This straightforward guide to configuring
is for anyone who didn't expect to see “Don't despair” and a “Quick
guide to EBNF” in the
Sudo (su "do") allows a system administrator to delegate authority to
give certain users (or groups of users) the ability to run some (or all)
commands as root or another user while providing an audit trail of the
commands and their arguments.
This guide is intended to supplement the manpage. The various
environment, security, and logging options are not covered; the
explanations in the manpage are easy to follow.
The first 90%
It's possible that the only sudo explanation you will ever need is:
%adm ALL=(ALL) NOPASSWD: ALL
This means “any user in the adm group on any host may run any
command as any user without a password”. The first ALL refers
to hosts, the second to target users, and the last to allowed commands.
The /etc/sudoers file contains “user specifications” that
define the commands that users may execute. When sudo is invoked, these
specifications are checked in order, and the last match is used. A user
specification looks like this at its most basic:
User Host = (Runas) Command
Read this as “User may run Command as the Runas user on Host”.
Any or all of the above may be the special keyword ALL, which
User and Runas may be usernames, group names prefixed
with %, numeric UIDs prefixed with #, or numeric GIDs
prefixed with %#. Host may be a hostname, IP address,
or a whole network (e.g., 192.0.2.0/24), but not
This optional clause controls the target user (and group) sudo will run
the Command as, or in other words, which combinations of the
-u and -g arguments it will accept.
If the clause is omitted, the user will be permitted to run commands
only as root. If you specify a username, e.g.,
(postgres), sudo will accept “-u postgres” and run
commands as that user. In both cases, sudo will not accept -g.
If you also specify a target group, e.g., (postgres:postgres),
sudo will accept any combination of the listed users and groups (see the
section on aliases below). If you specify only a target group, e.g.,
(:postgres), sudo will accept and act on “-g postgres”
but run commands only as the invoking user.
This is why you sometimes see (ALL:ALL) in the 90% example.
In the simplest case, a command is the full path to an executable, which
permits it to be executed with any arguments. You may specify a list of
arguments after the path to permit the command only with those exact
arguments, or write "" to permit execution only without any
A command may also be the full path to a directory (including a trailing
/). This permits execution of all the files in that directory,
but not in any subdirectories.
ams ALL=/bin/ls, /bin/df -h /, /bin/date "", \
/usr/bin/, sudoedit /etc/hosts, \
The keyword sudoedit is also recognised as a command name, and
arguments can be specified as with other commands. Use this instead of
allowing a particular editor to be run with sudo, because it runs the
editor as the user and only installs the editor's output file into place
as root (or other target user).
As shown above, comma-separated lists of commands and aliases may be
specified. Commands may also use shell wildcards either in the path or
in the argument list (but see the warning below about the latter).
Sudo is very flexible, and it's tempting to set up very fine-grained
access, but it can be difficult to understand the consequences of a
complex setup, and you can end up with
Try to keep things simple.
Before the command, you can specify zero or more options to control how
it will be executed. The most important options are NOPASSWD
(to not require a password) and SETENV (to allow the user to
set environment variables for the command).
ams ALL=(ALL) NOPASSWD: SETENV: /bin/ls
Other available options include NOEXEC, LOG_INPUT and
LOG_OUTPUT, and SELinux role and type specifications. These are
all documented in the manpage.
The path to a binary (i.e., not a directory or alias) may also be
prefixed with a digest:
ams ALL=(ALL) sha224:IkotndXGTmZtH5ZNFtRfIwkG0WuiuOs7GoZ+6g== /bin/ls
The specified binary will then be executed only if it matches the
digest. SHA-2 digests of 224, 256, 384, and 512-bits are accepted in hex
or Base64 format. The values can be generated using, e.g.,
sha512sum or openssl.
In addition to the things listed above, a User, Host,
Runas, or Command may be an alias, which is a named
list of comma-separated values of the corresponding type. An alias may
be used wherever a User, Host, Runas, or
Command may occur. They are always named in uppercase, and can
be defined as shown in these examples:
# Type_Alias NAME = a, b : NAME_2 = c, d, …
User_Alias TRUSTED = %admin, !ams
Runas_Alias LEGACYUSERS = oldapp1, oldapp2
Runas_Alias APPUSERS = app1, app2, LEGACYUSERS
Host_Alias PRODUCTION = www1, www2, \
Cmnd_Alias DBA = /usr/pgsql-9.4/bin, \
An alias definition can also include another alias of the same type
(e.g., LEGACYUSERS above). You cannot include options like
NOPASSWD: in command aliases.
Any term in a list may be prefixed with ! to negate it.
This can be used to include a group but exclude a certain user, or to
exclude certain addresses in a network, and so on. Negation can also be
used in command lists, but note the manpage's warning that trying to
“subtract” commands from ALL using ! is
Use aliases whenever you need rules involving multiple users, hosts, or
Sudo has a number of options whose values may be set in the
configuration file, overriding the defaults either unconditionally, or
only for a given user, host, or command. The defaults are sensible, so
you do not need to care about options unless you're doing something
Option values are specified in one or more "Defaults" lines. The example
below switches on env_reset, turns off insults (read
!insults as "not insults"), sets password_tries to 4,
and so on. All the values are set unconditionally, i.e. they apply to
every user specification.
Defaults env_reset, !insults, password_tries=4, \
Defaults passprompt="Password for %p:"
Options may also be set only for specific hosts, users, or commands,
as shown below.
Defaults@host sets options for a host,
Defaults:user for a (requesting) user,
Defaults!command for a command, and
Defaults>user for a target user.
You can also use aliases in these definitions.
Defaults:ams insults, !lecture
Defaults>root mail_always, mailto="email@example.com"
Cmnd_Alias FOO = /usr/bin/foo, /usr/bin/bar, \
Unconditional defaults are parsed first, followed by host and user
defaults, then runas defaults, then command defaults.
The many available options are explained well in the manpage.
In addition to the alias mechanism, a User, Host,
Runas, or Command may each be a
comma-separated list of things of the corresponding type. Also, a user
specification may contain multiple host and command sets for a single
User. Please be sparing in your use of this syntax, in case you
ever have to make sense of it again.
Users and hosts can also be a +netgroup or other more esoteric
things, depending on plugins. Host names may also use shell wildcards
(see the fqdn option).
If Runas is omitted but the () are not, sudo will
reject -u and -g and run commands only as the invoking
You can use wildcards in command paths and in arguments, but their
meaning is different. In a path, a * will not match a
/, so /usr/bin/* will match /usr/bin/who but
not /usr/bin/X11/xterm. In arguments, a *
does match /; also, arguments are matched as a single
string (not a list of separate words), so * can match across
words. The manpage includes the following problematic example, which
permits additional arguments to be passed to /bin/cat without
%operator ALL = /bin/cat /var/log/messages*
Warning: Sudo will not work if /etc/sudoers contains
syntax errors, so you should only ever edit it using visudo,
which performs basic sanity checks, and installs the new file only if
it parses correctly.
Another warning: if you take the EBNF in the manpage seriously enough,
you will discover that the implementation doesn't follow it. You can
avoid this sad fate by linking to this article instead of trying to
write your own. Happy sudoing!