Ubuntu Manpage: su - run a command with substitute user and group ID

Provided by: util-linux_2.40.2-1ubuntu1_amd64

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.

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 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. 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
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.

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/>.