What is computing with Unix?

Unix is a family of operating systems and environments that exploits the power of linguistic abstraction and composition to orchestrate tasks.

Unix users spend a lot of time at the command line.

The command line (often called the console, the shell or the terminal) looks something like this:

$ _

When you type in a command like ls , it will give you feedback:

$ ls Documents README.txt $ _

In this case, the ls command listed the files in the current directory.

Getting access to Unix

It was once a daunting task to get access to or install Unix. Fortunately, it's no longer a hassle.

These days, the two most accessible flavors of Unix are Apple's OS X (based on the BSD family of Unix) and Ubuntu Linux.

The easy option: OS X

OS X is a good way for many users to tip-toe into Unix.

What you're buying is a Unix machine that "just works."

To turn it into a complete Unix system, you'll need to install Apple's free Xcode development suite (available from the Mac App Store) and the (free) X11 server.

Xcode includes the compilers and interpreters you'll need to compile and run many open source applications.

You'll need the X11 server to run traditional graphical applications for Unix.

The DIY option: Linux

When I was in high school, before CD burning was widely available and when internet connections were slow, the best way to get Linux was to buy a book with an installation CD.

(Given that there wasn't much in the way of online resources for Linux back then, it was good to have a book that walked through the installation process and explained all the features, too.)

A quick search of amazon shows no shortages of such books today.

These days, there are plenty of free options:

If you're a CS major, you should have access to a Unix shell account at your school that you can acces with ssh . (See details on ssh below.)

. (See details on ssh below.) You can set up your computer to dual boot into either your preferred OS or Linux. Macs come with Boot Camp to make this easy. If you take the dual boot route with a Windows PC, FIRST BACK UP YOUR DATA. There is a chance you'll wipe out your files by accident. I recommend adding a second internal hard drive if you want to set up a dual boot with Linux for the first time.

You can burn a CD that lets you boot into or install Linux. I recommend Ubuntu Linux for this.

You can also create a USB thumb drive that boots into Linux. I recommend Ubuntu Linux for this as well.

You can run Linux inside your existing OS as a virtual machine. VirtualBox is a freely available virtualization tool. (I personally use VMware Fusion to run Windows on my Mac for those rare instances where I need to see what a page looks like in an old version of IE.)

to run Windows on my Mac for those rare instances where I need to see what a page looks like in an old version of IE.) You can run cygwin on Windows to get a "Unix-like" environment. (This isn't quite the real deal.)

You can buy a virtualized Linux server with remote access from a service like linode.com. (might.net runs on a linode.)

Getting to the command line

Once you're into Unix, you'll want to get to the command line.

On Linux, you can press CTRL+ALT+F1 to get to a raw terminal, and CTRL+ALT+F7 to get back to the window system. (In fact, CTRL+ALT+F1-F6 will get you different terminal on most systems, and of course, you can configure this.)

In most popular flavors of Linux, there is a way to open a graphical terminal in a window.

On a Mac, it's Applications > Utilities > Terminal.

The filesystem: ls and cd

If you find yourself at a command prompt, run ls to list the files in the current directory.

You might see something like this:

$ ls Documents README.txt $ _

With ls / , you can list the contents of the root directory.

You'll notice several directories that are common to almost all Unices:

/bin , standard executables.

, standard executables. /sbin , important system executables.

, important system executables. /etc , system configuration files.

, system configuration files. /var , frequently updated files like logs.

, frequently updated files like logs. /root , home directory of the administrator account.

, home directory of the administrator account. /home , a user account home directories. ( /Users on a Mac.)

, a user account home directories. ( on a Mac.) /opt , package-manager installed files.

, package-manager installed files. /tmp , temporary files; usually wiped between boots.

, temporary files; usually wiped between boots. /usr , user applications and utilities.

To change to a different directory, use the command cd .

For example:

cd Documents to change to the Documents sub-folder.

to change to the sub-folder. cd .. to change to the parent directory.

to change to the parent directory. cd to change to your home directory.

Paths

To print the path of the current working directory, use pwd :

$ pwd /home/matt $ _

On Unix, a path is a description of a file's location.

For example, /home/matt/README is a path to a file called README contained within the home directory for the user matt .

Notice that forward slashes ( / ) separate components of a path.

For example:

$ pwd /home/matt $ cd .. $ pwd /home $ cd matt $ pwd /home/matt $ _

Symbolic links

Unix filesystems make use of aliases for files known as symbolic links (symlinks).

(There are also hard links, in which there are two true, indistingushible references to the same file, but these are less commonly used.)

A symbolic link to a file is ordinarily treated identically to that file.

The command ln -s creates sym links.

The command ls -l can reveal where a symlink points.

For example:

$ ls bar foo $ ln -s bar baz $ ls -l total 8 -rw-r--r-- 1 matt staff 0 Jan 8 09:57 bar lrwxr-xr-x 1 matt staff 3 Jan 8 09:58 baz -> bar -rw-r--r-- 1 matt staff 0 Jan 8 09:57 foo $ _

Working with text: cat , less , emacs and vi

Suppose you want to look at a text file within the current directory.

A common way to take a glance at a text file is with the cat command:

$ cat README.txt * A README file for my home directory. Documents contains my files. $ _

(The command cat is actually meant to do more powerful things than just look at files, but that's what it's most commonly used for.)

If the file is too long to fit on a screen, use less instead of cat . (To quit less , press q .)

You're going to spend a lot of time editing text, so you'll want to learn one (or both) of the powerful Unix text editors: emacs and vim .

Both have a tutorial mode for teaching the essentials.

Help yourself: man up

If you don't know what a command does, use the command man command .

This will bring up the man(ual) page for command, which will document how the command operates; it frequently provides examples of usage.

Press q to quit the man page viewer.

The apropos can help you find the command you're looking for if you don't know it's name. It will search a database of command descriptions.

Some tools use an alternate documentation system called info

To look up documentation using info , try info topic .

For more info on info , run info info .

Search for it: grep and find

If you're looking for text within a file or a set of files, grep is the right tool.

The command grep pattern file ... will search the specified files for the pattern.

The pattern language used throughout Unix is called regular expressions, and there are many good introductions to these, including the man page for grep.

To list all files under the working directory (and its subdirectories), try the command find .

Pipes and redirection

Nearly every command in Unix makes use of a convention to have a "standard input" (also called stdin or STDIN) and standard output (also called stdout or STDOUT).

There is also a "standard error" (stderr or STDERR) output that is, by convention, reserved for error messages.

Many techniques in Unix rely on redirecting these channels.

If you want to dump the standard output into a file, use command > file .

For example:

$ pwd > pwd.txt $ ls pwd.txt $ cat pwd.txt /home/matt $ _

One of the most useful capabilities of Unix is the ability to redirect the STDOUT of one command into the STDIN of another.

To do this, you'll want to use a "pipe"; for example:

$ find . | grep READ ./Desktop/READINGLIST.txt ./README.txt $ _

This command found a file named READINGLIST.txt in the Desktop subdirectory, and a file named README in the current one.

It's customary for Unix programs to read from STDIN when no input file is specified, which is what grep is doing here.

It is also possible to send the contents of a file into the STDIN of a command using command < file .

Permissions: chmod , chown and chgrp

In Unix, every file and directory has an owner and a group.

Every user on a Unix machine can belong to one or more groups.

Every file also has three sets of permissions: what the owner can do, what the group can do and what anyone can do.

To see the owner, group and permissions associated with a file run ls -l . You'll see something like:

$ ls -l drwxr-xr-x 4 matt staff 136 Jun 23 2010 Documents drwxr-xr-x 143 matt staff 4862 Dec 30 2009 Desktop -rw-r--r-- 1 matt staff 4 Feb 7 11:16 README.txt $ _

The first column tells you about the permissions on the file.

The third and fourth tell you the owner and group respectively.

The very first character in the permissions column tells you what kind of file it is. A - means it's a regular file. A d means it's a directory.

The next nine characters come in three classes of three characters each. The three classes are owner permissions, group permissions and world permissions.

Inside a permission class, r means that class can read the file; w means that class can write the file; x means that class can execute the file.

If a file is a directory, x grants the permission to access inside the directory, while r grants permission to list its contents.

Package managers

Modern Unixes have package managers to that download install (free) software for you.

On a Mac, MacPorts is a popular package-management system, and Homebrew is gaining in popularity.

On Ubuntu, apt is the standard package manager, with both a command-line and graphical interface available.

It's a good idea to familiarize yourself with your package manager.

Remote access: ssh

If you have more than one machine, or you need remote access to another machine, ssh is a powerful "secure shell" utility.

ssh can grant you console access on a remote machine, while safely encrypting the session.

It is worth setting up passwordless key-based authentication using ssh-keygen .

Using ~/.ssh/config , many options, including aliases and per-host private keys, are available. (Try man ssh_config ).

To ssh into another computer, use ssh user@address where user is the user name of your account on the other machine, and address is host name or IP address of the other computer.

Windows users can use PuTTY to connect to another computer over ssh.

What's next?

Wikipedia has a list of Unix utilities to peruse.

You can also move onto my guide to settling into Unix.

Good books

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