SU(1) User Commands SU(1)
su - run a command with substitute user and group ID
su [options] [-] [user [argument...]]
su allows to run commands with a substitute user and group ID.
When called without arguments, su defaults to running an interactive
shell as root.
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 recom-
mended 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 implementa-
tions, such as support for a wheel group, have to be configured via
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
provide separate PAM configuration. If the PAM session is not required
at all then the recommend solution is to use command setpriv(1).
Pass command to the shell with the -c option.
Pass -f to the shell, which may or may not be useful, depending
on the shell.
Specify the primary group. This option is available to the root
Specify a supplemental 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 unspecified.
-, -l, --login
Start the shell as a login shell with an environment similar to
a real login:
o clears all the environment variables except TERM
o initializes the environment variables HOME, SHELL,
USER, LOGNAME, and PATH
o changes to the target user's home directory
o sets argv of the shell to '-' in order to make the
shell a login shell
-m, -p, --preserve-environment
Preserve the entire environment, i.e. it does not set HOME,
SHELL, USER nor LOGNAME. This option is ignored if the option
--login is specified.
Create pseudo-terminal for the session. The independent terminal
provides better security as user does not share terminal with
the original session. This allow to avoid TIOCSTI ioctl termi-
nal injection and another security attacks against terminal file
descriptors. The all session is also possible to move to back-
ground (e.g. "su --pty - usename -c application &"). If the
pseudo-terminal is enabled then su command works as a proxy
between the sessions (copy stdin and stdout).
This feature is EXPERIMENTAL for now and may be removed in the
Run the specified shell instead of the default. The shell to
run is selected according to the following rules, in order:
o the shell specified with --shell
o the shell specified in the environment variable SHELL,
if the --preserve-environment option is used
o the shell listed in the passwd entry of the target
If the target user has a restricted shell (i.e. not listed in
/etc/shells), the --shell option and the SHELL environment vari-
ables are ignored unless the calling user is root.
Same as -c but do not create a new session. (Discouraged.)
Display version information and exit.
Display help text and exit.
Upon receiving either SIGINT, SIGQUIT or SIGTERM, su terminates its
child and afterwards terminates itself with the received signal.
su reads the /etc/default/su and /etc/login.defs configuration files.
The following configuration items are relevant for su(1):
Delay in seconds in case of an authentication failure. The number
must be a non-negative integer.
Defines the PATH environment variable for a regular user. The
default value is /usr/local/bin:/bin:/usr/bin.
Defines the PATH environment variable for root. The default value
If set to yes and --login and --preserve-environment were not spec-
ified su initializes PATH.
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
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
allows to control su behavior by PAM configuration. If you want to use
the pam_lastlog module to print warning message about failed log-in
attempts then the pam_lastlog has to be configured to update the last-
log file as well. For example by:
session required pam_lastlog.so nowtmp
setpriv(1), login.defs(5), shells(5), pam(8), runuser(8)
This su command was derived from coreutils' su, which was based on an
implementation by David MacKenzie. The util-linux has been refactored
by Karel Zak.
The su command is part of the util-linux package and is available from
Linux Kernel Archive <https://www.kernel.org/pub/linux/utils/util-
util-linux July 2014 SU(1)