SU(1)                           User Commands                          SU(1)

NAME         top

su – run a command with substitute user and group ID

SYNOPSIS         top

su [options] [-] [user [argument…]]

DESCRIPTION         top

su allows to run commands with a substitute user and group ID.

When called without arguments, su defaults to running an interactive
shell as root.

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.

OPTIONS         top

-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.

-, -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

o      initializes the environment variables HOME, SHELL,

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.

-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

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.

Same as -c but do not create a new session.  (Discouraged.)

-V, –version
Display version information and exit.

-h, –help
Display help text and exit.

SIGNALS         top

Upon receiving either SIGINT, SIGQUIT or SIGTERM, su terminates its
child and afterwards terminates itself with the received signal.

CONFIG FILES         top

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_SUPATH (string)
Defines the PATH environment variable for root.  The default
value is /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:

If set to yes and –login and –preserve-environment were not
specified su initializes PATH.

EXIT STATUS         top

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         top

/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         top

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 configured to
update the lastlog file as well. For example by:

session  required nowtmp

SEE ALSO         top

runuser(8), pam(8), shells(5), login.defs(5)

HISTORY         top

This su command was derived from coreutils’ su, which was based on an
implementation by David MacKenzie.

AVAILABILITY         top

The su command is part of the util-linux package and is available
from Linux Kernel Archive

COLOPHON         top

This page is part of the util-linux (a random collection of Linux
utilities) project.  Information about the project can be found at
⟨⟩.  If you have a
bug report for this manual page, send it to  This page was obtained from the
project’s upstream Git repository
(git:// on
2014-12-30.  If you discover any rendering problems in this HTML ver‐
sion of the page, or you believe there is a better or more up-to-date
source for the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original man‐
ual page), send a mail to

util-linux                        July 2014                            SU(1)


Author: Jeg er en professionel system administrator og grundlægger af Jeg er en ivrig Linux-elsker og open source-entusiast. Jeg bruger Ubuntu og tror på at dele viden. Bortset fra Linux, elsker musik og dyr. Jeg er en stor fan af Dire straits.

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