When you log onto a UNIX somputer, the sustem runs a program called shell that gives you a prompt and wait for you type sommands, one per line. When you type a command and hit <enter>, the shell tries to execute the command. assuming that the first word in the command line is the name fothe command.A shell command can be built-in or external. The shell has the code for executing an build-in command, but the code for an external command is in the file.
The search path is stored in a shell variable called PATH  or path (for C shell)