# su

## NAME



           su - run a command with substitute user and group ID
    


## SYNOPSIS



           **su**[options] [**-**] [_user_ [_argument_...]]
    


## DESCRIPTION



           **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
           _argument_ s can be supplied, in which case they are passed to the
           shell.
    
           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,
           RLIMIT_RTPRIO, RLIMIT_FSIZE, RLIMIT_AS and RLIMIT_NOFILE.
    


## 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 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
               specified.
    
           **-** , **-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** ,
                   **COLORTERM** , **NO_COLOR** 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 -**_user_ **-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
    
           **--session-command=**_command_
               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**
               Display version 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 seconds 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** :
    
           **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:/sbin:/bin:/usr/sbin:/usr/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., pam_env(8)).
    


## 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 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 pam_lastlog.so nowtmp
    


## AVAILABILITY



           The **su** command is part of the util-linux package which can be
           downloaded from Linux Kernel Archive
           <https://www.kernel.org/pub/linux/utils/util-linux/>. This page is
           part of the _util-linux_ (a random collection of Linux utilities)
           project. Information about the project can be found at 
           ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have a
           bug report for this manual page, send it to
           util-linux@vger.kernel.org. This page was obtained from the
           project's upstream Git repository
           ⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
           2025-08-11. (At that time, the date of the most recent commit that
           was found in the repository was 2025-08-05.) 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 man-pages@man7.org
    
    util-linux 2.42-start-521-ec46  2025-08-09                          _SU_(1)
    


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