Useful Unix commands

One of the wonderful things about Unix is that there is a command to do just about anything you could ever conceive of doing. The tricky part is figuring out what the command is called and what switches it takes to do what you want. This page provides a primer on using some of the most important and useful commands to know.

Before you do much of anything else you need to know where you are and how to get somewhere else. Here are some commands to help you get around.

ls
cd
pwd

File manipulation

mv
cp
rm
mkdir
rmdir
touch

Text munging

An overarching theme in the design of Unix is that everything is a stream of characters. Consequently there are a number of commands that allow you to manipulate character streams.

| ("pipe")

Most Unix commands are short, but pipe is just a single character, |, which is usually located on the same key as the backslash. The pipe command allows you to send the output of one command to the input of another. You can string a number of commands this way to make a pipeline where the only the final result is printed.

more and less

The more and less commands output the contents of a file one screenful at a time. more is the older command and can only go forward through a file, i.e. once text has scrolled by you can't get back to it. less is a more sophisticated replacement that allows you to more forward and backward through the file and accepts vi-like commands. Hit h while less is running to see all the commands it accepts. On some systems (e.g. MacOS X) more has been aliased to less.

cat

The cat command is short for concatenate, although in practice that isn't too helpful to know. What it's useful for is outputting the contents of file to the output, which is generally then piped to another command.

grep

The grep command searches globally for a regular expression and prints matches. The two most common uses for it are to search for files that contain a specific string or match a regular expression, or to do the same for a stream piped into it.

sort

As you would expect, sort takes its input and outputs it in sorted order. Use sort -r to sort in reverse, and sort -f to fold upper and lower case together (meaning that 'A' and 'a' are treated the same; the default is for A-Z to come before a-z).

uniq

The uniq command filters its input such that if multiple lines are the same it only prints one instance to the output. To be useful the input needs to be sorted already, and as such it is usually used in a pipeline after sort.

wc

wc is short for word count. The default output is the number of lines, words, and bytes in a file. You can use the -l (lines), -w (words), -c (bytes) flags to limit what is shown. You can use the -m flag for number of characters, which is only useful for textfiles with multibyte characters, e.g. Chinese or Japanese.