Use of / when using cd

This is standard Unix behaviour.

The / at the beginning of the path represents the root of the disk (or the start/uppermost level of the filesystem tree). As Documents is not off the root, /Documents can't be found.

/Users is off the root directory, so this problem does not occur.

You could use cd /Users/lukas/Documents to change to that path.

Alternatively, you could use relative addressing. Unix based filesystems have 2 special directories:

• . which means "this directory",
• .. which means the parent directory.

If you are in /Users/lukas, cd ./Documents would take you to the correct place.

Let's assume you were in /Users/Lukas/Documents and wanted to go to /Users/Janes/Documents, you could issue a command cd ../../Janes/Documents - using .. would take you back a level - so you would do it twice, before going into the new relative path.

/Users/lukas is an "absolute" path. The leading / represents the root directory of your filesystem.

lukas is a "relative" path. As it is not anchored to the root, it means "look for this in the current directory". Unless the current directory is /Users (or some other directory with a lukas in it), this will fail.

So, let's explore your examples, assuming you're in /Users/lukas:

$ cd Documents/
/Users/lukas/Documents

Relative path given => change to the directory "Documents" that's inside /Users/lukas.

$ cd /Documents
-bash: cd: /Documents: No such file or directory

Absolute path given => change to the directory /Documents.

$ pwd
/

This shows that you've now changed the working directory to the root directory, / (though the cd command to do this was not shown).

$ cd Users
/Users

Relative path given => change to the directory "Users" that's inside /.

$ cd /Users
/Users

Absolute path given => change to the directory /Users.

The key each time is that leading /. With it, the path is absolute. Without it, the path is relative. This rule is unambiguous because all absolute paths begin with / (because the root directory is always called /).

Here's some pseudocode loosely describing that algorithm:

MakePathAbsolute(path):
   if <path> starts with '/'
      return <path>
   else
      return <current directory>/<path>

The argument you pass to cd goes through this algorithm; the directory you end up changing to is the path that the algorithm returns.

Further reading:

• Unix Files and Directories Tutorial
  Joseph L. Zachary
  "Introduction to Scientific Programming"