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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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Another version of this page is provided by the shadow-utils project
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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | SIGNALS | CONFIG FILES | EXIT STATUS | FILES | NOTES | HISTORY | SEE ALSO | REPORTING BUGS | AVAILABILITY |
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SU(1) User Commands SU(1)
su - run a command with substitute user and group ID
su [options] [-] [user [argument...]]
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.
-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.
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.
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)).
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
/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
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
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.
setpriv(1), login.defs(5), shells(5), pam(8), runuser(1)
For bug reports, use the issue tracker
<https://github.com/util-linux/util-linux/issues>.
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)
Pages that refer to this page: flock(1), homectl(1), login(1), login(1@@shadow-utils), machinectl(1), newgrp(1), runuser(1), setpriv(1), sg(1), updatedb(1), pam(3), pts(4), crontab(5), login.defs(5), passwd(5), passwd(5@@shadow-utils), shadow(5), suauth(5), credentials(7), environ(7), PAM(8), pam_rootok(8), pam_xauth(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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