É login .
A página man do login (1) do Linux diz:
The value for $HOME, $USER, $SHELL, $PATH, $LOGNAME, and $MAIL are set according to the appropriate fields in the password entry.
A página do login (1) do FreeBSD diz:
The login utility enters information into the environment (see environ(7)) specifying the user's home directory (HOME), command interpreter (SHELL), search path (PATH), terminal type (TERM) and user name (both LOGNAME and USER).
O NetBSD , OpenBSD e OS X dizem a mesma coisa.
Aqui está o código-fonte do util login -linux:
setenv("HOME", pwd->pw_dir, 0); /* legal to override */
setenv("USER", pwd->pw_name, 1);
setenv("SHELL", pwd->pw_shell, 1);
/* ... */
setenv("LOGNAME", pwd->pw_name, 1);
Veja a fonte código do login do FreeBSD:
(void)setenv("LOGNAME", username, 1);
(void)setenv("USER", username, 1);
(void)setenv("PATH", rootlogin ? _PATH_STDPATH : _PATH_DEFPATH, 0);