Patricia Mindanao wrote:
> Assume a Linux system where I have ONLY command line access (e.g. through telnet).

If you are using telnet for remote administration; I suggest you use
ssh instead. To determine whether or not ssh is installed try using
the which command (e.g. which ssh). Using ssh is far more secure than
the plain text sent via telnet.

> How can I find out which Linux version is currently installed (Linux, RedHat, Ubuntu, Debian,....)
> and which release number?

The uname -a command helps with kernel version info. Find what kind of
package management is available to you(if any at all). Check for
apt-get or rpm with the which command. If apt is installed then you
have a Debian system or some derivative.

With a Deb-system you can use dpkg -l ( I recommend dpkg -l | more) to
list everything installed on the system(if it is an ubuntu system you
will see ubuntu in many of the packages versions.

Also, on systems with rpm try rpm -qi package name

> How can I find out (from the command line as well) which eMail client/server software
> and other packages are currently used and installed ?

Definitely want to use the which command here too. However, if you're
not sure about names try using grep. (ls /bin /usr/bin | grep
'mail')--Or any words, wildcards etc. If it is installed and used it is
likely in /bin or /usr/bin. The ps -aux command may also come in handy
too.

There is more than one way to do this, but I'm not entirely sure how to
gather all the specifics you're seeking. I usually already know the
flavor and version of gnu/linux that I am running because I am the one
that installed it.