Linux screen command
On Unix-like operating systems, the screen command launches a screen manager with VT100/ANSI terminal emulation.
Using screen, you can create multiple terminal sessions, switch between them, and display them next to each other, all within a single terminal window.
Description
screen is a full-screen window manager that multiplexes a physical terminal between several processes (often interactive shells). Each virtual terminal provides the functions of a DEC VT100 terminal and, in addition, several control functions from the ISO 6429 (ECMA 48, ANSI X3.64) and ISO 2022 standards (e.g., insert/delete line and support for multiple character sets). There is a scrollback history buffer for each virtual terminal and a copy-and-paste mechanism that allows moving text regions between windows.
When screen is called, it creates a single window with a shell in it (or the specified command) and then gets out of your way so that you can use the program as you normally would. Then, at any time, you can create new (full-screen) windows with other programs in them (including more shells), kill existing windows, view a list of windows, turn output logging on and off, copy-and-paste text between windows, view the scrollback history, switch between windows in whatever manner you want, etc. All windows run their programs completely independent of each other. Programs continue to run when their window is currently not visible and even when the whole screen session is detached from the user’s terminal. When a program terminates, screen (per default) kills the window that contained it. If this window was in the foreground, the display switches to the previous window; if none are left, screen exits.
Everything you type is sent to the program running in the current window. The only exception to this is the one keystroke that is used to initiate a command to the window manager. By default, each command begins with a control-a (abbreviated C-a from now on), and is followed by one other keystroke. The command character and all the key bindings can be fully customized to be anything you like, though they are always two characters in length.
screen does not understand the prefix «C-» to mean control. Please use the caret notation («^A» instead of «C-a«) as arguments to e.g., the escape command or the -e option. screen also prints out control characters in caret notation.
The standard way to create a new window is to type «C-a c«. This creates a new window running a shell and switches to that window immediately, regardless of the state of the process running in the current window. Similarly, you can create a new window with a custom command in it by first binding the command to a keystroke (in your .screenrc file or at the «C-a :» command line) and then using it like the «C-a c» command. Also, new windows can be created by running a command like:
from a shell prompt within a previously created window. This will not run another copy of screen, but will instead supply the command name and its arguments to the window manager (specified in the $STY environment variable) who uses it to create the new window. The above example would start the emacs editor (editing prog.c) and switch to its window. Note that you cannot transport environment variables from the invoking shell to the application (emacs in this case), because it is forked from the parent screen process, not from the invoking shell.
If «/var/run/utmp» is writable by screen, an appropriate record will be written to this file for each window, and removed when the window is terminated. This is useful for working with «talk», «script», «shutdown», «rsend«, «sccs» and other similar programs that use the utmp file to determine who you are. As long as screen is active on your terminal, the terminal’s own record is removed from the utmp file. See also «C-a L«.
Syntax
screen [ -options ] [ cmd [ args ] ]
screen -r [[pid.]tty[.host]]
screen -r sessionowner/[[pid.]tty[.host]]
Options
| -a | include all capabilities (with some minor exceptions) in each window’s termcap, even if screen must redraw parts of the display to implement a function. |
| -A | Adapt the sizes of all windows to the size of the current terminal. By default, screen tries to restore its old window sizes when attaching to resizable terminals (those with «WS» in its description, e.g., suncmd or some xterm). |
| -c file | override the default configuration file from «$HOME/.screenrc» to file. |
| -d|-D [pid.tty.host] | does not start screen, but detaches the elsewhere running screen session. It has the same effect as typing «C-a d» from screen‘s controlling terminal. -D is the equivalent to the power detach key. If no session can be detached, this option is ignored. In combination with the -r/-R option more powerful effects can be achieved: |
| -d -r | Reattach a session and if necessary detach it first. |
| -d -R | Reattach a session and if necessary detach or even create it first. |
| -d -RR | Reattach a session and if necessary detach or create it. Use the first session if more than one session is available. |
| -D -r | Reattach a session. If necessary detach and logout remotely first. |
| -D -R | Attach here and now. In detail this means: If a session is running, then reattach. If necessary detach and logout remotely first. If it was not running create it and notify the user. This is the author’s favorite. |
| -D -RR | Attach here and now. Note: It is always a good idea to check the status of your sessions by means of «screen -list«. |
| -e xy | specifies the command character to be x and the character generating a literal command character to y (when typed after the command character). The default is «C-a» and ‘a‘, which can be specified as «-e^Aa«. When creating a screen session, this option sets the default command character. In a multiuser session all users added starts off with this command character. But when attaching to an already running session, this option changes only the command character of the attaching user. This option is equivalent to either the commands «defescape» or «escape» respectively. |
| -f, -fn, and -fa | turns flow-control on, off, or «automatic switching mode». This can also be defined through the «defflow» .screenrc command. |
| -h num | Specifies the history scrollback buffer to be num lines high. |
| -i | causes the interrupt key (usually C-c) to interrupt the display immediately when flow-control is on. See the «defflow» .screenrc command for details. The use of this option is discouraged. |
| -l and -ln | turns login mode on or off (for /var/run/utmp updating). This can also be defined through the «deflogin» .screenrc command. |
| -ls and -list | does not start screen, but prints a list of pid.tty.host strings and creation timestamps identifying your screen sessions. Sessions marked ‘detached‘ can be resumed with «screen -r«. Those marked ‘attached‘ are running and have a controlling terminal. If the session runs in multiuser mode, it is marked ‘multi‘. Sessions marked as ‘unreachable‘ either live on a different host or are ‘dead‘. An unreachable session is considered dead, when its name matches either the name of the local host, or the specified parameter, if any. See the -r flag for a description how to construct matches. Sessions marked as ‘dead‘ should be thoroughly checked and removed. Ask your system administrator if you are not sure. Remove sessions with the -wipe option. |
| -L | tells screen to turn on automatic output logging for the windows. |
| -m | causes screen to ignore the $STY environment variable. With «screen -m» creation of a new session is enforced, regardless whether screen is called from within another screen session or not. This flag has a special meaning in connection with the ‘-d‘ option: |
| -d -m | Start screen in «detached» mode. This creates a new session but doesn’t attach to it. This is useful for system startup scripts. |
| -D -m | This also starts screen in «detached» mode, but doesn’t fork a new process. The command exits if the session terminates. |
| -O | selects a more optimal output mode for your terminal rather than true VT100 emulation (only affects auto-margin terminals without ‘LP’). This can also be set in your .screenrc by specifying ‘OP’ in a «termcap» command. |
| -p number_or_name | Preselect a window. This is useful when you want to reattach to a specific window or you want to send a command via the «-X» option to a specific window. As with screen‘s select command, «—» selects the blank window. As a special case for reattach, «=» brings up the windowlist on the blank window. |
| -q | Suppress printing of error messages. In combination with «-ls» the exit value is as follows: 9 indicates a directory without sessions. 10 indicates a directory with running but not attachable sessions. 11 (or more) indicates 1 (or more) usable sessions. In combination with «-r» the exit value is as follows: 10 indicates that there is no session to resume. 12 (or more) indicates that there are 2 (or more) sessions to resume and should specify which one to choose. In all other cases «-q» has no effect. |
| -r [pid.tty.host] |
Getting Started
Before you begin to use screen, you need to make sure you have correctly selected your terminal type, as you would for any other termcap/terminfo program. You can do this using tset for example.
If you’re impatient and want to get started, remember this one command: «C-a ?«. Typing these two characters display a list of the available screen commands and their bindings. Each keystroke is discussed in the Default Key Bindings section. The Customization section deals with the contents of your .screenrc.
If your terminal is a «true» auto-margin terminal (it doesn’t allow the last position on the screen to be updated without scrolling the screen) consider using a version of your terminal’s termcap with automatic margins turned off. This ensures an accurate and optimal update of the screen in all circumstances. Most terminals have «magic» margins (automatic margins plus usable last column). This is the VT100 style type and perfectly suited for screen. If all you’ve got is a «true» auto-margin terminal, screen is content to use it, but updating a character put into the last position on the screen may not be possible until the screen scrolls or the character is moved into a safe position in some other way. This delay can be shortened using a terminal with insert-character capability.
Default key bindings
As mentioned, each screen command consists of a «C-a» followed by one other character. For your convenience, all commands that are bound to lowercase letters are also bound to their control character counterparts (with the exception of «C-a a«; see below), thus, «C-a c» and «C-a C-c» can create a window. See the Customization section for a description of the command.