PATH is a colon separated list of directories that your shell (
sh, etc) uses to find something to execute. If the first word of the line you type to the shell is not a Shell builtin or alias, the shell steps through the list of directories in
$PATH, looking for an executable file with the same name as the first word of the command you typed.
command not found errors for common commands (
ls, ...) is indicitive of a malformed
PATH. An emergency
PATH (enough to let you fix the problem can be set with
PATH errors are shell syntax errors (Space before or after
PATH=value, no spaces).
As far as "path corruption", if I have created a "special" version of
ls, stored in
/.evil/bin/ls, AND I can corrupt your path so that it has
/bin, when you type
ls, the shell will run
/.evil/bin/ls instead of
/bin/ls. A Well Known Gotcha, but your system is thoroughly penetrated by the time it becomes useful.