* understand the basic directory structure (/, /boot/, /var, /usr, /opt, /etc)
* file management (commands: tail, head, cat, awk, sed)
* process management (commands: ps, top, kill)
* package management (yum, dpkg)
* one configuration manager (puppet, chef, ansible)
* basics of tcp/ip
network management (ifconfig/ip, netstata/ss, ping, trace)
addiontal commands if you want to be more than a junior (strace, lsof, iostat, vmstat)
I'd like to point out that some sysadmins are focused on linux internals while others focus on application in production. So, of course, the list may vary according to the position you're looking to be hired.