$PATH is a default Linux variable that contains directories that will be searched for command line interfaces (CLIs).

echo or printf can be used to display the current directories in the $PATH variable.

echo $PATH

Which should return something like this.

/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin

For example, the which command can be used to determine if a Command Line Interface (CLI) exists in $PATH. For example, which can be used to determine if the java CLI exists in $PATH.

which java

If the java CLI does exist in $PATH, something like this should be returned, meaning that the java CLI is located at /usr/bin/java. In this example, since /usr/bin is one of the directories in $PATH, which was able to determine that the java CLI exists at /usr/bin/java.

/usr/bin/java

Directories can temporarily be appended to the PATH variable like this.

export PATH=$PATH:/opt/foo:/usr/bar

Better yet, PATH can be defined in your /home/username/.bash_profile file, so that every time you sign into the system, your PATH will contain the directories defined in your .bash_profile file.

PATH=$PATH:/home/john.doe/.local/bin:/home/john.doe/bin
export PATH

You would then want to log out of the system, log back in, and echo or printf should now contain the additional directories.