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Manual Pages  — SU


su – substitute user identity



su [-] [-c class] [-flms] [login [args]]


The su utility requests appropriate user credentials via PAM and switches to that user ID (the default user is the superuser). A shell is then executed.

PAM is used to set the policy su(1) will use. In particular, by default only users in the "wheel" group can switch to UID 0 ("root"). This group requirement may be changed by modifying the "pam_group" section of /etc/pam.d/su. See pam_group(8) for details on how to modify this setting.

By default, the environment is unmodified with the exception of USER, HOME, and SHELL. HOME and SHELL are set to the target login's default values. USER is set to the target login, unless the target login has a user ID of 0, in which case it is unmodified. The invoked shell is the one belonging to the target login. This is the traditional behavior of su. Resource limits and session priority applicable to the original user's login class (see login.conf(5)) are also normally retained unless the target login has a user ID of 0.

The options are as follows:
-c class
  Use the settings of the specified login class. The login class must be defined in login.conf(5). Only allowed for the super-user.
  If the invoked shell is csh(1), this option prevents it from reading the " .cshrc" file.
  Simulate a full login. The environment is discarded except for HOME, SHELL, PATH, TERM, and USER. HOME and SHELL are modified as above. USER is set to the target login. PATH is set to " /bin:/usr/bin". TERM is imported from your current environment. Environment variables may be set or overridden from the login class capabilities database according to the class of the target login. The invoked shell is the target login's, and su will change directory to the target login's home directory. Resource limits and session priority are modified to that for the target account's login class.
- (no letter) The same as -l.
  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.
  Set the MAC label to the user's default label as part of the user credential setup. Setting the MAC label may fail if the MAC label of the invoking process is not sufficient to transition to the user's default MAC label. If the label cannot be set, su will fail.

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

If the optional args are provided on the command line, they are passed to the login shell of the target login. Note that all command line arguments before the target login name are processed by su itself, everything after the target login name gets passed to the login shell.

By default (unless the prompt is reset by a startup file) the super-user prompt is set to " amp;#" to remind one of its awesome power.


Environment variables used by su:
HOME Default home directory of real user ID unless modified as specified above.
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).


  PAM configuration for su.


su -m operator -c poweroff
  Starts a shell as user operator, and runs the command poweroff. You will be asked for operator's password unless your real UID is 0. Note that the -m option is required since user "operator" does not have a valid shell by default. In this example, -c is passed to the shell of the user "operator", and is not interpreted as an argument to su.
su -m operator -c 'shutdown -p now'
  Same as above, but the target command consists of more than a single word and hence is quoted for use with the -c option being passed to the shell. (Most shells expect the argument to -c to be a single word).
su -m -c staff operator -c 'shutdown -p now'
  Same as above, but the target command is run with the resource limits of the login class "staff". Note: in this example, the first -c option applies to su while the second is an argument to the shell being invoked.
su -l foo
  Simulate a login for user foo.
su - foo
  Same as above.
su - Simulate a login for root.


csh(1), sh(1), group(5), login.conf(5), passwd(5), environ(7), pam_group(8)


A su command appeared in AT&T v1 .

SU (1) March 26, 2020

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