When you issue a command, the shell program parses the command line and either processes it directly or searches for a file with that name in any of the directories specified in your search path , which is controlled by the shell variable PATH. If the file is not found in any of the directories in your search path, the shell reports that the command was not found. The file may well be on the disk somewhere, but it is ``not in your path.''
We will attempt to provide an appropriate default path in a future
CERN UNIX Environment,
but you can always modify the value of PATH in your 'startup' file
(normally .cshrc or .login for the C shell family or
.profile for the Bourne shell family, see section 
).
If you are using the C, tcsh or zsh shells, for example, and you add a command to one of the directories in your search path, it may be necessary for you to either log out and log back in or to recreate the internal tables used by the C shell with the rehash command.