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Personalizing shell accounts that are used by multiple users

December 19th, 2011

Problem description

Although it is generally discouraged to share a shell account with multiple users, sometimes its necessary. When I recently had to share a login I wanted to recreate my personalized environment, but then again not mess with other peoples environment. Also I wanted to preferably create something they could use for themselves without messing in turn my environment up (for example by configuring emacs as the default text editor). I wanted to have my own .bash_history file, load my own .vimrc etc. I used Debian 6.0 with current ssh/sshd versions.

Overview

What I did was exporting an LC_* environment variable on the client machine; those get forwarded by the ssh client to the server. On the server I would check in the .bashrc file whether the variable was set, if it was set I would load the custom configuration files. Any of the users who share the account are then able to configure the server environment for themselves.

Implementation

I added this line to ~/.bashrc

.....
export LC_USER=michael
.....

Don’t forget to source the .bashrc file after it was edited, otherwise it will have no effect in the active session, this can be done with

$ source ~/.bashrc

You should have SendEnv set to forward the LC_* variables (this is usually enabled). This can be done in either the global ssh config /etc/ssh/ssh_config or the user ssh config: ~/.ssh/config

....
SendEnv LANG LC_*
....

On the server where you want to connect to, the file /etc/ssh/sshd_config should have the following option enabled (this is often enabled by default)

....
AcceptEnv LANG LC_*
....

To verify, ssh into the target machine and try this, the result should be similar. If you get no response check whether the variable was even exported locally (use the same command).

$ env | grep LC_
LC_USER=michael

Once this is done, upload your personalized .bashrc file to the remote server into ~/.yourusername/.bashrc .

Finally edit the ~/.bashrc file on the server so it sources the new custom file:


if [ `expr match "$LC_USER" '^[a-zA-Z0-9]\+$'` -gt 0 ] ;
then
echo "Welcome ${LC_USER}"
source ~/.${LC_USER}/.bashrc
fi

To keep a personal .bash_history and .vimrc file, put the following into the personal .bashrc file (don’t forget to upload your .vimrc file into ~/.${LC_USER}).

HISTFILE=./.bash_history
alias vim='vim -u ~/.${LC_USER}/.vimrc'

Conclusion

This proved to be a noninvasive and reliable way to share accounts with custom configurations. Other ways to solve this problem might have been the usage of IP addresses. I looked into this first, but then decided that its not very good since connecting through intermediate machines would render discrimination of users impossible.

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