su man page

su — run a command with substitute user and group ID

TL;DR

su {{username}}

su

su - {{username}}

Synopsis

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

Description

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.

su is mostly designed for unprivileged users, the recommended solution for privileged users (e.g. scripts executed by root) is to use non-suid 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).

Options

-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. The first specified supplementary group is also used as a primary group if the option --group is unspecified.
-, -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, 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. 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 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.)
-V, --version
Display version information and exit.
-h, --help
Display help text and exit.

Signals

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

Config Files

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_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.

Exit Status

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

/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

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

See Also

runuser(8), setpriv(1), pam(8), shells(5), login.defs(5)

History

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

Availability

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

Referenced By

at(1), ccze(1), credentials(7), crontab(5), dchroot(1), debuild(1), firehol(1), flock(1), game-data-packager(6), heimdal-krb5.conf(5), heimdal-login(1), kdesu(1), login(1), login.defs(5), lxqt-sudo(1), machinectl(1), mksh(1), newgrp(1), pam(3), PAM(8), pam_rootok(8), pam_xauth(8), passwd(5), pts(4), runuser(1), setpriv(1), sg(1), shadow(5), sucrack(1), sudo(8), sudoers(5), sudo_plugin(8), tcsh(1), x11vnc(1), xscreensaver(1).

July 2014 util-linux User Commands