Try the following:

sudo apt-get install rubygems

I had the same issue using zsh and this fixed it:

Copy$ echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.zshenv
$ echo 'eval "$(rbenv init -)"' >> ~/.zshenv
$ echo 'source $HOME/.zshenv' >> ~/.zshrc
$ exec $SHELL

I did confirm that the rubocop executable is in /usr/local/lib/ruby/gems/2.7.0/bin. Do I need to add this directory to $PATH?

Yes, the bin directory needs to be in your path. A little trick I use to keep the current version's directory in the PATH is something like this in .bashrc (or .zshrc whatever it is for you)

export PATH="$PATH:$(ruby -e 'puts Gem.user_dir')/bin"

check your rc files

• .zshenv
• .zshrc
• .zlogin
• .zprofile

most likely in one of those PATH is reset after RVM was sourced

also some oh-my-zsh plugins can break stuff, try disabling them and enabling one by one.

Your $PATH variable needs to include the exact path to your Ruby's bin directory. Adding a directory to the PATH does not include it's subfolders. Try adding the bin directory via:

export PATH=$PATH:/home/adam/.gem/ruby/1.8/bin

or if you installed the gem using sudo:

export PATH=$PATH:/usr/lib/ruby/gems/1.8/bin

You might want to add this to your .bashrc file, so that you don't have to set this manually every time your open up a new bash.

Thanks to historystamp's comment here is how the problem was solved (for bash shell):

1. Open/Launch Terminal
2. gem uninstall iStats (source)
3. gem install iStats --user-install (source)
4. export PATH="~/.gem/ruby/2.6.0/bin:$PATH" (source)
5. source ~/.bash_profile (source)
6. istats

If you use macOS Catalina, the default shell is zsh. The adapted instructions are the following:

1. Open/Launch Terminal
2. gem uninstall iStats (source)
3. gem install iStats --user-install (source)
4. Open or create ~/.zshrc
5. At the end of the file (or where appropriate) insert: export PATH=~/.gem/ruby/2.6.0/bin:$PATH (source)
6. Save file ~/.zshrc
7. Source the .zshrc file: . ~/.zshrc (source)
8. istats

It seems that either the rackup executable that is in your $PATH when you open a new terminal is not the same one that's in your path when you run your boot script, or else there are other aspects of the environment that are different in the two different scenarios.

You can test this by adding echo $PATH and env at the top of script and examining what is printed for each scenario.

If using the wrong $PATH is the issue, one quick and easy solution would be to call rackup using the full path instead, like so :

# replace /path/to with actual path

cd /Users/me/Sites/Ruby/mysite && /path/to/rackup &