su — run a command with substitute user and group ID


su [options] [] [ user [ argument... ] ]


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 of a wheel group, have to be configured via PAM.


−c, −−command=command

Pass command to the shell with the −c option.


Same as −c but do not create a new session (discouraged).

−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 for the root user only.

−G, −−supp−group=group

Specify a supplemental group. This option is available for the root user only.

, −l, −−login

Starts the shell as login shell with an environment similar to a real login:


clears all environment variables except TERM


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

Preserves the whole 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

Runs the specified shell instead of the default. 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



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.


Display help text and exit.


Display version information and exit.


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


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 authentication failure. 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:/usr/bin.


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


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:


Generic error before executing the requested command


The requested command could not be executed


The requested command could was not found



default PAM configuration file


PAM configuration file if −−login is specified


command specific logindef config file


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 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 lastlog file too. For example by:

session required nowtmp


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


Derived from coreutils' su which was based on an implementation from David MacKenzie.


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