what directory am i currently in?
>$ pwd

list all files in your current directory
>$ ls

list all files in your current directory plus more info
>$ ls -l

make a new directory called dir1
>$ mkdir dir1

move into a directory call dir1
>$ cd dir1

back out of a directory
>$ cd ..

delete an emtpy directory dir1
>$ rmdir dir1

delete a single file file1.txt (or any other)
>$ rm file1.txt

copy file1.txt to a new file called file2.txt
>$ cp file1.txt file2.txt

rename (or move) a file.txt to hello.txt
>$ mv file.txt hello.txt

text editors used to edit your file hello.txt (or any other file)
>$ pico hello.txt
>$ nano hello.txt
>$ vi hello.txt
>$ emacs hello.txt

compile a c program in hello.c
>$ gcc hello.c

compile a c++ program in hello.cpp
>$ g++ hello.cpp

execute a c/c++ program a.out
>$ ./a.out

compile a java program hello.java
>$ javac hello.java

execute a java program hello.class
>$ java hello