It all depends the job for the specific experiences the hiring mangers want to see. The #1 thing I believe they will looking for is something that does not need to have a GUI to be useful. If you a simple user, it means you are comfortable in a text based, shell environment and are at least familiar with, if not proficient with the UNIX tools such as, ed, vi or emacs, grep, tail, head, sed, awk, cut, join, tr, etc. You should be able to use one or more of the Bourne Shell/Korn Shell/Bash family or CShell. You should be familiar with the UNIX file tree and basic layout and its protection scheme. It helps if you understand the differences between BSD, SysV, Mac OSx, and Linux layout; but for many jobs in the UNIX community that may not be required. You should also understand how to use the Unix man command to get information on the tools you are using — i.e. you should have read, if not own a copy of Kernighan and Pike The Unix Programming Environment (Prentice-Hall Software Series): Brian W. Kernighan, Rob Pike: 9780139376818: Amazon.com: Books and be proficient in the first four chapters. If the job requires you writing scripts to be able to write Shell script (i.e. program the shell) using the Bourne Shell syntax i.e. Chapter 5 (Shell Programming).

If you are a programmer, then you need to be used to using the UNIX/Linux toolchains and probably not require an IDE - again as a programmer knowledge if not our proficiency of at least source code control system from SCCS, RCS, CVS, SVN, Mercurial, git or the like needed. Kernighan and Pike’s Chapter’s 6–8 should be common knowledge. But to be honest, you also should be familiar with the contents contained within it, if not own and keep a copy of the Rich Steven’s Advanced Programming in the UNIX Environment, 3rd Edition: W. Richard Stevens, Stephen A. Rago: 9780321637734: Amazon.com: Books (aka APUE) on your desk.

If you are system administrator, then the required set of tools get much larger and besides the different way to “generate” (build) a system is a good idea; but less tools for user maintenance. In this case you should be familiar with, again if not own have a copy on your desk of Evi Nemeth’s Amazon.com: UNIX and Linux System Administration Handbook, 4th Edition (8580001058917): Evi Nemeth, Garth Snyder, Trent R. Hein, Ben Whaley: Books - which is and has been one of if not the best UNIX admin work for many, many years.

Updated 05/07/18: to explain I am not shilling for anyone. I am trying to honestly answer the question and make helpful recommendations of how to learn what the person asked to help them be better equipped to be employed in the Unix world. I used Amazon’s URL’s because they are global and easy to use as a reference. But I am not suggesting you purchase from them. In fact, if you can borrow a copy from you library to start, that might be a good idea.