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 to run commands with a substitute user and group ID.
When called with no user specified, su defaults to running an interac-
tive 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 di-
rectory 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 implementa-
tions, 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
provide separate PAM configuration. If the PAM session is not required
at all then the recommend solution is to use command setpriv(1).
Note that su in all cases use PAM (pam_getenvlist()) to do final envi-
ronment modification. The command line options like --login or --pre-
serve-environment affect environment before it's 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 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 and
variables specified by --whitelist-environment
o initializes the environment variables HOME, SHELL,
USER, LOGNAME, and PATH
o changes to the target user's home directory
o 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., it does not set HOME,
SHELL, USER nor LOGNAME. This option is ignored if the option
--login is specified.
-P, --pty
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 - username -c application &"). If the
pseudo-terminal is enabled then su command works as a proxy be-
tween 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 pipe (e.g.,
echo "date" | su --pty) than 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:
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
user
o /bin/sh
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.
--session-command=command
Same as -c but do not create a new session. (Discouraged.)
-w, --whitelist-environment=list
Don't reset environment variables specified in comma separated
list when clears environment for --login. The whitelist is ig-
nored 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 sec-
onds 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(1):
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 de-
fault 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 spec-
ified 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 --lo-
gin command line option and PAM system setting (e.g. pam_env).
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
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 at-
tempts then the pam_lastlog has to be configured to update the lastlog
file as well. For example by:
session required pam_lastlog.so nowtmp
SEE ALSO
setpriv(1), login.defs(5), shells(5), pam(8), runuser(1)
HISTORY
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.
AVAILABILITY
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-
linux/>.
util-linux July 2014 SU(1)