su allows one user to become another user
login without logging out and in as the new user. If a
group is specified and
login is a member of
group, then the group is changed to
group rather than to
login's primary group. If
login is omitted and
group is provided (form two above), then
login is assumed to be the current username.
When executed by a user, the
login user's password is requested. When using Kerberos, the password for
login (or for “
login.root”, if no login is provided) is requested, and
su switches to that user and group ID after obtaining a Kerberos ticket granting ticket. A shell is then executed, and any additional
shell arguments after the login name are passed to the shell.
su will resort to the local password file to find the password for
login if there is a Kerberos error. If
su is executed by root, no password is requested and a shell with the appropriate user ID is executed; no additional Kerberos tickets are obtained.
Alternatively, if the user enters the password "s/key", authentication will use the S/Key one-time password system as described in
skey(1). S/Key is a Trademark of Bellcore.
By default, the environment is unmodified with the exception of
LOGNAME,
USER,
HOME,
SHELL, and
SU_FROM.
HOME and
SHELL are set to the target login's default values.
LOGNAME and
USER are set to the target login, unless the target login has a user ID of 0, in which case they are unmodified.
SU_FROM is set to the caller's login. The invoked shell is the target login's. With the exception of
SU_FROM this is the traditional behavior of
su.
The options are as follows:
-c
Specify a login class. You may only override the default class if you're already root. See
login.conf(5) for details.
-d
Same as -l, but does not change the current directory.
-f
If the invoked shell is
csh(1), this option prevents it from reading the “
.cshrc” file. If the invoked shell is
sh(1), or
ksh(1), this option unsets
ENV, thus preventing the shell from executing the startup file pointed to by this variable.
-K
Do not attempt to use Kerberos to authenticate the user.
-l
Simulate a full login. The environment is discarded except for HOME, SHELL, PATH, TERM, LOGNAME, USER, and SU_FROM. HOME, SHELL, and SU_FROM are modified as above. LOGNAME and USER are set to the target login. PATH is set to the path specified in the /etc/login.conf file (or to the default of “/usr/bin:/bin:/usr/pkg/bin:/usr/local/bin” ). TERM is imported from your current environment. The invoked shell is the target login's, and su will change directory to the target login's home directory.
-m
Leave the environment unmodified. The invoked shell is your login shell, and no directory changes are made. As a security precaution, if the target user's shell is a non-standard shell (as defined by
getusershell(3)) and the caller's real uid is non-zero,
su will fail.
The
-l and
-m options are mutually exclusive; the last one specified overrides any previous ones.
Only users in group “wheel” (normally gid 0), as listed in
/etc/group, can
su to “root”, unless group wheel does not exist or has no members. (If you do not want anybody to be able to
su to “root”, make “root” the only member of group “wheel”, which is the default.)
For sites with very large user populations, group “wheel” can contain the names of other groups that will be considered authorized to
su to “root”.
By default (unless the prompt is reset by a startup file) the super-user prompt is set to “
#” to remind one of its awesome power.