sudo(8)
NAME
sudo, sudoedit --- execute a command as another user
SYNOPSIS
sudo -h | -K | -k | -V
sudo -v [-AknS] [-a type] [-g group] [-h host] [-p prompt] [-u user]
sudo -l [-AknS] [-a type] [-g group] [-h host] [-p prompt] [-U user]
[-u user] [command]
sudo [-AbEHnPS] [-a type] [-C num] [-c class] [-g group] [-h host]
[-p prompt] [-r role] [-t type] [-u user] [VAR=value] [-i | -s]
[command]
sudoedit [-AknS] [-a type] [-C num] [-c class] [-g group] [-h host]
[-p prompt] [-u user] file ...
DESCRIPTION
sudo allows a permitted user to execute a command as the superuser or
another user, as specified by the security policy. The invoking user's
real (not effective) user ID is used to determine the user name with
which to query the security policy.
sudo supports a plugin architecture for security policies and
input/output logging. Third parties can develop and distribute their own
policy and I/O logging plugins to work seamlessly with the sudo front
end. The default security policy is sudoers, which is configured via the
file /etc/sudoers, or via LDAP. See the Plugins section for more
information.
The security policy determines what privileges, if any, a user has to run
sudo. The policy may require that users authenticate themselves with a
password or another authentication mechanism. If authentication is
required, sudo will exit if the user's password is not entered within a
configurable time limit. This limit is policy-specific; the default
password prompt timeout for the sudoers security policy is unlimited.
Security policies may support credential caching to allow the user to run
sudo again for a period of time without requiring authentication. The
sudoers policy caches credentials for 15 minutes, unless overridden in
sudoers(5). By running sudo with the -v option, a user can update the
cached credentials without running a command.
When invoked as sudoedit, the -e option (described below), is implied.
Security policies may log successful and failed attempts to use sudo. If
an I/O plugin is configured, the running command's input and output may
be logged as well.
The options are as follows:
-A, --askpass
Normally, if sudo requires a password, it will read it from
the user's 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 the SUDO_ASKPASS environment variable is
set, it specifies the path to the helper program. Otherwise,
if sudo.conf(5) contains a line specifying the askpass
program, that value will be used. For example:
# Path to askpass helper program
Path askpass /usr/X11R6/bin/ssh-askpass
If no askpass program is available, sudo will exit with an
error.
-b, --background
Run the given command in the background. Note that it is not
possible to use shell job control to manipulate background
processes started by sudo. Most interactive commands will
fail to work properly in background mode.
-C num, --close-from=num
Close all file descriptors greater than or equal to num
before executing a command. Values less than three are not
permitted. By default, sudo will close all open file
descriptors other than standard input, standard output and
standard error when executing a command. The security policy
may restrict the user's ability to use this option. The
sudoers policy only permits use of the -C option when the
administrator has enabled the closefrom_override option.
-E, --preserve-env
Indicates to the security policy that the user wishes to
preserve their existing environment variables. The security
policy may return an error if the user does not have
permission to preserve the environment.
-e, --edit Edit one or more files instead of running a command. In lieu
of a path name, the string "sudoedit" is used when consulting
the security policy. If the user is authorized by the
policy, the following steps are taken:
1. Temporary copies are made of the files to be edited with
the owner set to the invoking user.
2. The editor specified by the policy is run to edit the
temporary files. The sudoers policy uses the
SUDO_EDITOR, VISUAL and EDITOR environment variables (in
that order). If none of SUDO_EDITOR, VISUAL or EDITOR
are set, the first program listed in the editor
sudoers(5) option is used.
3. If they have been modified, the temporary files are
copied back to their original location and the temporary
versions are removed.
To help prevent the editing of unauthorized files, the
following restrictions are enforced unless explicitly allowed
by the security policy:
* Symbolic links may not be edited (version 1.8.15 and
higher).
* Symbolic links along the path to be edited are not
followed when the parent directory is writable by the
invoking user unless that user is root (version 1.8.16
and higher).
* Files located in a directory that is writable by the
invoking user may not be edited unless that user is root
(version 1.8.16 and higher).
Users are never allowed to edit device special files.
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 temporary file.
-g group, --group=group
Run the command with the primary group set to group instead
of the primary group specified by the target user's password
database entry. The group may be either a group name or a
numeric group ID (GID) prefixed with the '#' character (e.g.
#0 for GID 0). When running a command 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. In either case, the primary group will be set
to group.
-H, --set-home
Request that the security policy set the HOME environment
variable to the home directory specified by the target user's
password database entry. Depending on the policy, this may
be the default behavior.
-h, --help Display a short help message to the standard output and exit.
-h host, --host=host
Run the command on the specified host if the security policy
plugin supports remote commands. Note that the sudoers
plugin does not currently support running remote commands.
This may also be used in conjunction with the -l option to
list a user's privileges for the remote host.
-i, --login
Run the shell specified by the target user's password
database entry as a login shell. This means that login-
specific resource files such as .profile or .login will be
read by the shell. If a command is specified, it is passed
to the shell for execution via the shell's -c option. If no
command is specified, an interactive shell is executed. sudo
attempts to change to that user's home directory before
running the shell. The command is run with an environment
similar to the one a user would receive at log in. The
Command environment section in the sudoers(5) manual
documents how the -i option affects the environment in which
a command is run when the sudoers policy is in use.
-K, --remove-timestamp
Similar to the -k option, except that it removes the user's
cached credentials entirely and may not be used in
conjunction with a command or other option. This option does
not require a password. Not all security policies support
credential caching.
-k, --reset-timestamp
When used without a command, invalidates the user's cached
credentials. In other words, 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, this option will cause sudo to ignore the
user's cached credentials. As a result, sudo will prompt for
a password (if one is required by the security policy) and
will not update the user's cached credentials.
Not all security policies support credential caching.
-l, --list If no command is specified, list the allowed (and forbidden)
commands for the invoking user (or the user specified by the
-U option) on the current host. A longer list format is used
if this option is specified multiple times and the security
policy supports a verbose output format.
If a command is specified and is permitted by the security
policy, 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.
-n, --non-interactive
Avoid prompting the user for input of any kind. If a
password is required for the command to run, sudo will
display an error message and exit.
-P, --preserve-groups
Preserve the invoking user's group vector unaltered. By
default, the sudoers policy will initialize the group vector
to the list of groups the target user is a member of. The
real and effective group IDs, however, are still set to match
the target user.
-p prompt, --prompt=prompt
Use a custom password prompt with optional escape sequences.
The following percent ('%') escape sequences are supported by
the sudoers policy:
%H expanded to the host name including the domain name (on
if the machine's host name is fully qualified or the fqdn
option is set in sudoers(5))
%h expanded to the local host name without the domain name
%p expanded to the name of the user whose password is being
requested (respects the rootpw, targetpw, and runaspw
flags in sudoers(5))
%U expanded to the login name of the user the command will
be run as (defaults to root unless the -u option is also
specified)
%u expanded to the invoking user's login name
%% two consecutive '%' characters are collapsed into a
single '%' character
The custom prompt will override the system password prompt on
systems that support PAM unless the passprompt_override flag
is disabled in sudoers.
-r role, --role=role
Run the command with an SELinux security context that
includes the specified role.
-S, --stdin
Write the prompt to the standard error and read the password
from the standard input instead of using the terminal device.
The password must be followed by a newline character.
-s, --shell
Run the shell specified by the SHELL environment variable if
it is set or the shell specified by the invoking user's
password database entry. If a command is specified, it is
passed to the shell for execution via the shell's -c option.
If no command is specified, an interactive shell is executed.
-t type, --type=type
Run the command with an SELinux security context that
includes the specified type. If no type is specified, the
default type is derived from the role.
-U user, --other-user=user
Used in conjunction with the -l option to list the privileges
for user instead of for the invoking user. The security
policy may restrict listing other users' privileges. The
sudoers policy only allows root or a user with the ALL
privilege on the current host to use this option.
-u user, --user=user
Run the command as a user other than the default target user
(usually root). The user may be either a user name or a
numeric user ID (UID) prefixed with the '#' character (e.g.
#0 for UID 0). When running commands as a UID, many shells
require that the '#' be escaped with a backslash ('\'). Some
security policies may restrict UIDs to those listed in the
password database. The sudoers policy allows UIDs that are
not in the password database as long as the targetpw option
is not set. Other security policies may not support this.
-V, --version
Print the sudo version string as well as the version string
of the security policy plugin and any I/O plugins. If the
invoking user is already root the -V option will display the
arguments passed to configure when sudo was built and plugins
may display more verbose information such as default options.
-v, --validate
Update the user's cached credentials, authenticating the user
if necessary. For the sudoers plugin, this extends the sudo
timeout for another 15 minutes by default, but does not run a
command. Not all security policies support cached
credentials.
-- The -- option indicates that sudo should stop processing
command line arguments.
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 restrictions imposed by the security policy plugin. The
sudoers policy subjects variables passed on the command line 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 SETENV tag set or the command matched is ALL, the user may set
variables that would otherwise be forbidden. See sudoers(5) for more
information.
COMMAND EXECUTION
When sudo executes a command, the security policy specifies the execution
environment for the command. Typically, the real and effective user and
group and IDs are set to match those of the target user, as specified in
the password database, and the group vector is initialized based on the
group database (unless the -P option was specified).
The following parameters may be specified by security policy:
* real and effective user ID
* real and effective group ID
* supplementary group IDs
* the environment list
* current working directory
* file creation mode mask (umask)
* SELinux role and type
* scheduling priority (aka nice value)
Process model
When sudo runs a command, it calls fork(2), sets up the execution
environment as described above, and calls the execve system call in the
child process. The main sudo process waits until the command has
completed, then passes the command's exit status to the security policy's
close function and exits. If an I/O logging plugin is configured or if
the security policy explicitly requests it, a new pseudo-terminal
("pty") is created and a second sudo process is used to relay job control
signals between the user's existing pty and the new pty the command is
being run in. This extra process makes it possible to, for example,
suspend and resume the command. Without it, the command would be in what
POSIX terms an "orphaned process group" and it would not receive any job
control signals. As a special case, if the policy plugin does not define
a close function and no pty is required, sudo will execute the command
directly instead of calling fork(2) first. The sudoers policy plugin
will only define a close function when I/O logging is enabled, a pty is
required, or the pam_session or pam_setcred options are enabled. Note
that pam_session and pam_setcred are enabled by default on systems using
PAM.
Signal handling
When the command is run as a child of the sudo process, sudo will relay
signals it receives to the command. The SIGINT and SIGQUIT signals are
only relayed when the command is being run in a new pty or when the
signal was sent by a user process, not the kernel. This prevents the
command from receiving SIGINT twice each time the user enters control-C.
Some signals, such as SIGSTOP and SIGKILL, cannot be caught and thus will
not be relayed to the command. As a general rule, SIGTSTP should be used
instead of SIGSTOP when you wish to suspend a command being run by sudo.
As a special case, sudo will not relay signals that were sent by the
command it is running. This prevents the command from accidentally
killing itself. On some systems, the reboot(8) command sends SIGTERM to
all non-system processes other than itself before rebooting the system.
This prevents sudo from relaying the SIGTERM signal it received back to
reboot(8), which might then exit before the system was actually rebooted,
leaving it in a half-dead state similar to single user mode. Note,
however, that this check only applies to the command run by sudo and not
any other processes that the command may create. As a result, running a
script that calls reboot(8) or shutdown(8) via sudo may cause the system
to end up in this undefined state unless the reboot(8) or shutdown(8) are
run using the exec() family of functions instead of system() (which
interposes a shell between the command and the calling process).
If no I/O logging plugins are loaded and the policy plugin has not
defined a close() function, set a command timeout or required that the
command be run in a new pty, sudo may execute the command directly
instead of running it as a child process.
Plugins
Plugins may be specified via Plugin directives in the sudo.conf(5) file.
They may be loaded as dynamic shared objects (on systems that support
them), or compiled directly into the sudo binary. If no sudo.conf(5)
file is present, or it contains no Plugin lines, sudo will use the
traditional sudoers security policy and I/O logging. See the
sudo.conf(5) manual for details of the /etc/sudo.conf file and the
sudo_plugin(8) manual for more information about the sudo plugin
architecture.
EXIT VALUE
Upon successful execution of a command, the exit status from sudo will be
the exit status of the program that was executed. If the command
terminated due to receipt of a signal, sudo will send itself the signal
that terminated the command.
Otherwise, sudo exits with a 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 the standard
error. If sudo cannot stat(2) one or more entries in the user's PATH, an
error is printed to the standard error. (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.
SECURITY NOTES
sudo tries to be safe when executing external commands.
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.
Users should never be granted sudo privileges to execute files that are
writable by the user or that reside in a directory that is writable by
the user. If the user can modify or replace the command there is no way
to limit what additional commands they can run.
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 are not subject to sudo's security policy.
The same is true for commands that offer shell escapes (including most
editors). If I/O logging is enabled, subsequent commands will have their
input and/or output logged, but there will not be traditional logs for
those commands. 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
sudoers(5).
To prevent the disclosure of potentially sensitive information, sudo
disables core dumps by default while it is executing (they are re-enabled
for the command that is run). This historical practice dates from a time
when most operating systems allowed setuid processes to dump core by
default. To aid in debugging sudo crashes, you may wish to re-enable
core dumps by setting "disable_coredump" to false in the sudo.conf(5)
file as follows:
Set disable_coredump false
See the sudo.conf(5) manual for more information.
ENVIRONMENT
sudo utilizes the following environment variables. The security policy
has control over the actual content of the command's environment.
EDITOR Default editor to use in -e (sudoedit) mode if neither
SUDO_EDITOR nor VISUAL is set.
MAIL Set to the mail spool of the target user when the -i
option is specified or when env_reset is enabled in
sudoers (unless MAIL is present in the env_keep list).
HOME Set to the home directory of the target user when the -i
or -H options are specified, when the -s option is
specified and set_home is set in sudoers, when
always_set_home is enabled in sudoers, or when env_reset
is enabled in sudoers and HOME is not present in the
env_keep list.
LOGNAME Set to the login name of the target user when the -i
option is specified, when the set_logname option is
enabled in sudoers or when the env_reset option is
enabled in sudoers (unless LOGNAME is present in the
env_keep list).
PATH May be overridden by the security policy.
SHELL Used to determine shell to run with -s option.
SUDO_ASKPASS Specifies the path to a helper program used to read the
password if no terminal is available or if the -A option
is specified.
SUDO_COMMAND Set to the command run by sudo.
SUDO_EDITOR Default editor to use in -e (sudoedit) mode.
SUDO_GID Set to the group ID of the user who invoked sudo.
SUDO_PROMPT Used as the default password prompt.
SUDO_PS1 If set, PS1 will be set to its value for the program
being run.
SUDO_UID Set to the user ID of the user who invoked sudo.
SUDO_USER Set to the login name of the user who invoked sudo.
USER Set to the same value as LOGNAME, described above.
USERNAME Same as USER.
VISUAL Default editor to use in -e (sudoedit) mode if
SUDO_EDITOR is not set.
FILES
/etc/sudo.conf sudo front end configuration
EXAMPLES
Note: the following examples assume a properly configured security
policy.
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 shut down 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"
SEE ALSO
su(1), stat(2), passwd(5), sudo.conf(5), sudoers(5), sudo_plugin(8),
sudoreplay(8), visudo(8)
HISTORY
See the HISTORY file in the sudo distribution
(https://www.sudo.ws/history.html) for a brief history of sudo.
AUTHORS
Many people have worked on sudo over the years; this version consists of
code written primarily by:
Todd C. Miller
See the CONTRIBUTORS file in the sudo distribution
(https://www.sudo.ws/contributors.html) for an exhaustive list of people
who have contributed to sudo.
CAVEATS
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 the sudoers(5) plugin's noexec functionality.
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.
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).
BUGS
If you feel you have found a bug in sudo, please submit a bug report at
https://bugzilla.sudo.ws/
SUPPORT
Limited free support is available via the sudo-users mailing list, see
https://www.sudo.ws/mailman/listinfo/sudo-users to subscribe or search
the archives.
DISCLAIMER
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 https://www.sudo.ws/license.html for
complete details.
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