In Unix, folders are called directories. Files are still files. The location of a file is called its path. The very top folder (or bottom folder depending on how you think about it) is called the root directory. This root directory is the one you see when you double click your hard drive. It contains the Applications, Library, System, and Users folders.

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?

When discussing what a Unix command is, I mentioned that you can alter the function of a Unix command by using flags or arguments. For example, to list all the files in your current folder, you use the ls command. To reverse its order you add the “r” argument: ls -r. But how do you know which arguments to use?

Unix has a built-in manual called the man system. Each Unix command comes with a manual page which you can access by typing man and then the name of the command. For example, to get help with the ls command, open the Terminal and type man ls.

Each manual page shows the name, synopsis, description, examples, and more for that command. You can use the up and down arrows to scroll through the entry. To skip Forward a page hit “f”; to skip Back a page hit “b”. When you’re done reading the manual page, his “q” to quit.

The man command is one of the most important commands for learning and using Unix. I use it daily.

On a side note, these manual pages are also available on the Internet, including at Apple.com: Mac OS X Man Pages

A Unix command is like an application, but it usually performs a much simpler task. There are Unix commands to show the contents of a file, to sort, to filter, and even to browse the Internet. Every Apple computer comes with hundreds of Unix commands that each do something unique. Contrast that with the dozens of applications that come with every Mac, some of which perform very complicated workflows.

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.