Just a few off the top of my head
rm - removes a file or directory (directory must be empty)
mv - moves (renames) a file or directory
cp - copies a file
mkdir - creates a directory
cd - changes to a directory
pwd - shows the current directory
find - finds files in a directory tree
ls - lists contents of a directory
grep - finds text in a file or input stream
head - displays top of a file
tail - displays end of a file
wc - counts words, lines, characters in a file
ps - lists current processes
date - displays dates (current, or in relation to another date)
pg, more, less - displays files one page at a time
sort - sorts an input stream
cut - extracts columns from an input stream
who - shows currently logged in users, but can also (-b) display the last time the system was started
It is very important to understand the piping mechanism, which allows to direct the output of one command to the input of another, e.g.
ls | grep x
displays all directory entries having an x in their name, or
sort < a > b
reads file a, and writes the sorted output to file b.
Also learn the syntax of the shell (most often bash), so you can write scripts with conditions and loops.
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