Linux command line quick reference

The handful of commands you need to get around, compile, and run.

This is the same card that goes on the board in class, so you can read it from your own screen. Nothing here needs memorising — you will know all of it by about week three from using it.

Four keys that save you the most time

Learn these before you learn any commands.

Key	What it does
Tab	completes a name you have started typing — use it constantly
Up	brings back the last command, so you can edit and rerun it
Ctrl-C	stops a program that is running away
clear	tidies the screen

If your terminal ever fills with garbage and stops making sense, type reset and press Enter.

Getting around

Command	What it does
pwd	print the directory you are in
ls	list what is here
ls -a	…including hidden files
ls -l	…with sizes, dates and permissions
cd dir	change into dir
cd ..	go up one
cd	go back to your home directory
mkdir dir	make a directory
rm file	delete a file — there is no undo and no trash

Writing, compiling and running a program

The whole cycle, in the order you do it:

mkdir cmsc240          make somewhere to work (once)
cd cmsc240             go into it
nano hello.c           write the program
gcc hello.c -o hello   compile it into a program called hello
./hello                run it

In nano, ^ means the Control key: ^O then Enter saves, and ^X quits. Both are listed along the bottom of the screen.

The ./ in ./hello is not decoration. The shell only looks for programs in a fixed list of standard places, and the directory you are standing in is deliberately not one of them. ./ means “no, this one, the one right here.”

Once you are past the first week, compile with warnings turned on:

Command	What it does
gcc -Wall -g -o prog prog.c	compile prog.c into prog
./prog	run it
gcc -fopenmp …	compile with OpenMP enabled

Always compile with -Wall. A warning you ignore is usually the bug you spend the evening looking for.

Better nano

Some editing conveniences have to be explicitly turned on. Download this config file for nano into your home directory to do that, renaming it to .nanorc on the way — the leading dot is what makes nano look at it (the dot is missing here for technical reasons).

mv ~/Downloads/nanorc ~/.nanorc

Check the location is right by opening it in the terminal: nano ~/.nanorc.

When the compiler shouts at you

Read the message. It usually names a word that is not the broken one — expected ';' before 'return' is about the line above return, because a statement runs until its semicolon and the compiler has no reason to complain until it reads the next word. Recent versions of gcc point the ^ at the spot where the missing character belongs and print the character underneath as a suggestion, so the position is normally right even when the word quoted at you is not. If a message makes no sense where it points, look one line up.

Reading more

Dive Into Systems has a whole Unix reference in Appendix 2, and it is the first place to look. It is a reference, not a chapter — look things up in it rather than reading it through. The sections you will want this term, roughly in the order they become useful:

Section	What it covers
17.1	the command line and the filesystem
17.2	man, and how to look a command up
17.11	command history — stop retyping things
17.7	file permissions
17.6	finding files, and searching inside them
17.5	make and Makefiles
17.12, 17.13	redirection and pipes

man <command> answers most questions faster than any of the above, and the Bash reference manual is the authority when it doesn’t.