Manual: su

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.

       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 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

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 .

AVAILABILITY
       The su command is part of the util-linux package which can be downloaded from Linux Kernel Archive
       .

util-linux 2.41.3					 2025-12-15						       SU(1)