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 time stamp is updated and the user may then use sudo without a
password for a short period of time (
15 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 running sudo with the -v option,
a user can update the time stamp without running a command. If
a password is required, sudo will exit if the user's password
is not entered within a configurable time limit. The default
password prompt timeout is
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 option. 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
option 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:
Normally, if sudo requires a password, it will read it from the
current terminal. If the -A (askpass) option is specified,
a (possibly graphical) helper program is executed to read the
user's password and output the password to the standard output. If
SUDO_ASKPASS environment variable is set, it specifies the
path to the helper program. Otherwise, the value specified by the
askpass option in sudoers(5) is used.
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.
Normally, sudo will close all open file descriptors other than
standard input, standard output and standard error. The -C
(close from) option allows the user to specify a starting point
above the standard error (file descriptor three). Values less than
three are not permitted. This option is only available if the
administrator has enabled the closefrom_override option in
The -E (preserveenvironment) option will override the
env_reset option in sudoers(5)). It is only
available when either the matching command has the
or the setenv option is set in sudoers(5).
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
environment variables is run to edit the temporary files. If none
EDITOR are set, the first 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
Normally, sudo sets the primary group to the one specified by
the passwd database for the user the command is being run as (by
default, root). The -g (group) option causes sudo to run
the specified command with the primary group set to group. To
specify a gid instead of a group name, use #gid. When
running commands as a gid, many shells require that the '#' be
escaped with a backslash ('\'). If no -u option is specified,
the command will be run as the invoking user (not root). In either
case, the primary group will be set to group.
The -H (HOME) option sets the
HOME environment variable
to the homedir of the target user (root by default) as specified
in passwd(5). The default handling of the
variable depends on sudoers(5) settings. By default, sudo
HOME if env_reset or always_set_home are set, or
if set_home is set and the -s option is specified on the
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 target user as a login shell. This
means that login-specific resource files such as
.login will be read by the shell. If a command is specified,
it is passed to the shell for execution. Otherwise, an interactive
shell is executed. sudo attempts to change to that user's home
directory before running the shell. It also initializes the
environment, leaving DISPLAY and TERM unchanged, setting
HOME, MAIL, SHELL, USER, LOGNAME, and PATH, as well as
the contents of /etc/environment on Linux and AIX systems.
All other environment variables are removed.
The -K (sure kill) option is like -k except that it removes
the user's time stamp entirely and may not be used in conjunction
with a command or other option. This option does not require a
When used by itself, the -k (kill) option to sudo invalidates
the user's time stamp 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 file.
When used in conjunction with a command or an option that may require
a password, the -k option will cause sudo to ignore the user's
time stamp file. As a result, sudo will prompt for a password
(if one is required by sudoers) and will not update the user's
time stamp file.
The -L (list defaults) option will list the parameters that
may be set in a Defaults line along with a short description for
each. This option will be removed from a future version of sudo.
If no command is specified, the -l (list) option will list
the allowed (and forbidden) commands for the invoking user (or the
user specified by the -U option) on the current host. If a
command is specified and is permitted by sudoers, the
fully-qualified path to the command is displayed along with any
command line arguments. If command is specified but not allowed,
sudo will exit with a status value of 1. If the -l option is
specified with an l argument (i.e. -ll), or if -l
is specified multiple times, a longer list format is used.
The -n (non-interactive) option prevents sudo from prompting
the user for a password. If a password is required for the command
to run, sudo will display an error messages and exit.
The -P (preservegroup 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 -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 local host name including the domain name
(on if the machine's host name is fully qualified or the fqdnsudoers option is set)
expanded to the local host name without the domain name
expanded to the user whose password is being asked for (respects the
rootpw, targetpw and runaspw flags in sudoers)
expanded to the login name of the user the command will
be run as (defaults to root)
expanded to the invoking user's login name
% characters are collapsed into a single
The prompt specified by the -p option will override the system
password prompt on systems that support PAM unless the
passprompt_override flag is disabled in sudoers.
The -S (stdin) option causes sudo to read the password from
the standard input instead of the terminal device. The password must
be followed by a newline character.
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). If a command is specified, it is passed to the shell
for execution. Otherwise, an interactive shell is executed.
The -U (other user) option is used in conjunction with the -l
option to specify the user whose privileges should be listed. Only
root or a user with sudo
ALL on the current host may use this
The -u (user) option causes sudo to run the specified
command as a user other than root. To specify a uid instead
of a user name, use #uid. When running commands as a uid,
many shells require that the '#' be escaped with a backslash ('\').
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
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.
If given the -v (validate) option, sudo will update the
user's time stamp, 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 -- option indicates that sudo should stop processing command
Environment variables to be set for the command may also be passed
on the command line in the form of VAR=value, e.g.
LD_LIBRARY_PATH=/usr/local/pkg/lib. Variables passed on the
command line are subject to the same restrictions as normal environment
variables with one important exception. If the setenv option
is set in sudoers, the command to be run has the
set or the command matched is
ALL , the user may set variables
that would overwise be forbidden. See sudoers(5) for more information.
Upon successful execution of a program, the exit status from sudo
will simply be the exit status 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.
There are two distinct ways to deal with environment variables.
By default, the env_resetsudoers option is enabled.
This causes commands to be executed with a minimal environment
USERNAME in addition to variables from the invoking process
permitted by the env_check and env_keepsudoers options.
There is effectively a whitelist for environment variables.
If, however, the env_reset option is disabled in sudoers, any
variables not explicitly denied by the env_check and env_delete
options are inherited from the invoking process. In this case,
env_check and env_delete behave like a blacklist. Since it
is not possible to blacklist all potentially dangerous environment
variables, use of the default env_reset behavior is encouraged.
In all cases, environment variables with a value beginning with
() are removed as they could be interpreted as bash functions.
The list of environment variables that sudo allows or denies is
contained in the output of
sudo -V when run as root.
Note that the dynamic linker on most operating systems will remove
variables that can control dynamic linking from the environment of
setuid executables, including sudo. Depending on the operating
system this may include
SHLIB_PATH , and others. These type of variables are
removed from the environment before sudo even begins execution
and, as such, it is not possible for sudo to preserve them.
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.
sudo will check the ownership of its time stamp directory
(/var/lib/sudo by default) and ignore the directory's contents if
it is not owned by root or if it is writable by a user other than
root. On systems that allow non-root users to give away files via
chown(2), if the time stamp directory is located in a directory
writable by anyone (e.g., /tmp), it is possible for a user to
create the time stamp 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 time stamp dir. This is unlikely to happen
since once the time stamp 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 time stamps (/var/adm/sudo for
instance) or create /var/lib/sudo with the appropriate owner (root)
and permissions (0700) in the system startup files.
sudo will not honor time stamps 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 time stamp with a bogus
date on systems that allow users to give away files.
On systems where the boot time is available, sudo will also not
honor time stamps from before the machine booted.
Since time stamp files live in the file system, they can outlive a
user's login session. As a result, a user may be able to login,
run a command with sudo after authenticating, logout, login
again, and run sudo without authenticating so long as the time
stamp file's modification time is within
15 minutes (or
whatever the timeout is set to in sudoers). When the tty_tickets
option is enabled in sudoers, the time stamp has per-tty granularity
but still may outlive the user's session. On Linux systems where
the devpts filesystem is used, Solaris systems with the devices
filesystem, as well as other systems that utilize a devfs filesystem
that monotonically increase the inode number of devices as they are
created (such as Mac OS X), sudo is able to determine when a
tty-based time stamp file is stale and will ignore it. Administrators
should not rely on this feature as it is not universally available.
Please note that sudo will normally 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. For more
information, please see the
PREVENTING SHELL ESCAPES section in
sudo utilizes the following environment variables:
Default editor to use in -e (sudoedit) mode if neither
VISUAL is set
In -i mode or when env_reset is enabled in sudoers, set
to the mail spool of the target user
Set to the home directory of the target user if -i or -H are
specified, env_reset or always_set_home are set in sudoers,
or when the -s option is specified and set_home is set in
Set to a sane value if the secure_path sudoers option is set.
Used to determine shell to run with
Specifies the path to a helper program used to read the password
if no terminal is available or if the
-A option is specified.
Set to the command run by sudo
Default editor to use in -e (sudoedit) mode
Set to the group ID of the user who invoked sudo
Used as the default password prompt
PS1 will be set to its value for the program being run
Set to the user ID of the user who invoked sudo
Set to the login of the user who invoked sudo
Set to the target user (root unless the -u option is specified)
Default editor to use in -e (sudoedit) mode if
is not set
List of who can run what
Directory containing time stamps
Initial environment for -i mode on Linux and AIX
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 yaz on a machine where the
file system holding ~yaz is not exported as root:
$ sudo -u yaz ls ~yaz
To edit the index.html file as user www:
$ sudo -u www vi ~www/htdocs/index.html
To view system logs only accessible to root and users in the adm group:
$ sudo -g adm view /var/log/syslog
To run an editor as jim with a different primary group:
$ sudo -u jim -g audio vi ~jim/sound.txt
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 the 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.