su(1) — Linux manual page


SU(1)                           User Commands                          SU(1)

NAME         top

       su - run a command with substitute user and group ID

SYNOPSIS         top

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

DESCRIPTION         top

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

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

       For backward compatibility, su defaults to not change the current
       directory 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
       implementations, 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(3)) to do the final
       environment modification.  Command-line options such as --login and
       --preserve-environment affect the environment before it is modified
       by PAM.

OPTIONS         top

       -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 supplementary 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 not

       -, -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., do not set HOME, SHELL,
              USER or LOGNAME.  This option is ignored if the option --login
              is specified.

       -P, --pty
              Create a pseudo-terminal for the session. The independent
              terminal provides better security as the user does not share a
              terminal with the original session.  This can be used to avoid
              TIOCSTI ioctl terminal injection and other security attacks
              against terminal file descriptors.  The entire session can
              also be moved to the background (e.g., "su --pty - username -c
              application &").  If the pseudo-terminal is enabled, then su
              works as a proxy between 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 a pipe
              (e.g., echo "date" | su --pty), then the 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

                 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
              variables are ignored unless the calling user is root.

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

       -w, --whitelist-environment=list
              Don't reset the environment variables specified in the comma-
              separated list when clearing the environment for --login. The
              whitelist is ignored 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         top

       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
       seconds of delay the child is killed by SIGKILL.

CONFIG FILES         top

       su reads the /etc/default/su and /etc/login.defs configuration files.
       The following configuration items are relevant for su:

       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
           default 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:

       ALWAYS_SET_PATH (boolean)
           If set to yes and --login and --preserve-environment were not
           specified 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 the
       --login command-line option and the PAM system setting (e.g.,

EXIT STATUS         top

       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         top

       /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         top

       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 can be used to control su behavior by PAM configuration.  If
       you want to use the pam_lastlog(8) module to print warning message
       about failed log-in attempts then pam_lastlog(8) has to be configured
       to update the lastlog file as well. For example by:

              session  required nowtmp

HISTORY         top

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

SEE ALSO         top

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

AVAILABILITY         top

       The su command is part of the util-linux package and is available
       from Linux Kernel Archive 

COLOPHON         top

       This page is part of the util-linux (a random collection of Linux
       utilities) project.  Information about the project can be found at 
       ⟨⟩.  If you have a
       bug report for this manual page, send it to  This page was obtained from the
       project's upstream Git repository
       ⟨git://⟩ on
       2020-07-14.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-07-14.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

util-linux                        July 2014                            SU(1)

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