SU(1)

NAME
su - substitute user identity

SYNOPSIS
su [-Kdflm] [-c login-class] [ login[:group] [shell arguments]]
su [-Kdflm] [-c login-class] [ :group [shell arguments]]

DESCRIPTION
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:

-K  Do not attempt to use Kerberos to authenticate the user.

-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.

-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.

-   Same as -l.

-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.

CUSTOMIZATION

Changing required group
    For the pam(8) version of su the name of the required group can be changed by setting gname in pam.conf(5):

        auth requisite pam_group.so no_warn group=gname root_only fail_safe

    For the non pam(8) version of su the same can be achieved by compiling with SU_GROUP set to the desired group name.

Supplying own password
    su can be configured so that users in a particular group can supply their own password to become “root”. For the pam(8) version of su this can be done by adding a line to pam.conf(5) such as:

        auth sufficient pam_group.so no_warn group=gname root_only authenticate

    where gname is the name of the desired group. For the non pam(8) version of su the same can be achieved by compiling with SU_ROOTAUTH set to the desired group name.

Indirect groups
    This option is not available with the pam(8) version of su. For the non pam(8) version of su, if SU_INDIRECT_GROUP is defined, the SU_GROUP and SU_ROOTAUTH groups are treated as indirect groups. The group members of those two groups are treated as groups themselves.

EXIT STATUS
su returns the exit status of the executed subshell, or 1 if any error occurred while switching privileges.

ENVIRONMENT
Environment variables used by su:

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

EXAMPLES
To become user username and use the same environment as in original shell, execute:

    su username

To become user username and use environment as if full login would be performed, execute:

    su -l username

When a -c option is included after the login name it is not a su option, because any arguments after the login are passed to the shell. (See csh(1), ksh(1) or sh(1) for details.) To execute arbitrary command with privileges of user username, execute:

    su username -c "command args"

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

HISTORY
A su command existed in AT&T UNIX (and probably earlier).