SU(1)



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 with no user specified, su defaults to running an interac-
       tive shell as root.  When user is specified, additional  arguments  can
       be supplied, in which case they are passed to the shell.

       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).

       Note that su in all cases use PAM (pam_getenvlist()) to do final  envi-
       ronment  modification.  The command line options like --login or --pre-
       serve-environment affect environment before it's modified by PAM.

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, this variable is also affected by --lo-
       gin command line option and PAM system setting (e.g. pam_env).

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(1)

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)

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