C++ is not specific to GNU/Linux. About Shell scripting, you may have to
know what is POSIX and what is not (the "Bashisms").
But, as albertoefg wrote, the exam will certainly focus on the working of an
operating systems in general, on system administration (including the basic
configuration files: /etc/fstab, /etc/hostname, /etc/hosts, /etc/group,
/etc/shadow, etc.) ... maybe server administration but that would be quite
unfair since you do not know what kind of servers (Web?, Mail?, etc.) and
what server in particular (Apache or NGINX?, etc.).
Things definitely missing from albertoefg's list are TCP/IP networks and
commands relating to networking ('ping', 'tracepath', 'ftp', 'wget', etc.)
and POSIX commands for file management ('ls', 'cd', 'mv', 'rm', etc.) and
text processing ('tr', 'cut', 'sort', 'grep', etc.) and their main options.
There may be something on 'sed' (but probably only the 's' command) and/or
AWK because they are very useful and part of POSIX too.