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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
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 ses‐
sion 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 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
environment modification. The command line options like --login
or --preserve-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 simi‐
lar to a real login:
o clears all the environment variables except TERM
and variables specified by --whitelist-environ‐
ment
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 terminal injection and another security
attacks against terminal file descriptors. The all session
is also possible to move to background (e.g., "su --pty -
username -c application &"). If the pseudo-terminal is
enabled then su command works as a proxy between the ses‐
sions (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 tar‐
get 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 environ‐
ment 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 environment variables specified in comma sepa‐
rated list when clears environment for --login. The
whitelist is ignored 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 sig‐
nal. 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(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
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 --login 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 com‐
mand
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 attempts then the pam_lastlog has to be con‐
figured 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)