlevel 4, substituting your user name for <user>:

    # GNU Screen on runlevel 4
    scr2:4:respawn:/sbin/mingetty --autologin <user> tty1 linux

The above line uses mingetty to automatically login some user to a virtual console on startup. The inittab line segments are separated by colons. The first part (scr*) is simply an id. The second part is the runlevel: This should only happen on runlevel 4 (which is not used in any default setup - 3 is by default for a tty login and 5 is for X). 'Respawn' causes init to repeat the command (i.e. autologin) if the user logs out. We will need to see that nothing else happens on virtual console 1 when we use runlevel 4, so remove `4` from the the first of the agetty lines:

    c1:235:respawn:/sbin/agetty -8 38400 vc/1 linux

Once we are logged in, we want to ensure that screen is started. Add the following to the end of your `~/.bash_profile`:

    vico="$(tty | grep -oE ....$)"
    case "$vico" in
       tty1) TERM=screen; exec /usr/bin/screen -R arch;;
    esac

This checks for the current runlevel and will launch a screen session immediately after the automatic login if the runlevel is 4.

This can also be adapted to run screen on a virtual console next to X, simply checking for the current tty instead of the current runlevel. This checks to see if we are on virtual console 3:

    vico="$(tty | grep -oE ....$)"
    case "$vico" in
      vc/3) TERM=screen; exec /usr/bin/screen;;
    esac

Set inittab/mingetty to automatically log in to vc/3 on runlevel 5 and you are set.

### Setting a different bash prompt while in screen

If you want a different bash prompt when in a screen session, add the following to your .bashrc:

    if [ -z $STY ]
    then
            PS1="YOUR REGULAR PROMPT"
    else
            PS1="YOUR SCREEN PROMPT"
    fi

[1]

### Turn off visual bell

With this setting, screen will not make an ugly screen flash instead of a bell sound.

    ~/.screenrc

    vbell off

## Troubleshooting

### Fix Midnight Commander hard hang when starting in screen

In some cases (need deeper inspection) old gpm bug gets alive. So, then you try to run mc inside screen, you get a frozen screen window. Try to kill gpm daemon before starting mc and/or disable it in `/etc/rc.conf`.

### Fix for residual editor text

When you open a text editor like nano in screen and then close it, the text may stay visible in your terminal. To fix this, put the following:

    ~/.screenrc

    altscreen on

## See Also

- MacOSX Hints - Automatically using screen in your shell
- Gentoo Wiki - Tutorial for screen
- Arch Forums - Regarding 256 color issue with urxvt
- Arch Forums - .screenrc configs with screenshots
- Ratpoison - A window manager based on gnu screen
- Xpra - An utility to detach/reattach X programs, in a way similar as screen does for command line based programs

Category:

- System administration