An Introduction to the Command-Line (on Unix-like systems)

by Oliver; 2014

7. Where Are You? - Your Path and How to Navigate through the Filesystem

When you open up the terminal to browse through your filesystem, run a program, or do anything, you're always somewhere. Where? You start out in the designated home directory when you open up the terminal. The home directory's path is preset by a global variable called HOME. Again, it's /Users/username on a Mac.

As we navigate through the filesystem, there are some conventions. The current working directory (cwd)—whatever directory we happen to be in at the moment—is specified by a dot: Sometimes it's convenient to write this as: which is not to be confused with the root directory: When a program is run in the cwd, you often see the syntax:

$ ./myprogram

which emphasizes that you're executing a program from the current directory. The directory one above the cwd is specified by two dots: With the trailing slash syntax, that's: A tilde is shorthand for the home directory: or: To see where we are, we can print working directory:

$ pwd

To move around, we can change directory:

$ cd /some/path

By convention, if we leave off the argument and just type:

$ cd

we will go home. To make directory—i.e., create a new folder—we use:

$ mkdir

As an example, suppose we're in our home directory, /Users/username, and want to get one back to /Users. We can do this two ways:

$ cd /Users

or:

$ cd ..

This illustrates the difference between an absolute path and a relative path. In the former case, we specify the complete address, while in the later we give the address with respect to our cwd. We could even accomplish this with:

$ cd /Users/username/..

or maniacally seesawing back and forth:

$ cd /Users/username/../username/..

if our primary goal were obfuscation. This distinction between the two ways to specify a path may seem pedantic, but it's not. Many scripting errors are caused by programs expecting an absolute path and receiving a relative one instead or vice versa. Use relative paths if you can because they're more portable: if the whole directory structure gets moved, they'll still work.

Let's mess around. We know cd with no arguments takes us home, so try the following experiment:

$ echo $HOME      # print the variable HOME
/Users/username
$ cd              # cd is equivalent to cd $HOME
$ pwd             # print working directory shows us where we are
/Users/username 

$ unset HOME      # unset HOME erases its value
$ echo $HOME      

$ cd /some/path   # cd into /some/path
$ cd              # take us HOME?
$ pwd
/some/path

What happened? We stayed in /some/path rather than returning to /Users/username. The point? There's nothing magical about home—it's merely set by the variable HOME. More about variables soon!

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