2.9BSD/usr/man/man1/su.1

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.TH SU 1
.UC
.SH NAME
su  \-  substitute user id temporarily
.SH SYNTAX
.B su
[ \- ]
[ userid ]
.SH DESCRIPTION
.I Su
demands the password of the specified
.I userid,
and if it is given,
changes to that 
.I userid
and invokes a shell
without changing the current directory or the
user environment.
The new user ID stays in force until the shell exits.
If the `SHELL' environment variable is set, it is used as
the name of the new shell, and, if necessary, the previous
user environment is restored for that shell. This allows
for the use of multiple shells with \fIsu\fP.
If the optional
.B -
flag is given, the shell of the specified user will be
the new shell, the environment is set as for that user,
and the new shell is started in the new home directory.
This can be used to simulate the login environment of a specified user.
.PP
If no 
.I userid
is specified, `root' is assumed.
To remind the super-user of his responsibilities,
the shell substitutes `#' for its usual prompt.
.SH "SEE ALSO"
csh(1), newgrp(1), sh(1)