sudo - execute a command as another user
sudo -V | -h | -l | -L | -v | -k | -K | -s | [ -H ] [-P ]
[-S ] [ -b ] | [ -p prompt ] [ -c class|- ] [ -a auth_type
] [ -u username|#uid ] command
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
(the group vector is also initialized when the target user
is not root). By default, sudo requires that users
authenticate themselves with a password (NOTE: by default
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).
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 entered within 5 minutes (unless overridden
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 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.
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:
-V 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.
-l The -l (list) option will list out the allowed (and
forbidden) commands for the user on the current host.
-L 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).
-h The -h (help) option causes sudo to print a usage message
-v 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 5 minutes (or whatever the timeout is set to
in sudoers) but does not run a command.
-k 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
-K The -K (sure kill) option to sudo removes the user's
timestamp entirely. Likewise, this option does not
require a password.
-b 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
-p The -p (prompt) option allows you to override the
default password prompt and use a custom one. The
following percent (`%') escapes are supported:
%u expanded to the invoking user's login name
%U expanded to the login name of the user the
command will be run as (defaults to root)
%h expanded to the local hostname without the
%H expanded to the local hostname including the
domain name (on if the machine's hostname is
fully qualified or the fqdn sudoers option is
%% two consecutive % characters are collaped into
a single % character
-c 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 - indicates 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
-a 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.
-u 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.
-s The -s (shell) option runs the shell specified by the
SHELL environment variable if it is set or the shell
as specified in passwd(5).
-H 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 HOME.
-P The -P (preserve group vector) option causes sudo to
preserve the 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.
-S The -S (stdin) option causes sudo to read the password
from standard input instead of the terminal device.
-- 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 unreachable.
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 LD_*, _RLD_*, SHLIB_PATH (HP-UX only),
and LIBPATH (AIX only) environment variables are removed
from the environment passed on to all commands executed.
sudo will also remove the IFS, ENV, BASH_ENV, KRB_CONF,
KRBCONFDIR, KRBTKFILE, KRB5_CONFIG, LOCALDOMAIN,
RES_OPTIONS, HOSTALIASES, NLSPATH, PATH_LOCALE, TERMINFO,
TERMINFO_DIRS and 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 LC_* or
LANGUAGE variables contain the / or % characters, they are
ignored. If sudo has been compiled with SecurID support,
the VAR_ACE, USR_ACE and 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 actual 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 *
TIMEOUT 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.
Note: the following examples assume suitable sudoers(5)
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 filesystem holding ~yazza is not exported as
% 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 subshell
to make the cd and file redirection work.
% sudo sh -c "cd /home ; du -s * | sort -rn > USAGE"
sudo utilizes the following environment variables:
PATH Set to a sane value if SECURE_PATH
SHELL Used to determine shell to run
with -s option
USER Set to the target user (root unless the -u option
HOME In -s or -H mode (or if sudo was
the --enable-shell-sets-home option), set to
homedir of the target user.
SUDO_PROMPT Used as the default password
SUDO_COMMAND Set to the command run by sudo
SUDO_USER Set to the login of the user who
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
/etc/sudoers List of who can run what
/var/run/sudo Directory containing timestamps
Many people have worked on sudo over the years; this version
consists of code written primarily by:
See the HISTORY file in the sudo distribution or visit
http://www.sudo.ws/sudo/history.html for a short history
If you feel you have found a bug in sudo, please submit a
bug report at http://www.sudo.ws/sudo/bugs/
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 for complete details.
There is no easy way to prevent a user from gaining a root
shell if that user has access to commands allowing shell
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 supports the /dev/fd/ directory,
setuid shell scripts are generally safe).
grep(1), su(1), stat(2), login_cap(3), sudoers(5),
1.6.7 March 13, 2003 7 [ Back ]