Home arrow Help Docs arrow SCREEN Tutorial
Main Menu
Home
Help Docs
Newbies
Corporates
Gnu/Linux Distros
Programming
Databases
Links
Search
Regular
News
Interviews
Aaj Ka Tip
Aaj Ka Command
Community
Blogs
Wiki
Forums
Services
Free Software

SCREEN Tutorial Print E-mail
SCREEN Tutorial

While tabbed terminals might be a regular feature in GUI based terminals like Gnome Terminal and KDE Konsole, it is not a possibility in text mode console. Screen helps us overcome this backdraw and also offers many advanced features. Screen has a sharp learning curve, but once learnt it can optimise your time and effort like never before. Screen doesnt come default installed with many distros, but installing it is as easy as typing in "apt-get install screen" in debian or "yum install screen" in Fedora/Red Hat. You can also build from source. The applications of Screen include:
1. Create new full screen windows
2. Kill existing windows
3. View a list of windows
4. Turn output logging on/off
5. copy and paste text between virtual terminals
6. View scroll back history
7. Switch between windows and much much more

Notations Used:
 C-x   -  The 'C' character in the notation is not the C alphabet but the Ctrl key, which is abbreviated as C (in the true spirit of an emacs fan). C-a A  -  The Alphabet following the Ctrl+a is 'A' and not 'a' i.e. The commands are all case sensitive. For instance C-a A is a different command from C-a a

Italilisation - All the commands for screen and associated commands are italised for easy viewing. e.g Ctrl+a is italised as C-a

Command Line arguments:

The manual page define the screen options as:
                            screen [ -options ] [ cmd [ args ] ]
                            screen -r [[pid.]tty[.host]]


To run screen all you have to do is have screen installed (of course!!), and then run "screen cmd args", Where cmd and args the command name and its respective arguments. for e.g,  
                                screen  
emacs  prog.c
will open a screen terminal from inside the same window with emacs editor opening the prog.c file. The command line options to screen are many, we will look at some of the most used options here. The '-r' option for screen stands for "resume operation" and will be covered shortly.

Command Line option Meaning
-c FILE override default config file with specified file
-d (or) -D [pid.tty.host] detaches an elsewhere running screen session
-e xy changes control character to 'x' and literal character to 'y'. Default is 'C-a' and 'C-a a' for literal. This will be changed to 'C-x' and 'C-x y' respectively.
-h num Specifies the history scrollback buffer length
-ls (or) -list [MATCH] specifies all the detached screen sessions in the pid.tty.host format
-r [pid.tty.host] -r sessionowner/[pid.tty.host] This resumes a detached screen process. The pid.tty.host is optional and can be specified to choose if more than one screen process is running. All the screen sessions can be obtained by the '-ls'  option
-R Resumes first detached screen
This is a small compilation of the command line arguments that you will probably use. Many more are available, but the usage of which will be very limited. The two most important commands are:

 -r  --- Resume session

 C-a d --- Detach session

Default Key Bindings:

[This list is not to be memorised but to know that the options exist]
Key Binding Action
C-a ' Prompt for window number
C-a " Presents Window list
C-a 0....9 Switches between windows
C-a Switches input focus
C-a C-a Move to last window
C-a a sends literal Ctrl+a
C-a A Allows change of title
C-a c C-a C-c Creates new window session within the present screen session
C-a C clears screen
C-a d C-a C-d Detaches screen session
C-a C-g Toggles visual bell
C-a H Start logging to screenlog.N, where N is the window number
C-a h sends hardcopy of screen to hardcopy.N where N is the window number. (something like a screenshot)
C-a i C-a C-i Shows information about the present screen
C-a C-a n C-a C-n Toggles to next window
C-a C-a p C-a C-p Toggle previous window
C-a S Split region into 2 windows
C-a w C-a C-w Shows a lit of windows
C-a x C-a C-x Locks window
C-a z Suspend screen
C-a C-\ Terminate all screens
C-a : Enter a command line
C-a { C-a } view history of commands
C-a * shows list of attached displays

This is a rather huge set of commands (although not exhaustive). So the simplest way to view a command even if you forget is to use the command "C-a ?" or view "man screen" or "info screen".

Screen Commands:
All these commands for screen have to be entered after using the "C-a :" command in screen. They can also be inputted into the .screenrc file, which will be present in the home folder of the user. If it is inputted into the .screenrc configuration file, those commands will be executed by default each time screen is started.

[This list is not to be memorised but to know that the options exist]
Command Description
autodetach STATE Automatically detach the session on exit
bind KEY [COMMAND [ARGS]] Bind a command to a key
bindkey [OPTS] [STRING [CMD ARGS]] Bind a string to a series of keystrokes
chdir [DIRECTORY] Change the current directory for future windows
clear Clear the window screen
colon Enter a `screen' command
detach [-h] Disconnect `screen' from the terminal
displays List currently active user interfaces
echo [-n] MESSAGE Display a message on startup
escape XY Set the command and `meta' characters
hardcopy [-h] [FILE] Write out the contents of the current window
help Display current key bindings
kill Destroy the current window
log [STATE] Log all output in the current window
only Kill all other regions
other Switch to the window you were in last
password [CRYPTED_PW] Set reattach password
quit Kill all windows and exit
sessionname [NAME] Name this session
setenv [VAR [STRING]] Set an environment variable for new windows
split Split region into two parts
su [USERNAME [PASSWORD [PASSWORD2]]] log in as username
time [STRING] display time
vbell [STATE] Use visual bell
windows List active windows

OK Until now was the sharp learning curve that I mentioned. It was an orgy of commands, but you would not need to know all of this. Just read through this to know that all these commands exist. The complete information is provided in the manual pages. For the actual usage of screen read on...

Using Screen
This section will describe some of the most used commands and it's applications in detail.

First and foremost, you will be working with different virtual terminals, so how do you know which window you are working on? well Screen defines the environmental variable 'WINDOW'. so a simple `echo $WINDOW` will tell you which window you are working on.

Another useful feature of screen is the ability to title windows. You can set the title of a window by using the command 'C-a A' and can be verified using the command 'C-a w'

To create a new window just type in 'C-a c' and to move between windows you can use 'C-a num' where num is the window number (obtained from $WINDOW). To move between named windows, you can use the command 'C-a '' to name the session you want to move to.

Screen has the useful ability to define user defined Key bindings. To create a key binding, you have to edit the .screenrc file. In the file, add the following commands:

bind  

e.g. bind  o title

What this does is that, the command key 'C-a o' will cause the title for the window to be asked. Where as, had we given the command as

bind  o title  "New Title"

this would modify the title of the window as "New Title".

The most useful feature of screen is its ability to be detached and reattached as and when required. What this means is that you can work on a terminal within screen, suspend it and then come back to it at a later stage exactly as you left it. To do this automatically you can set the command 'autodetach on' in .screenrc or at the prompt (C-a :). If you want to do this manually, just use the commands 'C-a d' or 'C-a C-d'. Now to resume the detached session, i.e. to reattach the session, you have to run the command 'screen -r' at the prompt. You can have more than one detached session. To view a list of all the detached sessions, type in 'screen -ls' at the prompt.

Screen also provides the same lock feature as Window managers provide, i.e. all activity will be locked unless a password is specified. To set the password use the command 'password' at the prompt (C-a :) or set the line 'password ' in your .screenrc file. Now that you have the password, you can lock the screen by using the command 'lockscreen'. It is that simple. You can also use the key bindings 'C-a x' or 'C-a C-x' for doing the same. The screenlock program is based on PAM and can be changes using the environmental variable $LOCKPRG

You can name the sessions too as would be named when you do 'screen -ls'. You can suspend the activity of screen by using the key binding 'C-a z' or 'C-a C-z'.

Some of the useful environmental variables used by screen are PATH - locating program to run (similar to the terminal's PATH), LOCKPRG - to locate the locking program of screen, SCREENDIR - used to specify alternate socket path (where the detached sessions are saved), SCREENRC - to specify an alternate configuration file apart from the usual .screenrc file in the user's home folder, WINDOW - used to specify the current window's number.

A really useful application of screen is when you ssh or telnet into a remote machine. All you have with you is the friendly CLI (command line interface). In such a case, screen comes in very handy, making wonderful use of bandwidth, resources and time. Using screen, you can avoid making multiple connections to the server. All you need is that screen be installed on the server machine.

Another friendly application of screen is in Instant Messaging. In the case of IRC for instance, you can use a CLI based IRC client such as irssi, and log in from a remote server. Suppose you want to go out, and log out of your machine, you can set the status to away in the client, and then detach the screen session and log out from the remote machine. The screen session continues to run on the remote server. So the next time you log in, you can resume the screen session and see all the activity that took place as history, thereby losing no data in spite of your lack of presence.

If you are used to emacs and love the splitting of windows. You can use that in screen too. All you have to do is 'C-a S' and there you have it 2 windows in the same window. You can switch focus between these windows using 'C-a and of course you can split the splitted windows into even more windows.

Screen is a powerful CLI tool. Starting to use CLI based tools might be initially hard. But the power of CLI can be realised only upon usage. It has many advanced options, though only the most useful ones have been explained here. All information can be obtained from the manual pages.

Prashanth Mohan

prashmohan [at] gmail [dot] com


< Previous   Next >



gnulinuxclub
Join Us
About Us
Contact Us
Support Us
Acknowlegdement
Login(only for dev.)

Royalty Free Images

O'Reilly User Group discount!




Powered By GIMP GIMP
Contact Webmaster Copyleft 2011 gnulinuxclub.org