su

Examples (TL;DR)

• Switch to superuser (admin password required): su
• Switch to user username (password required): su username
• Switch to user username and simulate a full login shell: su - username

tldr.sh

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-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).

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:

• clears all the environment variables except TERM and variables specified by --whitelist-environment
• 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

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.

-P, --pty

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 mostly designed for interactive sessions. If the standard input is not a terminal, but for example pipe (e.g. echo "date" | su --pty) than ECHO flag for the pseudo-terminal is disabled to avoid messy output.

-s, --shell=shell

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 --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
• /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.)

-w, --whitelist-environment=list

Don't reset environment variables specified in comma separated list when clears environment for --login. The whitelist is ignored for the environment variables HOME, SHELL, USER, LOGNAME, and PATH.

-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. The child is terminated by SIGTERM, after unsuccessful attempt and 2 seconds of delay the child is killed by SIGKILL.

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.  ENV_SUPATH takes precedence.  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.

The environment variable PATH may be different on systems where /bin and /sbin are merged into /usr.

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:

See Also

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

History

This su command was derived from coreutils' su, which was based on an implementation by David MacKenzie. The util-linux has been refactored by Karel Zak.

Availability

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

Referenced By

at(1), beesu(1), ccze(1), credentials(7), crontab(5), debuild(1), flock(1), game-data-packager(6), hamlib-primer(7), heimdal-krb5.conf(5), kdesu(1), login(1), login.defs(5), lxqt-sudo(1), machinectl(1), mksh(1), newgrp(1), ntp-keygen(8), pam(3), PAM(8), pam_rootok(8), pam_xauth(8), passwd(5), pts(4), runuser(1), setpriv(1), sg(1), shadow(5), sudo(8), sudoers(5), sudo_plugin(8), tcsh(1), termy-setup(1), termy-ssh(1), termy-systemd-setup(1), x11vnc(1), xscreensaver(1).