Friday, November 14, 2014

Taking a screen test.

Starting Linux Screen

Screen is started from the command line just like any other command:

[root@office ~]# screen

Note: you do not have to be root to use it and good security suggests you only use root as a last resort.

You are now inside of a window within screen. This functions just like a normal shell except for a few special characters.
Control Command

Command: “Ctrl-a”

Screen uses the command “Ctrl-a” that’s the control key and a lowercase “a” as a signal to send commands to screen instead of the shell.

For example, “Ctrl-a” then “?”.

Key bindings are the commands the screen accepts after you hit “Ctrl-a”. You can reconfigure these keys to your liking using a .screenrc file, but I just use the defaults.
Creating Windows

Command: “Ctrl-a” “c”.

To create a new window, you just use “Ctrl-a” “c”.

This will create a new window for you with your default prompt. Your old window is still active.

For example, I can be running top and then open a new window to do other things. Top stays running! It is still there. To try this for yourself, start up screen and then run top.

Now open a new window with: “Ctrl-a” “c”

Your top window is still running you just have to switch back to it.
Switching Between Windows

Command: “Ctrl-a” “n”

Screen allows you to move forward and back. In the example above, you could use “Ctrl-a “n” to get back to top. This command switches you to the next window.

The windows work like a carousel and will loop back around to your first window.

You can create several windows and toggle through them with “Ctrl-a” “n” for the next window or “Ctrl-a” “p” for the previous window.

Each process will keep running until you kill that window.
Detaching From Screen

Command: “Ctrl-a” “d”

Detaching is the most powerful part of screen. Screen allows you to detach from a window and reattach later. If your network connection fails, screen will automatically detach your session! Note this is great when you have run screen on another system and you have to get away or to shut your system down without bothering the system you were attached to.

You can detach from the window using “Ctrl-a” “d”.

This will drop you into your shell.

All screen windows are still there and you can re-attach to them later.

This is great when you are using rsync for server migration.

Reattach to Screen

If your connection drops or you have detached from a screen, you can re-attach by just running:

$ screen -r

This will re-attach to your screen.

However, if you have multiple screens you may get this:

If you get this, just specify the screen you want.

$ screen -r   27153.pts-11.oelt02

Logging Your Screen Output

As a consultant, I find it important to keep track of what I do to someone’s server. Fortunately, screen makes this easy.

Using “Ctrl-a” “H”, creates a running log of the session.

Screen will keep appending data to the file through multiple sessions. Using the log function is very useful for capturing what you have done, especially if you are making a lot of changes. If something goes awry, you can look back through your logs.

Getting Alerts

Screen can monitor a window for activity or inactivity. This is great if you are downloading large files, compiling, or waiting for output.

If you are waiting for output from a long running program, you can use “Ctrl-a” “M” to look for activity. Screen will then flash an alert at the bottom of the page when output is registered on that screen.

I use this when running a command that takes a long time to return data. I can just fire up the command, switch to another window and not have to keep switching back to check the status.

You can also monitor for inactivity. Why use this?

If you are downloading a large file or compiling a program, you can be notified when there is no more output. This is a great signal to when that job is done. To monitor for silence or no output use “Ctrl-A” “_”.
Locking Your Screen Session

If you need to step away from your computer for a minute, you can lock your screen session using “Ctrl-a” “x”. This will require a password to access the session again.

Stopping Screen

When you are done with your work, I recommend you stop the session instead of saving it for later. To stop screen you can usually just type exit from your shell. This will close that screen window. You have to close all screen windows to terminate the session.

You should get a message about screen being terminated once you close all windows. [screen is terminating]

Alternatively, you can use “Ctrl-a” “k”. You should get a message if you want to kill the screen.

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