|SUDO(8)||System Manager's Manual||SUDO(8)|
sudoallows 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
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
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
Security policies may support credential caching to allow the user
sudo again for a period of time without
requiring authentication. The sudoers policy caches
15 minutes, unless overridden in
sudoers(5). By running
-v option, a user can update the cached
credentials without running a command.
When invoked as
-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:
- Normally, if
sudorequires 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_ASKPASSenvironment 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,
sudowill exit with an error.
- 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.
- Close all file descriptors greater than or equal to
num before executing a command. Values less than
three are not permitted. By default,
sudowill 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
-Coption when the administrator has enabled the closefrom_override option.
- 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.
- Indicates to the security policy that the user wishes to add the comma-separated list of environment variables to those preserved from the user's environment. The security policy may return an error if the user does not have permission to preserve the environment.
- 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
- Temporary copies are made of the files to be edited with the owner set to the invoking user.
- The editor specified by the policy is run to edit the temporary files.
The sudoers policy uses the
EDITORenvironment variables (in that order). If none of
EDITORare set, the first program listed in the editor sudoers(5) option is used.
- 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,
sudois 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.
- 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.,
#0for GID 0). When running a command as a GID, many shells require that the ‘
#’ be escaped with a backslash (‘
\’). If no
-uoption is specified, the command will be run as the invoking user. In either case, the primary group will be set to group. The sudoers policy permits any of the target user's groups to be specified via the
-goption as long as the
-Poption is not in use.
- Request that the security policy set the
HOMEenvironment variable to the home directory specified by the target user's password database entry. Depending on the policy, this may be the default behavior.
- Display a short help message to the standard output and exit.
- 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
-loption to list a user's privileges for the remote host.
- 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, .bash_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
-coption. If no command is specified, an interactive shell is executed.
sudoattempts 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. Note that most shells behave differently when a command is specified as compared to an interactive session; consult the shell's manual for details. The Command environment section in the sudoers(5) manual documents how the
-ioption affects the environment in which a command is run when the sudoers policy is in use.
- Similar to the
-koption, 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.
- When used without a command, invalidates the user's cached credentials. In
other words, the next time
sudois run a password will be required. This option does not require a password and was added to allow a user to revoke
sudopermissions from a .logout file.
When used in conjunction with a command or an option that may require a password, this option will cause
sudoto ignore the user's cached credentials. As a result,
sudowill 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.
- If no command is specified, list the allowed (and
forbidden) commands for the invoking user (or the user specified by the
-Uoption) 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 a command is specified but not allowed by the policy,
sudowill exit with a status value of 1.
- Avoid prompting the user for input of any kind. If a password is required
for the command to run,
sudowill display an error message and exit.
- 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.
- Use a custom password prompt with optional escape sequences. The following
%’) escape sequences are supported by the sudoers policy:
- 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))
- expanded to the local host name without the domain name
- expanded to the name of the user whose password is being requested (respects the rootpw, targetpw, and runaspw flags in sudoers(5))
- expanded to the login name of the user the command will be run as
(defaults to root unless the
-uoption is also specified)
- expanded to the invoking user's login name
- two consecutive ‘
%’ characters are collapsed into a single ‘
The custom prompt will override the default prompt specified by either the security policy or the
SUDO_PROMPTenvironment variable. On systems that use PAM, the custom prompt will also override the prompt specified by a PAM module unless the passprompt_override flag is disabled in sudoers.
- Run the command with an SELinux security context that includes the specified role.
- Write the prompt to the standard error and read the password from the standard input instead of using the terminal device.
- Run the shell specified by the
SHELLenvironment 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
-coption. If no command is specified, an interactive shell is executed. Note that most shells behave differently when a command is specified as compared to an interactive session; consult the shell's manual for details.
- 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.
- Used in conjunction with the
-loption 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
ALLprivilege on the current host to use this option.
- Used to set a timeout for the command. If the timeout expires before the command has exited, the command will be terminated. The security policy may restrict the ability to set command timeouts. The sudoers policy requires that user-specified timeouts be explicitly enabled.
- 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.,
#0for 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.
- Print the
sudoversion 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
-Voption will display the arguments passed to configure when
sudowas built and plugins may display more verbose information such as default options.
- Update the user's cached credentials, authenticating the user if
necessary. For the sudoers plugin, this extends the
sudotimeout for another
15minutes by default, but does not run a command. Not all security policies support cached credentials.
--option indicates that
sudoshould 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
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
sudoexecutes 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
-Poption 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¶There are two distinct ways
sudocan run a command.
If an I/O logging plugin is configured or if the security policy
explicitly requests it, a new pseudo-terminal (“pty”) is
allocated and fork(2) is used to create a second
sudo process, referred to as the
monitor. The monitor creates a new
terminal session with itself as the leader and the pty as its controlling
terminal, calls fork(2), sets up the execution environment
as described above, and then uses the execve(2) system
call to run the command in the child process. The monitor
exists to relay job control signals between the user's existing terminal and
the pty the command is being run in. This makes it possible to suspend and
resume the command. Without the monitor, the command would be in what POSIX
terms an “orphaned process group” and it would not receive any
job control signals from the kernel. When the command exits or is terminated
by a signal, the monitor passes the command's exit status
to the main
sudo process and exits. After receiving
the command's exit status, the main
sudo passes the
command's exit status to the security policy's close function and exits.
If no pty is used,
fork(2), sets up the execution environment as described
above, and uses the execve(2) system call to run the
command in the child process. The main
waits until the command has completed, then passes the command's exit status
to the security policy's close function and exits. As a special case, if the
policy plugin does not define a close function,
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
sudowill relay signals it receives to the command. The
SIGQUITsignals 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
SIGINTtwice each time the user enters control-C. Some signals, such as
SIGKILL, cannot be caught and thus will not be relayed to the command. As a general rule,
SIGTSTPshould be used instead of
SIGSTOPwhen you wish to suspend a command being run by
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
all non-system processes other than itself before rebooting the system. This
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
close() function, set a command timeout or
required that the command be run in a new pty,
may execute the command directly instead of running it as a child
Plugins¶Plugins may be specified via
Plugindirectives in the sudo.conf(5) file. They may be loaded as dynamic shared objects (on systems that support them), or compiled directly into the
sudobinary. If no sudo.conf(5) file is present, or it contains no
sudowill 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(5) manual for more information about the
EXIT VALUE¶Upon successful execution of a command, the exit status from
sudowill be the exit status of the program that was executed. If the command terminated due to receipt of a signal,
sudowill send itself the same signal that terminated the command.
-l option was specified without a
sudo will exit with a value of 0 if the
user is allowed to run
sudo and they authenticated
successfully (as required by the security policy). If a command is specified
-l option, the exit value will only be 0 if
the command is permitted by the security policy, otherwise it will be 1.
If there is an authentication failure, a configuration/permission
problem or if the given command cannot be executed,
sudo exits with a value of 1. 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
sudotries to be safe when executing external commands.
To prevent command spoofing,
"." 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
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
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
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.
sudoutilizes the following environment variables. The security policy has control over the actual content of the command's environment.
- Default editor to use in
-e(sudoedit) mode if neither
- Set to the mail spool of the target user when the
-ioption is specified or when env_reset is enabled in sudoers (unless
- Set to the home directory of the target user when the
-Hoptions are specified, when the
-soption 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.
- Set to the login name of the target user when the
-ioption is specified, when the set_logname option is enabled in sudoers or when the env_reset option is enabled in sudoers (unless
LOGNAMEis present in the env_keep list).
- May be overridden by the security policy.
- 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
-Aoption is specified.
- Set to the command run by sudo.
- Default editor to use in
- Set to the group ID of the user who invoked sudo.
- Used as the default password prompt unless the
-poption was specified.
- If set,
PS1will 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 name of the user who invoked sudo.
- Set to the same value as
LOGNAME, described above.
- Default editor to use in
-e(sudoedit) mode if
SUDO_EDITORis not set.
sudofront 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:
$ sudoedit -u www ~www/htdocs/index.html
To view system logs only accessible to root and users in the adm group:
$ sudo -g adm more /var/log/syslog
To run an editor as jim with a different primary group:
$ sudoedit -u jim -g audio ~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"
DIAGNOSTICS¶Error messages produced by
editing files in a writable directory is not permitted
- By default,
sudoeditdoes not permit editing a file when any of the parent directories are writable by the invoking user. This avoids a race condition that could allow the user to overwrite an arbitrary file. See the sudoedit_checkdir option in sudoers(5) for more information.
editing symbolic links is not permitted
- By default,
sudoeditdoes not follow symbolic links when opening files. See the sudoedit_follow option in sudoers(5) for more information.
effective uid is not 0, is sudo installed setuid root?
sudowas not run with root privileges. The
sudobinary must be owned by the root user and have the Set-user-ID bit set. Also, it must not be located on a file system mounted with the ‘nosuid’ option or on an NFS file system that maps uid 0 to an unprivileged uid.
effective uid is not 0, is sudo on a file system with the 'nosuid' option set or an NFS file system without root privileges?
sudowas not run with root privileges. The
sudobinary has the proper owner and permissions but it still did not run with root privileges. The most common reason for this is that the file system the
sudobinary is located on is mounted with the ‘nosuid’ option or it is an NFS file system that maps uid 0 to an unprivileged uid.
fatal error, unable to load plugins
- An error occurred while loading or initializing the plugins specified in sudo.conf(5).
invalid environment variable name
- One or more environment variable names specified via the
-Eoption contained an equal sign (‘
=’). The arguments to the
-Eoption should be environment variable names without an associated value.
no password was provided
sudotried to read the password, it did not receive any characters. This may happen if no terminal is available (or the
-Soption is specified) and the standard input has been redirected from /dev/null.
no tty present and no askpass program specified
sudoneeds to read the password but there is no mechanism available to do so. A terminal is not present to read the password from,
sudohas not been configured to read from the standard input, and no askpass program has been specified either via the
-Aoption or the
no writable temporary directory found
sudoeditwas unable to find a usable temporary directory in which to store its intermediate files.
sudo must be owned by uid 0 and have the setuid bit set
sudowas not run with root privileges. The
sudobinary does not have the correct owner or permissions. It must be owned by the root user and have the Set-user-ID bit set.
sudoedit is not supported on this platform
- It is only possible to run
sudoediton systems that support setting the effective user-ID.
timed out reading password
- The user did not enter a password before the password timeout (5 minutes by default) expired.
you do not exist in the passwd database
- Your user ID does not appear in the system passwd database.
you may not specify environment variables in edit mode
- It is only possible to specify environment variables when running a command. When editing a file, the editor is run with the user's environment unmodified.
SEE ALSO¶su(1), stat(2), login_cap(3), passwd(5), sudo.conf(5), sudo_plugin(5), sudoers(5), sudoreplay(8), visudo(8)
HISTORY¶See the HISTORY file in the
sudodistribution (https://www.sudo.ws/history.html) for a brief history of sudo.
AUTHORS¶Many people have worked on
sudoover the years; this version consists of code written primarily by:
See the CONTRIBUTORS file in the
distribution (https://www.sudo.ws/contributors.html) for an exhaustive list
of people who have contributed to
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
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
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.
sudois 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
sudoor https://www.sudo.ws/license.html for complete details.
|November 25, 2018||Sudo 1.8.27|