Make sure the shell script is executable:
$ chmod +x /path/to/gh.sh
In your ~/.bash_profile add an alias for the script (you may alias to anything you want... I chose 'gh'):
alias gh='~/path/to/gh.sh'
Restart terminal after saving.
To open the GitHub page for your repo, run the script in the terminal anywhere in the repository:
$ gh
To open a specific subpage on GitHub for your repo, just add the identifier (url) of the page. For Pull Requests, it would look like this:
$ gh pulls
For the wiki:
$ gh wiki
If you use something often, you could further alias these commands to something like:
$ ghp
by adding this to your `~/.bash_profile:
alias ghp='gh pulls'