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 provides separate PAM configuration. If the
       PAM session is not required at all then the recommended solution
       is to use command setpriv(1).

       Note that su in all cases uses 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.

       Since version 2.38 su resets process resource limits RLIMIT_NICE,

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.

           Note that on systemd-based systems, a new session may be
           defined as a real entry point to the system. However, su does
           not create a real session (by PAM) from this point of view.
           You need to use tools like systemd-run or machinectl to
           initiate a complete, real session.

           su does:

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

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

           •   changes to the target user’s home directory

           •   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 (sync
           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. 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.

           The shell to run is selected according to the following
           rules, in order:

           •   the shell specified with --shell

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

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

           •   /bin/sh

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

       -T, --no-pty*
           Do not create a pseudo-terminal, opposite of --pty and -P.
           Note that running without a pseudo-terminal opens the
           security risk of privilege escalation through
           TIOCSTI/TIOCLINUX ioctl command injection.

       -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.

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

       -V, --version
           Print version 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

       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., pam_env(8)).

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:

           Generic error before executing the requested command

           The requested command could not be executed

           The requested command was not found

FILES         top

           default PAM configuration file

           PAM configuration file if --login is specified

           command specific logindef config file

           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)

REPORTING BUGS         top

       For bug reports, use the issue tracker at

AVAILABILITY         top

       The su command is part of the util-linux package which can be
       downloaded from Linux Kernel Archive
       <>. 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
       2024-06-14. (At that time, the date of the most recent commit
       that was found in the repository was 2024-06-10.) 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 2.41.devel-537-e... 2024-04-04                          SU(1)

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