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.

For more information you can use the man command on Unix (e.g. man wc for help on the wc command), although sometimes man pages can be difficult to understand. Another good resource is to search for the command on [http://en.wikipedia.org/wiki/Main_Page Wikipedia].

1. Navigation

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.

1.1. ls

The ls command lists the contents of the current directory. Useful flags include -l to output the listing in long format, which gives more info than just filenames, -h, which in concert with -l shows file sizes in a more human readable format (i.e. bytes, kilobytes, megabytes, etc. instead of disc blocks or byte count), and -a, which shows hidden files.

1.2. cd

cd is short for change directory. The directory can be specified as relative to the current directory (e.g. to go to the foo subdirectory of the current directory you'd type cd foo), relative to the root directory of the system (e.g. to go to the bin subdirectory of the usr directory you'd type cd /usr/bin), or relative to your home directory (e.g. to go to the foo subdirectory of your home directory from anywhere on the filesystem you'd type cd ~/foo), or relative to another user's home directory (e.g. to go to the foo subdirectory of a user named bob's home directory you'd type cd ~bob/foo).

There are also a couple of nice shortcuts. If you type cd with no arguments it always takes you back to your home directory. If you type cd - it will take you to the last directory you were in before the current one. This can be useful if you were working in a directory like /Library/Frameworks/Python.framework/Versions/Current/lib/python2.4/, went back to your home directory to do something, and want to get back without typing all that again.

1.3. pwd

Probably the most simple command that will be discussed here. All pwd does is print the working directory, i.e. show you where you are in the file system.

2. File manipulation

• mv
• cp
• rm
• mkdir
• rmdir
• touch

3. 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.

3.1. | ("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.

3.2. 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.

3.3. 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.

3.4. 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.

3.5. 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).

3.6. uniq

The uniq command filters its input such that if multiple consecutive 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.

3.7. 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.

3.8. cut

The cut command is used to cut chunks out of lines in a file. The most common flags to use are -f, which is by field (tab being the default field separator) and -c, which is by character or character range.

3.9. paste

The paste command takes an arbitrary number of filenames as arguments and outputs sequentially correspondent lines concatenated together, separated by tabs. In essence, the inverse of cut -f.

3.10. sed

sed is short for stream editor. It's actually a full-fledged programming language, but it's often used to write one line scripts to manipulate a character stream. Highly useful to know how to use, but too complex to discuss in detail here.

3.11. awk

Like sed above, awk (short for Aho, Weinberger, and Kernighan, last names of its creators) is a full programming language that can also be used inline for character stream manipulation. Again, you should learn it, but it's too complex for this page.