TL;DR

Where we talk about ssh, a shared account in the lab and how to still get some personalization.

Sometimes I log in a lab host through a shared account. I know it’s not the best practice in the world, but at the end of the day it’s indeed a lab host, where speed of solving problems takes precedence over tight security and rigid processes.

I’m not young any more, and I had my first ventures with the command line using DOS. So, for example, I use dir to view the list of files in a directory, and I like to get all the details, like ls -l does. Which brings me to type this over and over:

$ alias dir='ls -l'

In my hosts, this is called from ~/.bashrc so I don’t need to type it every time. When logging in a shared account, though, I don’t have it and I also don’t want to clutter everyone’s space with this alias.

My first step is to put a function like the following in ~/.bashrc:

polettix() {
   alias dir='ls -l'
   # other personal customizations...
}

I use a function name that would not clash with something meaningful and it’s clearly tied to me. This means that I can call the function after logging in, and get all my customizations in one shot.

But.

Why should I type this command every time? My next mental stop is to have the system do this for me. My initial thought was to set an environment variable and use it in ~/.bashrc, like this:

if [ -n "$IS_POLETTIX" ] ; then
   alias dir='ls -l'
   # ...
fi

Alas, this does not work as expected in the general case, because sshd (in OpenSSH, at least) does not allow (by default) to carry whatever environment variable you set, for security reasons. Bummer!

There is still some hope anyway. A few environment variables related to the locale are usually let through, so… why not leverage one of them?!? These variables begin with LC_, here’s a list I got in my system:

LC_CTYPE
LC_NUMERIC
LC_TIME
LC_COLLATE
LC_MONETARY
LC_MESSAGES
LC_PAPER
LC_NAME
LC_ADDRESS
LC_TELEPHONE
LC_MEASUREMENT
LC_IDENTIFICATION
LC_ALL

I guess most of them aren’t really used if not by some program I don’t have, and I’m particularly intrigued by LC_IDENTIFICATION because it resonates so well with what I want to do!

Don’t get me wrong: I know that this variable is meant for other, and using it would be abusing it. It just happens that this variable is not used by any program I know of, and I’m willing to take the risk.

So here’s how the thing is modified:

if [ "$LC_IDENTIFICATION" = 'polettix' ] ; then
   alias dir='ls -l'
   # ...
fi

Now that this should (normally) get through, we can set things up client-side. On reasonably recent versions of OpenSSH (the client), the configuration file supports the SetEnv variable, so the configuration would look something like this:

Host foobar
   HostName foobar.example.com
   IdentityFile ~/.ssh/id_rsa-foobar
   SetEnv LC_IDENTIFICATION=polettix

If you have an older version, the trick is to use SendEnv instead, but at this point you should set the LC_IDENTIFICATION variable in your shell:

Host foobar
   HostName foobar.example.com
   IdentityFile ~/.ssh/id_rsa-foobar
   SendEnv LC_IDENTIFICATION

which is a but of a bummer if you want to set it and forget it because it will be one value only. It’s life.