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sudo - Unix, Linux Command
NAME
sudo, sudoedit - execute a command as another userSYNOPSIS
sudo -h | -K | -k | -L | -l | -V | -vsudo [-bEHPS] [-p prompt] [-u username|#uid] [VAR=value] {-i | -s | command}
sudoedit [-S] [-p prompt] [-u username|#uid] file ...
DESCRIPTION
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 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).When invoked as sudoedit, the -e option (described below), is implied.
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 via sudoers).
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 flags. 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 flag 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_USER.
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.
OPTIONS
sudo accepts the following command line options:| Tag | Description | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| -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 the process. | ||||||||||||||
| -E | The -E (preserve environment) option will override the env_reset option in sudoers(5)). It is only available when either the matching command has the SETENV tag or the setenv option is set in sudoers(5). | ||||||||||||||
| -e |
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:
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. | ||||||||||||||
| -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 (see set_home and always_set_home in sudoers(5)). | ||||||||||||||
| -h | The -h (help) option causes sudo to print a usage message and exit. | ||||||||||||||
| -i | The -i (simulate initial login) option runs the shell specified in the passwd(5) entry of the user that the command is being run as. The command name argument given to the shell begins with a ‘-’ to tell the shell to run as a login shell. sudo attempts to change to that user’s home directory before running the shell. It also initializes the environment, leaving TERM unchanged, setting HOME, SHELL, USER, LOGNAME, and PATH, and unsetting all other environment variables. Note that because the shell to use is determined before the sudoers file is parsed, a runas_default setting in sudoers will specify the user to run the shell as but will not affect which shell is actually run. | ||||||||||||||
| -K | The -K (sure kill) option is like -k except that it removes the user’s timestamp entirely. Like -k, this option does not require a password. | ||||||||||||||
| -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 .logout file. | ||||||||||||||
| -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). | ||||||||||||||
| -l | The -l (list) option will list out the allowed (and forbidden) commands for the invoking user on the current host. | ||||||||||||||
| -P | The -P (preserve group 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. | ||||||||||||||
| -p |
The -p (prompt) option allows you to override the default
password prompt and use a custom one. The following percent (‘%’)
escapes are supported:
| ||||||||||||||
| -S | The -S (stdin) option causes sudo to read the password from the standard input instead of the terminal device. | ||||||||||||||
| -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). | ||||||||||||||
| -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. 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 password database. | ||||||||||||||
| -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. | ||||||||||||||
| -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. | ||||||||||||||
| -- | The -- flag indicates that sudo should stop processing command line arguments. It is most useful in conjunction with the -s flag. |
RETURN VALUES
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.
SECURITY NOTES
sudo tries to be safe when executing external commands.There are two distinct ways to deal with environment variables. By default, the env_reset sudoers option is enabled. This causes commands to be executed with a minimal environment containing TERM, PATH, HOME, SHELL, LOGNAME, USER and USERNAME in addition to variables from the invoking process permitted by the env_check and env_keep sudoers 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 _RLD*, DYLD_*, LD_*, LDR_*, LIBPATH, 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 actual PATH environment variable is not modified and is passed unchanged to the program that sudo executes.
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 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 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 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 sudoers(5).
ENVIRONMENT
sudo utilizes the following environment variables:| Tag | Description |
|---|---|
| EDITOR | Default editor to use in -e (sudoedit) mode if VISUAL is not set |
| HOME | In -s or -H mode (or if sudo was configured with the --enable-shell-sets-home option), set to homedir of the target user |
| PATH | Set to a sane value if the secure_path sudoers option is set. |
| SHELL | Used to determine shell to run with -s option |
| SUDO_PROMPT | Used as the default password prompt |
| SUDO_COMMAND | Set to the command run by sudo |
| SUDO_USER | Set to the login of the user who invoked sudo |
| 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 value |
| USER | Set to the target user (root unless the -u option is specified) |
| VISUAL | Default editor to use in -e (sudoedit) mode |
FILES
| Tag | Description |
|---|---|
| /etc/sudoers | List of who can run what |
| /var/run/sudo | Directory containing timestamps |
EXAMPLES
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 yazza on a machine where the file system holding ~yazza is not exported as root:
$ 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 sub-shell to make the cd and file redirection work.
$ sudo sh -c "cd /home ; du -s * | sort -rn > USAGE"
SEE ALSO
grep(1), su(1), stat(2),
passwd(5), sudoers(5), visudo(8)
AUTHORS
Many people have worked on sudo over the years; this
version consists of code written primarily by:
Todd C. Miller
Chris Jepeway
See the HISTORY file in the sudo distribution or visit http://www.sudo.ws/sudo/history.html for a short history of 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 sudo’s noexec functionality. See the sudoers(5) manual for details.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).