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=commandPass command to the shell
with the −c
option.
−−session−command=commandSame as −c but do
not create a new session (discouraged).
−f, −−fastPass −f to the
shell, which may or may not be useful, depending on the
shell.
−g, −−group=groupSpecify the primary group. This option is available for the root user only.
−G, −−supp−group=groupSpecify a supplemental group. This option is available for the root user only.
−, −l, −−loginStarts the shell as login shell with an environment similar to a real login:
oclears all environment variables except
TERMoinitializes the environment variables
HOME,SHELL,USER,LOGNAME, andPATHochanges to the target user's home directory
osets argv[0] of the shell to '
−' in order to make the shell a login shell
−m, −p, −−preserve−environmentPreserves 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=shellRuns the specified shell instead of the default. The shell to run is selected according to the following rules, in order:
othe shell specified with
−−shellothe shell specified in the environment variable
SHELL, if the−−preserve−environmentoption is usedothe 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.
−−helpDisplay help text and exit.
−−versionDisplay 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_ROOTPATH (string)
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.
ALWAYS_SET_PATH
(boolean)
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:
1Generic error before executing the requested command
126The requested command could not be executed
127The requested command could was not found
/etc/pam.d/sudefault PAM configuration file
/etc/pam.d/su-lPAM configuration file if −−login is specified
/etc/default/sucommand specific logindef config file
/etc/login.defsglobal 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 pam_lastlog.so nowtmp
The su command is part of the util-linux package and is available from Linux Kernel Archive
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