Next few lines of code:

if [ -z "${RBENV_ROOT}" ]; then
  RBENV_ROOT="${HOME}/.rbenv"
else
  RBENV_ROOT="${RBENV_ROOT%/}"
fi
export RBENV_ROOT

Setting RBENV_ROOT

We’ve seen the -z flag in conditional logic before- it checks whether a value has a length of zero.

So if the RBENV_ROOT variable has not yet been set, then we give it a default value of ${HOME}/.rbenv, i.e. the .rbenv hidden directory located as a subdir of our UNIX home directory. If it has been set, then we just trim off any trailing “/” character. Either way, we export it as a environment variable.

What does $RBENV_ROOT do? According to the “Environment Variables” section of the README, it:

Defines the directory under which Ruby versions and shims reside.

And what does $HOME do? This resolves to the root directory of a particular user (specifically, you) in your Bash terminal. When you see me type ~/Workspace/OpenSource in some of the example code, the ~ (aka tilde) character expands to "$HOME", or /Users/myusername.

Setting RBENV_DIR

Next few lines of code:

if [ -z "${RBENV_DIR}" ]; then
  RBENV_DIR="$PWD"
else
...
fi
export RBENV_DIR

Let’s examine everything except the code inside the else block, which we’ll look at next.

This block of code is similar to the block before it. We check if a variable has not yet been set (in this case, RBENV_DIR instead of RBENV_ROOT). If it’s not yet set, then we give it a default value of the current working directory. Once we’ve exited the if/else block, we export RBENV_DIR as an environment variable.

Now the code inside the else block:

  [[ $RBENV_DIR == /* ]] || RBENV_DIR="$PWD/$RBENV_DIR"

The code[[ $RBENV_DIR == /* ]] is an attempt to pattern-match, not an equality check. The particular pattern that we’re matching against returns true if $RBENV_DIR starts with the / character. If the pattern does not match, we set RBENV_DIR equal to "$PWD/$RBENV_DIR".

Since a leading / in a filepath means we’re dealing with an absolute path, this means we’re checking to see if $RBENV_DIR is a string that represents an absolute path. And since "$PWD" is always an absolute path pointing to our current directory, this line of code means that:

• We check whether RBENV_DIR is an absolute path.
• If it isn’t, we update its value to be prefixed with the absolute path to our current directory.

The next block of code is:

cd "$RBENV_DIR" 2>/dev/null || abort "cannot change working directory to \`$RBENV_DIR'"
RBENV_DIR="$PWD"
cd "$OLDPWD"

Here we’re attempting to cd into our latest version of $RBENV_DIR, sending any error message to /dev/null, and aborting with a helpful error message if that cd attempt fails. We then set the value of RBENV_DIR to the value of $PWD (the directory we’re currently in), before cding into OLDPWD, an environment variable that Bash maintains and which stores the directory we were in prior to our current one.

This sequence of code is doing two things:

• It ensures that the value we store in RBENV_DIR is a valid directory, by attempting to cd into it and aborting if this fails.
• It normalizes the value of RBENV_DIR, “…remov(ing) unneeded /./ and ../dir sequences.”

Next line of code is:

export RBENV_DIR

Here we just make the result of our RBENV_DIR setting into an environment variable, so that it’s available elsewhere in the codebase.

Setting RBENV_ORIG_PATH

Next line of code is:

[ -n "$RBENV_ORIG_PATH" ] || export RBENV_ORIG_PATH="$PATH"

Here we check if $RBENV_ORIG_PATH has been set already. If not, we set it equal to our current path and export it as an environment variable.

Nothing too special there. Let’s move on.