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I'm in my home directory on my mac:

$ pwd

/Users/lukas

When I cd around, I do not (and can't) start the path with /:

$ cd Documents/

/Users/lukas/Documents



$ cd /Documents

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

Except when I'm in /:

$ pwd

/

$ cd Users

/Users



$ cd /Users

/Users

Why can I use / in front of the path when going from / to /Users but not in any subsequent levels? Is this specific to macOS or is this standard Unix behavior?

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"