Unix
commands:
ls  - list the files of the working directory
pwd - print the working directory
cd  - change directory
files:
.   - the current directory
..  - the parent directory
.name - an invisible file
~/  - your home directory
Emacs
C- - (short for Ctrl button)
M- - (short for Alt  button)
C-x f - find/create a file
C-x c - quit
C-x s - save the current file
C-x b - change buffer
M-%   - query-replace (search and replace)

buffer - an open file