sudo allows a permitted user to execute a command as the
superuser or another user, as specified in the sudoers file.
The real and effective uid and gid are set to match those of the
target user as specified in the passwd file and the group vector
is initialized based on the group file (unless the -P option was
specified). If the invoking user is root or if the target user is
the same as the invoking user, no password is required. Otherwise,
sudo requires that users authenticate themselves with a password
by default (NOTE: in the default configuration this is the user's
password, not the root password). Once a user has been authenticated,
a timestamp is updated and the user may then use sudo without a
password for a short period of time (
5 minutes unless
overridden in sudoers).
When invoked as sudoedit, the -e option (described below),
sudo determines who is an authorized user by consulting the file
/etc/sudoers. By giving sudo the -v flag a user
can update the time stamp without running a command. The password
prompt itself will also time out if the user's password is not
5 minutes (unless overridden via
If a user who is not listed in the sudoers file tries to run a
command via sudo, mail is sent to the proper authorities, as
defined at configure time or in the sudoers file (defaults to
root ). Note that the mail will not be sent if an unauthorized
user tries to run sudo with the -l or -v flags. This allows
users to determine for themselves whether or not they are allowed
to use sudo.
If sudo is run by root and the
SUDO_USER environment variable
is set, sudo will use this value to determine who the actual
user is. This can be used by a user to log commands through sudo
even when a root shell has been invoked. It also allows the -e
flag to remain useful even when being run via a sudo-run script or
program. Note however, that the sudoers lookup is still done for
root, not the user specified by
sudo can log both successful and unsuccessful attempts (as well
as errors) to syslog(3), a log file, or both. By default sudo
will log via syslog(3) but this is changeable at configure time
or via the sudoers file.
sudo accepts the following command line options:
The -H (HOME) option sets the
HOME environment variable
to the homedir of the target user (root by default) as specified
in passwd(5). By default, sudo does not modify
(see set_home and always_set_home in sudoers(5)).
The -K (sure kill) option is like -k except that it removes
the user's timestamp entirely. Like -k, this option does not
require a password.
The -L (list defaults) option will list out the parameters
that may be set in a Defaults line along with a short description
for each. This option is useful in conjunction with grep(1).
The -P (preserve group vector) option causes sudo to
preserve the invoking user's group vector unaltered. By default,
sudo will initialize the group vector to the list of groups the
target user is in. The real and effective group IDs, however, are
still set to match the target user.
The -S (stdin) option causes sudo to read the password from
the standard input instead of the terminal device.
The -V (version) option causes sudo to print the version
number and exit. If the invoking user is already root the -V
option will print out a list of the defaults sudo was compiled
with as well as the machine's local network addresses.
The -a (authentication type) option causes sudo to use the
specified authentication type when validating the user, as allowed
by /etc/login.conf. The system administrator may specify a list
of sudo-specific authentication methods by adding an ``auth-sudo''
entry in /etc/login.conf. This option is only available on systems
that support BSD authentication where sudo has been configured
with the --with-bsdauth option.
The -b (background) option tells sudo to run the given
command in the background. Note that if you use the -b
option you cannot use shell job control to manipulate the process.
The -c (class) option causes sudo to run the specified command
with resources limited by the specified login class. The class
argument can be either a class name as defined in /etc/login.conf,
or a single '-' character. Specifying a class of
that the command should be run restricted by the default login
capabilities for the user the command is run as. If the class
argument specifies an existing user class, the command must be run
as root, or the sudo command must be run from a shell that is already
root. This option is only available on systems with BSD login classes
where sudo has been configured with the --with-logincap option.
The -e (edit) option indicates that, instead of running
a command, the user wishes to edit one or more files. In lieu
of a command, the string ``sudoedit'' is used when consulting
the sudoers file. If the user is authorized by sudoers
the following steps are taken:
Temporary copies are made of the files to be edited with the owner
set to the invoking user.
The editor specified by the
variables is run to edit the temporary files. If neither
EDITOR are set, the program listed in the editorsudoers
variable is used.
If they have been modified, the temporary files are copied back to
their original location and the temporary versions are removed.
If the specified file does not exist, it will be created. Note
that unlike most commands run by sudo, the editor is run with
the invoking user's environment unmodified. If, for some reason,
sudo is unable to update a file with its edited version, the
user will receive a warning and the edited copy will remain in a
The -h (help) option causes sudo to print a usage message and exit.
The -i (simulate initial login) option runs the shell specified
in the passwd(5) entry of the user that the command is
being run as. The command name argument given to the shell begins
- to tell the shell to run as a login shell. sudo
attempts to change to that user's home directory before running the
shell. It also initializes the environment, leaving TERM
unchanged, setting HOME, SHELL, USER, LOGNAME, and
PATH, and unsetting all other environment variables. Note that
because the shell to use is determined before the sudoers file
is parsed, a runas_default setting in sudoers will specify
the user to run the shell as but will not affect which shell is
The -k (kill) option to sudo invalidates the user's timestamp
by setting the time on it to the epoch. The next time sudo is
run a password will be required. This option does not require a password
and was added to allow a user to revoke sudo permissions from a .logout
The -l (list) option will list out the allowed (and
forbidden) commands for the user on the current host.
The -p (prompt) option allows you to override the default
password prompt and use a custom one. The following percent (`
escapes are supported:
expanded to the invoking user's login name
expanded to the login name of the user the command will
be run as (defaults to root)
expanded to the local hostname without the domain name
expanded to the local hostname including the domain name
(on if the machine's hostname is fully qualified or the fqdn
sudoers option is set)
% characters are collapsed into a single
The -s (shell) option runs the shell specified by the SHELL
environment variable if it is set or the shell as specified
The -u (user) option causes sudo to run the specified command
as a user other than root. To specify a uid instead of a
username, use #uid. Note that if the targetpw Defaults
option is set (see sudoers(5)) it is not possible
to run commands with a uid not listed in the password database.
If given the -v (validate) option, sudo will update the
user's timestamp, prompting for the user's password if necessary.
This extends the sudo timeout for another
(or whatever the timeout is set to in sudoers) but does not run
The -- flag indicates that sudo should stop processing command
line arguments. It is most useful in conjunction with the -s flag.
Upon successful execution of a program, the return value from sudo
will simply be the return value of the program that was executed.
Otherwise, sudo quits with an exit value of 1 if there is a
configuration/permission problem or if sudo cannot execute the
given command. In the latter case the error string is printed to
stderr. If sudo cannot stat(2) one or more entries in the user's
PATH an error is printed on stderr. (If the directory does not
exist or if it is not really a directory, the entry is ignored and
no error is printed.) This should not happen under normal
circumstances. The most common reason for stat(2) to return
``permission denied'' is if you are running an automounter and one
of the directories in your
PATH is on a machine that is currently
sudo tries to be safe when executing external commands. Variables
that control how dynamic loading and binding is done can be used
to subvert the program that sudo runs. To combat this the
SHLIB_PATH (HP-UX only), and
only) environment variables are removed from the environment passed
on to all commands executed. sudo will also remove the
TERMPATH variables as they too can pose a threat. If the
TERMCAP variable is set and is a pathname, it too is ignored.
Additionally, if the
LANGUAGE variables contain the
% characters, they are ignored. Environment variables
with a value beginning with
() are also removed as they could
be interpreted as bash functions. If sudo has been
compiled with SecurID support, the
DLC_ACE variables are cleared as well. The list of environment
variables that sudo clears is contained in the output of
sudo -V when run as root.
To prevent command spoofing, sudo checks ``.'' and "" (both denoting
current directory) last when searching for a command in the user's
PATH (if one or both are in the PATH). Note, however, that the
PATH environment variable is not modified and is passed
unchanged to the program that sudo executes.
For security reasons, if your OS supports shared libraries and does
not disable user-defined library search paths for setuid programs
(most do), you should either use a linker option that disables this
behavior or link sudo statically.
sudo will check the ownership of its timestamp directory
(/var/run/sudo by default) and ignore the directory's contents if
it is not owned by root and only writable by root. On systems that
allow non-root users to give away files via chown(2), if the timestamp
directory is located in a directory writable by anyone (e.g.: /tmp),
it is possible for a user to create the timestamp directory before
sudo is run. However, because sudo checks the ownership and
mode of the directory and its contents, the only damage that can
be done is to ``hide'' files by putting them in the timestamp dir.
This is unlikely to happen since once the timestamp dir is owned
by root and inaccessible by any other user the user placing files
there would be unable to get them back out. To get around this
issue you can use a directory that is not world-writable for the
timestamps (/var/adm/sudo for instance) or create /var/run/sudo
with the appropriate owner (root) and permissions (0700) in the
system startup files.
sudo will not honor timestamps set far in the future.
Timestamps with a date greater than current_time + 2 *
will be ignored and sudo will log and complain. This is done to
keep a user from creating his/her own timestamp with a bogus
date on systems that allow users to give away files.
Please note that sudo will only log the command it explicitly
runs. If a user runs a command such as
sudo su or
sudo sh ,
subsequent commands run from that shell will not be logged, nor
will sudo's access control affect them. The same is true for
commands that offer shell escapes (including most editors). Because
of this, care must be taken when giving users access to commands
via sudo to verify that the command does not inadvertently give
the user an effective root shell.
sudo utilizes the following environment variables:
EDITOR Default editor to use in -e (sudoedit) mode if
VISUAL is not set
HOME In -s or -H mode (or if sudo was configured with
the --enable-shell-sets-home option), set to
homedir of the target user
PATH Set to a sane value if sudo was configured with
the --with-secure-path option
SHELL Used to determine shell to run with -s option
SUDO_PROMPT Used as the default password prompt
SUDO_COMMAND Set to the command run by sudo
SUDO_USER Set to the login of the user who invoked sudo
SUDO_UID Set to the uid of the user who invoked sudo
SUDO_GID Set to the gid of the user who invoked sudo
SUDO_PS1 If set, PS1 will be set to its value
USER Set to the target user (root unless the -u option
VISUAL Default editor to use in -e (sudoedit) mode
/etc/sudoers List of who can run what
/var/run/sudo Directory containing timestamps
Note: the following examples assume suitable sudoers(5) entries.
To get a file listing of an unreadable directory:
$ sudo ls /usr/local/protected
To list the home directory of user yazza on a machine where the
file system holding ~yazza is not exported as root:
$ sudo -u yazza ls ~yazza
To edit the index.html file as user www:
$ sudo -u www vi ~www/htdocs/index.html
To shutdown a machine:
$ sudo shutdown -r +15 "quick reboot"
To make a usage listing of the directories in the /home
partition. Note that this runs the commands in a sub-shell
to make the
cd and file redirection work.
$ sudo sh -c "cd /home ; du -s * | sort -rn > USAGE"
There is no easy way to prevent a user from gaining a root shell
if that user is allowed to run arbitrary commands via sudo.
Also, many programs (such as editors) allow the user to run commands
via shell escapes, thus avoiding sudo's checks. However, on
most systems it is possible to prevent shell escapes with sudo's
noexec functionality. See the sudoers(5) manual
It is not meaningful to run the
cd command directly via sudo, e.g.
$ sudo cd /usr/local/protected
since when whe command exits the parent process (your shell) will
still be the same. Please see the EXAMPLES section for more information.
If users have sudo
ALL there is nothing to prevent them from
creating their own program that gives them a root shell regardless
of any '!' elements in the user specification.
Running shell scripts via sudo can expose the same kernel bugs that
make setuid shell scripts unsafe on some operating systems (if your OS
has a /dev/fd/ directory, setuid shell scripts are generally safe).
Sudo is provided ``AS IS'' and any express or implied warranties,
including, but not limited to, the implied warranties of merchantability
and fitness for a particular purpose are disclaimed. See the LICENSE
file distributed with sudo or http://www.sudo.ws/sudo/license.html
for complete details.