login[options]
Sign on and identify yourself to the system. At the beginning of each terminal session, the system prompts you for your username and, if relevant, a password. The options are usually not used.
userSign on as user (instead of being prompted).
ttySpecify the pathname of the tty that serves as the login port.
var=valueWhen specified after the username,
assign a value to one or more environment variables.
PATH and SHELL can't be changed.)