tsh (tiny shell)

tsh is a tiny shell for Unix systems. tsh is written in Python. It's use is to give users very restricted access to a system. One can create a user which has tsh as his/her login shell.

This small shell might serve as an example of how to write a more elaborate shell in Python. In practice, this shell is not really very useful since the Unix system itself can provides good security. Also note that most systems provide a restricted bash rbash.

Installation

Requirement: python 2.5

Unpack the tarball, and just copy the executable tsh into /usr/local/bin (or wherever you which to have it).

Description

tsh has the only the following buildtin commands:

alias [name='value']
unalias name
cd [path]
exit
show_commands

They work in the way one would expect them to work in bash, except the command show_commands, e.g.:

/home/joe tsh> show_commands
    alias           builtin
    cat             OS
    cd              builtin
    exit            builtin
    l               aliased 'ls -l'
    ls              OS, aliased 'ls --color'
    show_commands   builtin
    unalias         builtin

The only commands imported from the OS are specified in the file /etc/tsh.conf. In the example above, only cat and ls are the only non-builtin commands, the user is allowed to execute. It is important not to include commands line nice or time into this list, since they would leave a backdoor for all other commands, e.g.

/home/joe tsh> nice rm -rf *

or even:

/home/joe tsh> time bash

Also, tsh does not allow multiple commands to be executed in a call, e.g.

/home/joe tsh> ls ; rm -rf *
/home/joe tsh> ls || rm -rf *
...

does not work, because tsh will interpret all a words (separated with whitespace, after the command itself as arguments to that command. This means that tsh does not know anything about, pipes, IO redirecion, combining commands and quoting.

Files

• /etc/tsh.conf contains the list of commands allowed for execution. See the example file tsh.conf.

• ~/.tshrc run-control file. I have included an example Note the example file tshrc.

• ~/.tsh-history history file.

Download

Source: tsh.tar.gz (current version is 0.02)

Platform independent. (I've tested it on Linux and Mac OS X, but it should run an any Unix system where Python runs.)

Older versions: tsh-0.01.tar.gz