Unix and Unix-like
On POSIX and Unix-like operating systems, the $PATH variable is specified as a list of one or more directory names separated by colon (:) characters.
The /bin, /usr/bin, and /usr/local/bin directories are typically included in most users' $PATH setting (although this varies from implementation to implementation). The current directory (.) is sometimes included as well, allowing programs residing in the current working directory to be executed directly. Superuser (root) accounts as a rule do not include it in $PATH, however, in order to prevent the accidental execution of scripts residing in the current directory. Executing such a program requires the deliberate use of a directory prefix (./) on the command name.
When a command name is specified by the user or an exec call is made from a program, the system searches through $PATH, examining each directory from left to right in the list, looking for a filename that matches the command name. Once found, the program is executed as a child process of the command shell or program that issued the command.
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