Section (1) su
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
LOGNAME if the target
user is not root). It is
recommended to always use the
−−login option (instead of its
−) 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).
Pass command to the shell with the
−fto the shell, which may or may not be useful, depending on the shell.
Specify the primary group. This option is available to the root user only.
Specify a supplemental 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
−, −l, −−login
Start the shell as a login shell with an environment similar to a real login:
clears all the environment variables except
TERMand variables specified by
initializes the environment variables
changes to the target user_zsingle_quotesz_s home directory
sets argv of the shell to _zsingle_quotesz_
−_zsingle_quotesz_ in order to make the shell a login shell
−m, −p, −−preserve−environment
Preserve the entire environment, i.e. it does not set
LOGNAME. This option is ignored if the option
Create pseudo-terminal for the session. The independent terminal provides better security as user does not share terminal with the original session. This allow to avoid TIOCSTI ioctl terminal injection and another security attacks against terminal file descriptors. The all session is also possible to move to background (e.g. su --pty - username -c application &). If the pseudo-terminal is enabled then su command works as a proxy between the sessions (copy stdin and stdout).
This feature is EXPERIMENTAL for now and may be removed in the next releases.
Run the specified shell instead of the default. The shell to run is selected according to the following rules, in order:
the shell specified with
the shell specified in the environment variable
SHELL, if the
−−preserve−environmentoption 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
−−shelloption and the
SHELLenvironment variables are ignored unless the calling user is root.
−cbut do not create a new session. (Discouraged.)
Don_zsingle_quotesz_t reset environment variables specified in comma separated list when clears environment for
−−login. The whitelist is ignored for the environment variables
Display version information and exit.
Display help text and exit.
Upon receiving either
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
files. The following configuration items are relevant for
Delay in seconds in case of an authentication failure. The number must be a non-negative integer.
Defines the PATH environment variable for a regular user. The default value is
Defines the PATH environment variable for root. The default value is
If set to yes and −−login and −−preserve−environment were not specified su initializes
The environment variable PATH may be different on systems where /bin and /sbin are merged into /usr.
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 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 the lastlog file as well. For example by:
session required pam_lastlog.so nowtmp
This su command was derived from coreutils_zsingle_quotesz_ su, which was based on an implementation by David MacKenzie. The util-linux has been refactored by Karel Zak.
The su command is part of the util-linux package and is available from Linux Kernel Archive