-.I Su
-demands the password of the specified
-.I userid,
-and if it is given,
-changes to that
-.I userid
-and invokes the Shell
-.IR sh (1)
-or
-.IR csh (1)
-without changing the current directory.
-The user environment
-is unchanged except for HOME and SHELL,
-which are taken from the password file
-for the user being substituted
-(see
-.IR environ (7)).
-The new user ID stays in force until the Shell exits.
+\fISu\fP requests the password for \fIlogin\fP (or for ``root'', if no
+login is provided), and switches to that user and group ID. A shell is
+then invoked.
+.PP
+By default, your environment is unmodified with the exception of
+\fIUSER\fP, \fIHOME\fP, and \fISHELL\fP. \fIHOME\fP and \fISHELL\fP
+are set to the target login's \fI/etc/passwd\fP values. \fIUSER\fP
+is set to the target login, unless the target login has a UID of 0,
+in which case it is unmodified. The invoked shell is the target
+login's. This is the traditional behavior of \fIsu\fP.
+.PP
+The \fI-l\fP option simulates a full login. The environment is discarded
+except for \fIHOME\fP, \fISHELL\fP, \fIPATH\fP, \fITERM\fP, and \fIUSER\fP.
+\fIHOME\fP and \fISHELL\fP are modified as above. \fIUSER\fP is set to
+the target login. \fIPATH\fP is set to ``/usr/ucb:/bin:/usr/bin''.
+\fITERM\fP is imported from your current environment. The invoked shell
+is the target login's, and \fIsu\fP will change directory to the target
+login's home directory.
+.PP
+The \fI-m\fP option causes the environment to remain unmodified, and
+the invoked shell to be your login shell. No directory changes are
+made. As a security precaution, if the target shell is not a standard
+shell (as defined by \fIgetusershell\fP(3)) it is invoked instead of
+your login shell.