Month: August 2012

Killing Me Softly

Not sure how it was possible that I have not done this before but I recently realized I needed to forcibly remove a user from a login session on a remote Linux system and didn’t have a better idea than simply killing off all their individual system processes one at a time.  Thankfully, Linux provides a much more useful way of dealing with kicking users from terminal sessions (and thereby shutting down their entire process tree as well.)

$who -u

Will give you a list of user sessions based on which terminal they are logged into.  This includes X sessions, virtual terminals, remote sessions, and any text mode logins.  The output should looks something like this:

bobby    :0              2011-04-21 20:01   ?            12122
bobby    pts/0        2011-04-21 20:01   .             12405 (:0)
bobby    pts/1        2011-04-21 20:01 02:10       12322 (:0)
root        pts/2       2011-04-21 22:19 .                13887 (10.0.0.101)

You can then kill the session login by looking at the last column and killing that process ID.  In the example above you can see there are two virtual terminals (i.e. the pts/X sessions), a remote session (the ssh session I am remotely accessing the machine on from host 10.0.0.101), and a single local login on terminal session :0.  Because the terminal session must be responsible for starting the virtual terminals, you can simply kill the process 12122 force a logout of all three sessions.

$kill 12122

Entirely too easy.  If you would like to be kind (I am NOT) and actually warn your users that you are bout to kick them off, you can send them a system message using the standard Unix wall command.  If you type wall you will get an open text area to type your message (end the message by clicking Ctrl+d) or you can pipe a message to standard input like so:

$echo “My name is Inigo Montoya. You killed my father. Prepare to die.” |wall

Wall will send a system message to every terminal session that allows messages (if you are root, that means everybody.)

scientia potentia est

What most people are objecting to is that the market gives people what the people want instead of what the person talking thinks the people aught to want.
–Milton Friedman

Milton Friedman is easily the most influential economist since John Maynard Keynes. What makes him such a powerful voice for the free market is his ability to distill complex macro economic theory into chunks non-economists can easily understand.  He is so influential, and understandable, that PBS actually produced a series with him explaining economic ideas and debating these thoughts with other prominent scholars, politicians, and businessmen. The series was called “Free To Choose.”

Unfortunately most of us don’t have hours worth of time to watch all the episodes (although you should.)  To get a quick overview of each of his core concepts Trent Liberty has produced a series of 7 videos called The Friedman Series.  The background audio ranges from amazing to annoying, but the topic selection is outstanding.  If you get nothing else from the video, always remember that the biggest danger to liberty isn’t inequality, but the sincerity of the well intentioned.

To Any Place Worth Going

One of the best parts of Unix systems is that fundamentally they are built as development platforms.  The most common text command interface for Unix is call Bash (the Bourne Again Shell)and it is a full blown script-able interface allowing direct interaction with command line programs and giving the user the ability to string together these programs into really powerful applications.  Because of the power of this interface, developers have over many years improved the ability to use it directly as well.  Things like <tab> completion are well known, but how about reverse command searches, built-in text editor mode, and shortcuts galore.  I have been trying to use more and more of this “built-in” bash functionality and so below are some of my favorite shortcuts and functionality.

Shortcuts:

Ctrl + A Go to the beginning of the line you are currently typing on
Ctrl + E Go to the end of the line you are currently typing on
Ctrl + L Clears the Screen, similar to the clear command
Ctrl + U Clears the line before the cursor position. If you are at the end of the line, clears the entire line.  Especially useful when you know you’ve mis-typed a password and want to start again.
Ctrl + K Cut the line after the cursor, inverse of the Ctrl + U
Ctrl + Y Pastes the content from a previous Ctrl + K or Ctrl + U cut.
Ctrl + H Same as backspace
Ctrl + R Search through previously used commands
Ctrl + C Sends SIGINT to whatever you are running (effectively terminating the program.)
Ctrl + D Exit the current shell
Ctrl + Z Puts whatever you are running into a suspended background process. fg restores it.
Ctrl + W Delete the word before the cursor
Ctrl + T Swap the last two characters before the cursor
Alt + T Swap the last two words before the cursor
Alt + F Move cursor forward one word on the current line
Alt + B Move cursor backward one word on the current line
Tab Auto-complete files and folder names (f there is a multiple option match hitting Tab twice will list all possible values.)
Alt + . Paste the previous commands final argument (great for running different commands on the same file path.)

To see a complete list of all bound bash shortcuts you can type

bind -P |less

but you may need to look-up some bash hex character values to understand all of them.  What is more you can actually set bound shortcuts to almost anything you can think of, including actual applications, for example:

$ bind -x ˜\C-e˜:firefox.

will launch the Firefox web browser from the command line when you hit Ctrl + e.

Another one of my favorite commands is fc (fix command.) If you simply type

fc

FC will copy your most recent bash history into your preferred editor (vi by default on most systems) and allow you to edit it within the editor.  If you save and exit the editor it will automatically copy it the contents into the bash session and hit enter.  Additionally if you are interested in editing some other history item you can type

fc -l

to get a full history with numbers beside them.  Then type

fc <num>

where<num> is the history number you want to edit.  In a former life my bash terminal and fc was all I needed for most SQL testing.