whoami displays the login name corresponding to the current effective user ID. If you have used su to temporarily adopt another user, whoami will report the login name associated with that user ID. whoami gets its information from the geteuid and getpwuid library routines (see getuid and getpwnam(3C), respectively).

/etc/passwd

username data base

See attributes(5) for descriptions of the following attributes:

su(1M), who(1), getuid(2), getpwnam(3C), attributes(5)