*Note — This is not an original sudobash.net article — credit goes completely to
http://www.gratisoft.us/sudo/man/1.8.0/sudo.man.html. But I know people use search engines on ‘sudo bash’ and end up here so why not provide the answer as well? 😉
sudo, sudoedit — execute a command as another user
sudo [-D level] -h | -K | -k | -V sudo -v [-AknS] [-a auth_type] [-D level] [-g group name|#gid] [-p prompt] [-u user name|#uid] sudo -l[l] [-AknS] [-a auth_type] [-D level] [-g group name|#gid] [-p prompt] [-U user name] [-u user name|#uid] [command] sudo [-AbEHnPS] [-a auth_type] [-C fd] [-D level] [-c class|-] [-g group name|#gid] [-p prompt] [-r role] [-t type] [-u user name|#uid] [VAR=value] [-i | -s] [command] sudoedit [-AnS] [-a auth_type] [-C fd] [-c class|-] [-D level] [-g group name|#gid] [-p prompt] [-u user name|#uid] file …
sudo allows a permitted user to execute a command as the superuser or another user, as specified by the security policy. The real and effective uid and gid 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).
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 modules to work seemlessly 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 5 minutes.
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 5 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.
sudo accepts the following command line options:
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 /etc/sudo.conf contains a line specifying the askpass program, that value will be used. For example:
Path askpass /usr/X11R6/bin/ssh-askpass
If no askpass program is available, sudo will exit with an error.
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.
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. Most interactive commands will fail to work properly in background mode.
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. The security policy may restrict the user’s ability to use the -C option. The sudoers policy only permits use of the -C option when the administrator has enabled the closefrom_override option.
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.
Enable debugging of sudo plugins and sudo itself. The level may be a value from 1 through 9.
The -E (preserve environment) option indicates to the security policy that the uses wishes to preserve their existing environment variables. The security policy may return an error if the -E option is specified and the user does not have permission to preserve the environment.
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 security policy. If the user is authorized by the policy, 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 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.
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 temporary file.
Normally, sudo runs a command with the primary group set to the one specified by the password database for the user the command is being run as (by default, root). The -g (group) option causes sudo to run the command with the primary group set to group instead. 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 requests that the security policy set the HOME environment variable to the home directory of the target user (root by default) as specified by the password database. Depending on the policy, this may be the default behavior.
The -h (help) option causes sudo to print a short help message to the standard output and exit.
The -i (simulate initial login) option runs the shell specified by the password database entry of the target user 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 security policy shall initialize the environment to a minimal set of variables, similar to what is present when a user logs in.
The -K (sure kill) option is like -k 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 alone, the -k (kill) option to sudo invalidates the user’s cached credentials. 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. Not all security policies support credential caching.
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 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.
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 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. 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 (preserve group vector) option causes sudo to 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 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 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 -u option is also specified)
expanded to the invoking user’s login name
two consecutive % characters are collapsed into a single % character
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 -r (role) option causes the new (SELinux) security context to have the role specified by role.
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 the password database. 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.
The -t (type) option causes the new (SELinux) security context to have the type specified by type. If no type is specified, the default type is derived from the specified role.
The -U (other user) option is used in conjunction with the -l option to specify the user whose privileges should be listed. 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.
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 (‘\’). 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.
The -V (version) option causes sudo to print its version string and 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.
When given the -v (validate) option, sudo will update the user’s cached credentials, authenticating the user’s password if necessary. For the sudoers plugin, this extends the sudo timeout for another 5 minutes (or whatever the timeout is set to in sudoers) 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 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 overwise be forbidden. See sudoers(5) for more information.
Plugins are dynamically loaded based on the contents of the /etc/sudo.conf file. If no /etc/sudo.conf file is present, or it contains no Plugin lines, sudo will use the traditional sudoers security policy and I/O logging, which corresponds to the following /etc/sudo.conf file.
<pre class=”brush: bash;”>Plugin policy_plugin sudoers.so<br>
Plugin io_plugin sudoers.so</pre>
A Plugin line consists of the Plugin keyword, followed by the symbol_name and the path to the shared object containing the plugin. The symbol_name is the name of the struct policy_plugin or struct io_plugin in the plugin shared object. The path may be fully qualified or relative. If not fully qualified it is relative to the /usr/local/libexec directory. Any additional parameters after the path are ignored. Lines that don’t begin with Plugin or Path are silently ignored
For more information, see the sudo_plugin(8) manual.
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 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 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.
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.
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).
sudo utilizes the following environment variables. The security policy has control over the content of the command’s environment.
Default editor to use in -e (sudoedit) mode if neither SUDO_EDITOR nor 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 sudoers
May be overridden by the security policy.
Used to determine shell to run with -s option
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
If set, 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 SUDO_EDITOR is not set
sudo plugin and path configuration
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 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"
grep(1), su(1), stat(2), login_cap(3), passwd(5), sudoers(5), sudo_plugin(8), sudoreplay(8), visudo(8)
Many people have worked on sudo over the years; this version consists of code written primarily by:
Todd C. Miller
See the HISTORY file in the sudo distribution or visit http://www.sudo.ws/sudo/history.html for a short history of sudo.
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) module’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).
If you feel you have found a bug in sudo, please submit a bug report at http://www.sudo.ws/sudo/bugs/
Limited free support is available via the sudo-users mailing list, see http://www.sudo.ws/mailman/listinfo/sudo-users to subscribe or search the archives.
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.