su

SU(1)                            User Commands                           SU(1)

NAME
       su - run a command with substitute user and group ID

SYNOPSIS
       su [options] [-] [user [argument...]]

DESCRIPTION
       su allows to run commands with a substitute user and group ID.

       When  called  without  arguments, su defaults to running an interactive
       shell as root.

       For backward compatibility, su defaults to not change the  current  di-
       rectory  and to only set the environment variables HOME and SHELL (plus
       USER and LOGNAME if the target user is not root).  It is recommended to
       always use the --login option (instead of its shortcut -) to avoid side
       effects caused by mixing environments.

       This version of su uses PAM for  authentication,  account  and  session
       management.   Some  configuration options found in other su implementa-
       tions, such as support for a wheel group, have  to  be  configured  via
       PAM.

       su  is mostly designed for unprivileged users, the recommended solution
       for privileged users (e.g. scripts executed by root) is to use non-set-
       user-ID  command  runuser(1)  that  does not require authentication and
       provide separate PAM configuration. If the PAM session is not  required
       at all then the recommend solution is to use command setpriv(1).

OPTIONS
       -c, --command=command
              Pass command to the shell with the -c option.

       -f, --fast
              Pass  -f to the shell, which may or may not be useful, depending
              on the shell.

       -g, --group=group
              Specify the primary group.  This option is available to the root
              user only.

       -G, --supp-group=group
              Specify  a  supplemental group.  This option is available to the
              root user only.  The first specified supplementary group is also
              used as a primary group if the option --group is unspecified.

       -, -l, --login
              Start  the shell as a login shell with an environment similar to
              a real login:

                 o      clears all the environment variables except  TERM  and
                        variables specified by --whitelist-environment

                 o      initializes  the  environment  variables  HOME, SHELL,
                        USER, LOGNAME, and PATH

                 o      changes to the target user's home directory

                 o      sets argv[0] of the shell to '-' in order to make  the
                        shell a login shell

       -m, -p, --preserve-environment
              Preserve  the  entire  environment,  i.e.  it does not set HOME,
              SHELL, USER nor LOGNAME.  This option is ignored if  the  option
              --login is specified.

       -P, --pty
              Create pseudo-terminal for the session. The independent terminal
              provides better security as user does not  share  terminal  with
              the  original session.  This allow to avoid TIOCSTI ioctl termi-
              nal injection and another security attacks against terminal file
              descriptors.  The  all session is also possible to move to back-
              ground (e.g. "su --pty - username -c  application  &").  If  the
              pseudo-terminal  is enabled then su command works as a proxy be-
              tween the sessions (copy stdin and stdout).

              This feature is mostly designed for interactive sessions. If the
              standard  input  is  not  a terminal, but for example pipe (e.g.
              echo "date" | su --pty) than ECHO flag for  the  pseudo-terminal
              is disabled to avoid messy output.

       -s, --shell=shell
              Run  the  specified  shell instead of the default.  The shell to
              run is selected according to the following rules, in order:

                 o      the shell specified with --shell

                 o      the shell specified in the environment variable SHELL,
                        if the --preserve-environment option is used

                 o      the  shell  listed  in  the passwd entry of the target
                        user

                 o      /bin/sh

              If the target user has a restricted shell (i.e.  not  listed  in
              /etc/shells), the --shell option and the SHELL environment vari-
              ables are ignored unless the calling user is root.

       --session-command=command
              Same as -c but do not create a new session.  (Discouraged.)

       -w, --whitelist-environment=list
              Don't reset environment variables specified in  comma  separated
              list  when  clears environment for --login. The whitelist is ig-
              nored for the environment variables HOME, SHELL, USER,  LOGNAME,
              and PATH.

       -V, --version
              Display version information and exit.

       -h, --help
              Display help text and exit.

SIGNALS
       Upon  receiving  either  SIGINT,  SIGQUIT or SIGTERM, su terminates its
       child and afterwards terminates itself with the received  signal.   The
       child  is  terminated by SIGTERM, after unsuccessful attempt and 2 sec-
       onds of delay the child is killed by SIGKILL.

CONFIG FILES
       su reads the /etc/default/su and /etc/login.defs  configuration  files.
       The following configuration items are relevant for su(1):

       FAIL_DELAY (number)
           Delay  in  seconds in case of an authentication failure. The number
           must be a non-negative integer.

       ENV_PATH (string)
           Defines the PATH environment variable for a regular user.  The  de-
           fault value is /usr/local/bin:/bin:/usr/bin.

       ENV_ROOTPATH (string)
       ENV_SUPATH (string)
           Defines  the  PATH environment variable for root.  ENV_SUPATH takes
           precedence.  The default value  is  /usr/local/sbin:/usr/local/bin:
           /sbin:/bin:/usr/sbin:/usr/bin.

       ALWAYS_SET_PATH (boolean)
           If set to yes and --login and --preserve-environment were not spec-
           ified su initializes PATH.

       The environment variable PATH may be different on  systems  where  /bin
       and /sbin are merged into /usr.

EXIT STATUS
       su normally returns the exit status of the command it executed.  If the
       command was killed by a signal, su returns the  number  of  the  signal
       plus 128.

       Exit status generated by su itself:

                 1      Generic error before executing the requested command

                 126    The requested command could not be executed

                 127    The requested command was not found

FILES
       /etc/pam.d/su    default PAM configuration file
       /etc/pam.d/su-l  PAM configuration file if --login is specified
       /etc/default/su  command specific logindef config file
       /etc/login.defs  global logindef config file

NOTES
       For  security reasons su always logs failed log-in attempts to the btmp
       file, but it does not write to the lastlog file at all.  This  solution
       allows to control su behavior by PAM configuration.  If you want to use
       the pam_lastlog module to print warning message about failed log-in at-
       tempts  then the pam_lastlog has to be configured to update the lastlog
       file as well. For example by:

              session  required  pam_lastlog.so nowtmp

SEE ALSO
       setpriv(1), login.defs(5), shells(5), pam(8), runuser(8)

HISTORY
       This su command was derived from coreutils' su, which was based  on  an
       implementation  by  David MacKenzie. The util-linux has been refactored
       by Karel Zak.

AVAILABILITY
       The su command is part of the util-linux package and is available  from
       Linux   Kernel   Archive  <https://www.kernel.org/pub/linux/utils/util-
       linux/>.

util-linux                         July 2014                             SU(1)