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Wed, 01 Sep 2010

/Admin/commandLine: Screen: An Easy Way to Open Multiple Terminals on a Linux Server

I have been hearing buzz about the "screen"[1] utility for some time, and having tried it, I am now a serious devotee. Wish I had looked at it earlier....

Screen basically gives access and control of multiple terminal sessions in one open SSH session to a server. Ie. instead of using SSH to login to a server from multiple terminals, just use one terminal and invoke "screen" once logged in.

Screen becomes useful with only a very short list of keystrokes:

Ctrl-a c - create a new "screen"
Ctrl-a n - cycle to the next "screen"
Ctrl-a p - cycle to the previous "screen"
Ctrl-a d - detach from screen program, which continues to run in background
Ctrl-a [ - permit scrolling through the terminal's history buffer
Ctrl-c - exit scroll mode and return to command mode

If you only have two or three "screens" open, that is about all you really need.

Killer Feature #1: No More Clobbered SSH Sessions

If you detach from screen and logout, or are forcibly detached by an interrupted SSH session, screen continues to run on the server in the background. That means, for instance, that you can allow a big file transfer to continue on the server without needing an open SSH session. Or, in the case of a network problem resulting in an interrupted SSH session, just log back into the server, and enter:

screen -r -d

to be returned to all your screen session, with all your open terminals, EXACTLY THE WAY YOU LEFT THEM. No more having to navigate back to where you were before. Absolutely indispensable in the crappy excuse for a network we have here in China.

Killer Feature #2: Terminal Sharing!!

Sharing your terminal (both input and output) is simple with screen, as long as both people can SSH into the server and su to the same user:

First one person starts screen, then the other person su's to the same username on the server and attaches to the other screen session as follows:

screen -x

And now both people are looking at exactly the same screen / terminal session, and both can type into that terminal. Sometimes when the second person tries to attach, there is an error message like the following:

Cannot open your terminal '/dev/pts/3' - please check.

To fix, just chmod the file /dev/pts/3 to make permissions more permissive.

[1] http://savannah.gnu.org/projects/screen

posted at: 00:09 | path: /Admin/commandLine | permanent link to this entry

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Wed, 01 Sep 2010