su - run a command with substitute user and group ID
su [options] [-] [user
[argument...]]
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 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(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.
- -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:
- 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., 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 (copy 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:
- 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
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.
- -V, --version
- Display version information and exit.
- -h, --help
- Display help text and exit.
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.
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)).
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
- /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 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
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.