substitute user identity
su utility allows a user to run a
shell with the user and group ID of another user without having to log out
and in as that other user. All of the real, effective, and saved user and
group IDs as well as all supplementary group IDs are always set according to
the target user. If the target login name is not
specified, “root” is used.
By default, the shell of the target login is invoked and the
variables are set according to the target login, whereas the current working
directory remains unchanged. If the target login has a user ID of 0,
PATH and the
umask(2) value are set according to
USER are set to the target login and
PATH and the
umask(2) value are preserved. The
environment variable is always preserved. The rest of the environment
remains unmodified by default.
The options are as follows:
- Same as the
- Specify an authentication type such as “skey” or “radius”.
- Specify a login class. You may only override the default class if you're already root.
- If the invoked shell is csh(1), this option prevents it from reading the “.cshrc” file.
- This is shorthand for “
-apasswd”, provided for backwards compatibility.
- Loop until a correct username and password combination is entered, similar
to login(1). Note that in this mode target login
must be specified explicitly, either on the command line or interactively.
suwill prompt for the password even when invoked by root.
- Simulate a full login. The shell of the target login is invoked and the
current working directory is changed to the home directory of the target
USERare set to the default values for the target login.
PATHand the umask(2) value are set according to login.conf(5). Except for preserving
TERM, the rest of the environment is discarded.
- Leave the environment unmodified. The login shell of the invoking user is
started, and the current working directory is not changed. As a security
precaution, if the target user's shell is a non-standard shell (as defined
getusershell(3)) and the caller's real UID is non-zero,
- Specify the path to an alternate login shell. You may only override the
shell if you're already root. This option will override the shell even if
-moption is specified.
options are mutually exclusive; the last one specified overrides any
If the optional shell arguments are provided
on the command line, they are passed to the login shell of the target login.
This allows it to pass arbitrary commands via the
option as understood by most shells. Note that
usually expects a single argument only; you have to quote it when passing
If group 0 (normally “wheel”) has users listed then
only those users can
su to “root”. It
is not sufficient to change a user's /etc/passwd
entry to add them to the “wheel” group; they must explicitly
be listed in /etc/group. If no one is in the
“wheel” group, it is ignored, and anyone who knows the root
password is permitted to
The following list provides the values of environment variables in
the new shell that is started by
- The home directory of the target login, except that it remains unchanged
- The target login by default, but unchanged if the target login has a UID
of 0 or if
- The search path. It remains unchanged by default, but is set according to
the target login if the target login has a UID of 0 or if
- The current working directory. It remains unchanged by default, but is set
to the home directory of the target login with
- The new shell that is started. It is the shell of the target login by
default, but the shell of the invoking user with
- The terminal type. It is always retained from the invoking process.
- Same as
Run the command “makewhatis” as user “bin”. You will be asked for bin's password unless your real UID is 0.
$ su bin -c makewhatis
Same as above, but the target command consists of more than a single word:
$ su bin -c 'makewhatis /usr/local/man'
Same as above, but the target command is run with the resource
limits of the login class “staff”. Note that the first
-c option applies to
while the second is an argument to the shell.
$ su -c staff bin -c 'makewhatis /usr/local/man'
Pretend a login for user “foo”:
$ su -l foo
Same as above, but use S/Key for authentication:
$ su -a skey -l foo
doas(1), login(1), setusercontext(3), group(5), login.conf(5), passwd(5), environ(7)
su command first appeared in
Version 1 AT&T UNIX.
The login name is not optional for root if there are shell arguments.