== A Linux _su_ surprise

I found out about Linux _su_'s '_-p_' option from comments on a a
[[previous entry RedHatPATHAnnoyance]]. The option is also known as _-m_
and _--preserve-environment_; the manpage describes it tersely as 'do
not reset environment variables', but the info document tells the full
story:

> Do not change the environment variables `HOME', `USER', `LOGNAME',
> or `SHELL'. Run the shell given in the environment variable
> `SHELL' instead of the shell from USER's passwd entry, unless
> the user running `su' is not the superuser and USER's shell is
> restricted. A "restricted shell" is one that is not listed in the file
> `/etc/shells', or in a compiled-in list if that file does not exist.
> Parts of what this option does can be overridden by `--login' and
> `--shell'.

I'm used to old-fashioned _su_'s that *always* run the target user's
login shell, no matter what. Being able to override the login shell is a
bit alarming; although there are safeguards, it does make what you put
in _/etc/shells_ somewhat more dangerous. (Clearly you never want to put
_/bin/false_ or the like in there, in order to get something else to
work right.)

At the same time, being able to run alternate shells for 'restricted'
accounts as root is a useful trick (whether done through _-p_ or
directly through _--shell_). I'll have to remember it for the next
time I want to fiddle with an environment owned by such an account; it
certainly beats trying to do it as root and then _chown_'ing the setup
all over.