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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 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
       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.

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:

           •   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 (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:

           •   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

       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.

       --session-command=command
           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
       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

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

REPORTING BUGS
       For bug reports, use the issue tracker at https://github.com/karelzak/util-linux/issues.

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/>.



util-linux 2.37.2                            2021-06-02                                        SU(1)