From MEPIS Documentation Wiki
The word "path" has two distinct uses in MEPIS Linux. The first is the everyday meaning of: the absolute location of a file within the root structure; e.g., /home/UserName/Desktop/File. The second "is an environment variable that is a list of directories that the operating system looks in to find commands issued by the user. For example, when you type ls to list files, you are actually executing /bin/ls because the /bin directory is in your path by default" (source). This second use is the topic of this article.
The PATH environment variable
When you type a command, MEPIS searches in all the directories listed in the PATH environment variable ("a set of dynamic named values that can affect the way running processes will behave on a computer," from Wikipedia). Many executable files, for instance, are stored in /usr/bin, which is one of the locations specified by default in MEPIS. The others are /usr/local/bin, /bin, and /usr/games.
To see which directories are specified on your system, open a terminal and type:
If you need to add a different directory, then you have two choices: 1) create a new path, or 2) make a symlink between the directory and one of the existing path locations.
To add a new path to a directory, open a terminal and type
PATH=$PATH:/path/to/new/directory export PATH
This change is temporary, only commands typed in this particular terminal window are affected. For permanent change, add these commands to your ~/.bashrc file (if it doesn't exist, use a text editor such as KWrite to create it).
For more complicated situations, see documentation.