Escaping from shell-scripts

Chris Brennan xaero at
Thu Nov 18 16:27:34 UTC 2010

 On Thu, Nov 18, 2010 at 8:52 AM, Julian Fagir <gnrp at>wrote:

> Hi,
> I'm planning a service with a login-user-interface. Thus, I want to
> restrict
> the user somehow to this script and to do nothing else.
> The straight-forward way would be to write this script, have all input
> parsed
> by read and then let the script act according to this input (let's assume
> that these tools are secure, it's just cp'ing and writing to
> non-sensitive files.
> Are there possibilities to escape from such a script down to a prompt?
> On the other hand, if I would take python for this, so a python-script is
> executed, are there ways to get to a generic python-prompt?
> The restriction to that script would be done by either setting the
> login-shell to that script, setting the ssh-command for that account/key
> (and
> ensuring that it can't be altered), or both.
> All in all, this is a more general question I have for quite a time: Can
> you
> use shell-scripts for security-relevant environments? Does an attacker have
> the possibility to escape from a script down to a prompt?
> I'm not that into shell-programming and there are too many legacies about
> terminals (some time ago, I had to cope with termcap...) and shells which
> one
> just can't all know.
> E.g., it was just a few days ago I found out what a terminal-stop means and
> that it is still interpreted by screen, though using it for several years
> now.
> Regards, Julian

While I cannot answer your question fully, you could use python as the
user's shell and call the python script in question by passing the script to
the parser as login, how specifically that is done, I am not sure. I've
never used py as a login shell but I am fairly certain it can be done....

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