How to add a directory to the $PATH in Ubuntu?

let us start by first outputting the content of $PATH variable:

$ echo $PATH

And you'll get the result something like this which is a list of directories separated by colon:
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games

Now here comes the important part to make your terminal programs executable without writing full path.

1. Exporting PATH variable to /etc/environment
   One of the fastest way to permanently add directory to $PATH environment variable is by using the following command:

#first append the new directory to path

$ PATH = /usr/local/sbin:/usr/local/bin:/pathToMyDirectory
$ source /etc/environment && export PATH

(OR)

1. Using ~/.profile file
   Another way is use the .profile file by adding the export command and then run the source command:
   #add this command to ~/.profile file
   $ export PATH=$PATH:/myNewDir
   #then run the source command
   $ source ~/.profile
   Here $PATH refers to content already set by system, so that we don't have to write previous directories path.