SU(1)

NAME
su - substitute user identity

SYNOPSIS
su [-fKLlm] [-a auth-type] [-c login-class] [-s login-shell] [login [shell arguments]]

DESCRIPTION
The 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.

If Kerberos is in use, 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 access. A shell is then executed, and any additional shell arguments after the login name are passed to the shell. If Kerberos is not configured or if there is a Kerberos error, su falls back to local password authentication to validate the password for login. 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.

By default, the environment is unmodified with the exception of LOGNAME, HOME, SHELL, and USER. 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 and the -l flag was not specified, in which case it is unmodified. The invoked shell is the target login's. This is the traditional behavior of su.

If not using -m and the target login has a user ID of 0 then the PATH variable and umask value (see umask(2)) are always set according to the /etc/login.conf file (see login.conf(5)).

The options are as follows:

-   Same as the -l option (deprecated).

-a auth-type
    Specify an authentication type such as “skey”, “securid”, or “krb5”.

-c login-class
    Specify a login class. You may only override the default class if you're already root.

-f  If the invoked shell is csh(1), this option prevents it from reading the “.cshrc” file.

-K  Do not attempt to use Kerberos to authenticate the user. This is shorthand for “su -a passwd”, provided for backwards compatibility.

-L  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. Additionally, su will prompt for the password even when invoked by root.

-l  Simulate a full login. The environment is discarded except for HOME, SHELL, PATH, TERM, LOGNAME, and USER. HOME and SHELL are modified as above. LOGNAME and USER are set to the target login. PATH is set to the value specified by the “path” entry in login.conf(5). 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.

-s login-shell
    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 the -m option is specified.

The -l and -m options are mutually exclusive; the last one specified overrides any previous ones.

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 -c option as understood by most shells. Note that -c usually expects a single argument only; you have to quote it when passing multiple words.

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 su to “root”.

By default (unless the prompt is reset by a startup file) the superuser prompt is set to “#” to remind one of its awesome power.

ENVIRONMENT

HOME
    Default home directory of real user ID unless modified as specified above.

LOGNAME
    The user ID is always the effective ID (the target user ID) after an su unless the user ID is 0 (root).

PATH
    Default search path of real user ID unless modified as specified above.

TERM
    Provides terminal type which may be retained for the substituted user ID.

USER
    Same as LOGNAME.

EXAMPLES
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 su 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

SEE ALSO
csh(1), kinit(1), login(1), sh(1), skey(1), setusercontext(3), group(5), login.conf(5), passwd(5), environ(7), sudo(8)

HISTORY
A su command appeared in AT&T UNIX.

BUGS
The login name is not optional for root if there are shell arguments.