2. Directories and File System Paths

---

Making and changing directories

• ls is the name of a built in program that lists the contents of the current directory
  • To run it, type ls in the command line and press enter:

machine_name:~$ ls

• The program mkdir accepts one argument: a new directory name, and creates a new directory with that name in the current working directory:

machine_name:~$ mkdir test_dir

A shell program’s potentially multiple arguments are separated by spaces, not specified as a comma-separated list enclosed by parentheses.

• To change your current working directory, pass the name of a new directory to the program cd as an argument:

machine_name:~$ cd test_dir

• After running the command above, the new command line prompt should list ~/test_dir as the new working directory:

machine_name:~/test_dir$

File system paths

• The working directory ~/test_dir above is an example of a file system path, a string that maps to a particular location on the file system.
  • Paths are strings of directory names separated by /
• Try making another directory inside ~/test_dir called test2, confirm you created the new directory inside test_dir with ls, and then change directories to ~/test_dir/test2:

machine_name:~/test_dir$ mkdir test2
machine_name:~/test_dir$ ls
machine_name:~/test_dir$ cd test2

• After running the last command above, your command line prompt should now look like so:

machine_name:~/test_dir/test2$

If we don’t put ~ or / at the beginning of the path to the new directory that we pass to cd, the cd program looks for the new directory relative to the current working directory.

Aside: escaping characters

• Suppose instead we had run the following commands:

machine_name:~/test_dir$ mkdir test 2
machine_name:~/test_dir$ ls

• We’d see that there are two directories named test and 2!
  • Remember, arguments are separated by spaces in shell programming, so mkdir makes as many directories as there are arguments.
• To make a directory called test 2 (DON’T!), you have to escape special characters by prefacing them with a \ or by enclosing the whole argument in single (') or double (") quotes:

machine_name:~/test_dir$ mkdir test\ 2
machine_name:~/test_dir$ mkdir 'test 2'
machine_name:~/test_dir$ mkdir "test 2"

• Escaping characters means telling the shell to interpret that character literally, rather than as something with programmatic meaning.

Relative vs. absolute file system paths

• Actually ~ is a special file system shortcut or alias for another path; use pwd to see full current working directory:

machine_name:~/test_dir/test2$ pwd

The output of pwd is a path that starts with a /, so it’s an absolute path.

• The path / points to the top of the file system, i.e. the directory that contains all others (called the “root”).

• / is not the “home” directory for a user because files related to the operating system live in / that users shouldn’t (usually) touch.

• Paths that don’t start with / are relative paths; wherever they’re used (directly in the shell or in code), they start from the current working directory, not /.
  • The shell assumes paths that don’t start with / are meant to be relative to the current working directory.
• The following file system diagram illustrates an example directory structure as a tree:

• The directories highlighted in orange form the absolute path /Users/nrapstin, which is aliased by ~ for my machine (my “home directory”).

Relative path helpers

• If you use the special directory name .. somewhere in the path, it will replace the path up to the .. with the “parent” directory of the directory before the ..
  • E.g. the path /this/is/a/path/.. is equivalent to the path /this/is/a
• You can also use ls to list the contents of any directory (not just the current one) by passing it a path as an argument; try passing it the path ..:

machine_name:~/test_dir/test2$ ls ..

Since .. doesn’t start with /, the shell assumes the path is relative to the current directory, so the output should be the contents of ~/test_dir (i.e. the directory test2)