As always, if you wanted to get a more elaborate description on a command or service, you can always use man (shortened for: manual). This will display a full description, and various options of a command which you have found using apropos.

For example, suppose you have a text file full of email addresses, with one email address per line. Suppose it’s named “emails.txt” and it’s on your Desktop.

To view the emails, type the following command in the Terminal:
cat ~/Desktop/emails.txt

To view the emails sorted alphabetically, type the following:
cat ~/Desktop/emails.txt | sort

The pipe causes the output of the cat command to be fed into the sort command, then outputted to the screen. You can tack on as many pipe commands as you need.

To view the emails sorted alphabetically and with a prompt between each page, type:
cat ~/Desktop/emails.txt | sort | more

The list of emails “flows” from the cat command to the sort command to the more command, as if going through water pipes.

Here is how you refer to various parts of the file system in Unix:

• The forward slash (/) refers to the root folder.
• A path name beginning with a slash (/) is an absolute path. For example, the path /Applications refers to the Applications folder, no matter where you currently are. To see a list of Applications, type ls /Applications. It doesn’t matter where you currently are because it’s an absolute path.
• A path name beginning without a slash is a relative path. For example, if you are currently in your Home folder, the path Documents refers to the Documents folder under your Home folder. While in your Home folder, type ls Documents to see a list of your documents. If you were to type ls Applications (without the slash), you’d likely see an error since you (probably) don’t have an Application folder inside your Home folder, and the path is relative to where you currently are.
• The tilde (~) refers to your Home folder. (It’s also an absolute reference, despite not having a preceding slash.) To see what’s in your Home folder, type ls ~. To see your own Documents type ls ~/Documents. It’s an absolute reference that works from anywhere. For me, the path /Users/richard/Documents is the same as ~/Documents.
• You can use the tilde with someone else’s username to refer to their Home folder. For example, to see what’s in Mary’s Home folder, type ls ~mary or to see what’s in John’s Documents folder, type ls ~john/Documents.

You should now be able to refer to any path on your hard drive. Did I leave anything out?

Mac applications have Preferences for selecting how you’d like them to run. Unix commands, on the other hand, don’t (usually*) have any preferences or persistent settings. To specify how a Unix command runs, you use what are called flags or arguments. These flags or arguments are specified each time you run the command.

Arguments are specified with one dash and a letter (e.g. -a -b -c), or two dashes and a word (--help.) Sometimes certain settings or parameters can follow an argument (-u "stevejobs" or --user "stevejobs".) Whenever you use multiple single-letter parameters, you can combine them (-abc is the same as -a -b -c.)

Here’s an example. The ls command lists the files in a folder. Open Terminal, type ls, and hit Enter. You’ll see a list of the files and folders in your Home folder. Each name is listed one after another.

If, instead, you’d like to see all the files listed in columns, type ls -l and his Enter. You’ll see all the files listed in one column. It will also show the dates the files were created, and other information. This is the “long” format, as specified by the l flag.

You’ll notice that the list of files is in alphabetical order. If you want to reverse it, you can use the r flag. Type ls -l -r. The files will be listed in reverse alphabetical order. Since you can combine single letter flags, ls -lr will produce the same result. Most of the time the order of the flags doesn’t matter either, so ls -rl also produces the same result.

On Unix systems, there are actually multiple shells — bash, sh, csh, tcsh, and many others. The shell you use determines how you interact with the command line interface, but they’re all similar. The default shell on Mac OS X is bash, or “Bourne Again Shell”. Bash is the one we’ll be using.

To find out what shell you’re using, you can type echo $0 into Terminal and push Enter. It should return “bash”.

There isn’t much practical knowledge to be gained by knowing about the different shells, but I just didn’t want you to be surprised if you heard the terms shell or bash.

Further reading:
Wikipedia: Shell
Wikipedia: bash

Is anything unclear? I’ll explain!

As long as you’re going to start using Terminal regularly, you might as well make it at home. You can drag the Terminal to your Dock for easy access. Also, you might want to change the look of Terminal by going to Window Settings in the Terminal menu. Switch the drop down menu to Color, then change the color to your liking. I personally like White on Black with a little transparency, but you can do whatever you like.

You might also want to switch to the Display menu and choose a different font and font size. When you’re done, click Use Settings as Defaults.

If you really want to get fancy, you can also access the command line interface through a 3rd party application like iTerm. iTerm adds some niceties that Terminal doesn’t have, such as tabbed windows. I know people that really like iTerm, but I personally just use Terminal.

Further reading:
Wikipedia: Terminal Application
Wikipedia: Text terminal
Wikipedia: Graphical user interface
Wikipedia: Command line interface

Is anything unclear? I’ll explain!

Since then, many variants have descended from the first Unix, including Linus Torvald’s college project Linux. Unix was originally spelled “UNICS”, an acronym for Uniplexed Information and Computing System.

Mac OS X is built on a version of Unix called FreeBSD, which means that every modern Mac inherits the strength of a powerful, time-tested operating system. FreeBSD is widely regarded as one of the most secure operating systems. Apple combines this with a beautiful interface. Mac OS X is like the engine of a classic muscle car in a shiny, modern body.

Further reading:
Apple: Solid as a Rock
Wikipedia: Unix
The Unix family tree (diagram)

Is anything unclear? I’ll explain!