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:
• 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 (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. 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.
-h, --help
Display help text and exit.
-V, --version
Print 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 at
https://github.com/util-linux/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 2.38.1 2022-05-11 SU(1)
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