sudo – In Linux it is used to execute a command as another user.
The sudo command is frequently used to execute a command that requires root privileges.
Say for example, if you want to run a shell script setup.sh that installs a program into a directory that only root can modify you can use sudo as follows: sudo -u root ./setup.sh
You may be required to enter the root password before the command is executed. After you have logged in, you can continue to execute commands through sudo for a few minutes without having to specify the login (-u root) with every command. If possible, it is better to do your regular work using an account with restricted privileges to avoid causing serious damage to the system by accident.
You can list the files of a protected directory with the following command: sudo ls /usr/local/classified
Use the following line to reboot the system in 20 minutes:
sudo shutdown -r +20 “rebooting to fix network issue”
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 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.
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:
- 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.
- The -l (list) option will list out the allowed (and forbidden) commands for the user on the current host.
- 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).
- The -h (help) option causes sudo to print a usage message and exit.
- 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 -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.
- The -K (sure kill) option to sudo removes the user’s timestamp entirely. Likewise, this option does not require a password.
- 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.
- The -p (prompt) option allows you to override the default password prompt and use a custom one. If the password prompt contains the %u
will be replaced with the user’s login name. Similarly, %h
will be replaced with the local hostname.
- 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 where sudo has been configured with the –with-logincap option.
- 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 where sudo has been configured with the –with-bsdauth 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 username, use #uid.
- 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).
- 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
- The -P (preserve group vector) option causes sudo to preserve the 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 -S (stdin) option causes sudo to read the password from standard input instead of the terminal device.
- The — flag indicates that sudo should stop processing command line arguments. It is most useful in conjunction with the -s flag.
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.
sudo tries to be safe when executing external commands. Variables that control how dynamic loading and binding is done can be used to subvert the program that sudo runs. To combat this the LD_*
(HP-UX only), and LIBPATH
(AIX only) environment variables are removed from the environment passed on to all commands executed. sudo will also remove the IFS
variables as they too can pose a threat. If the TERMCAP
variable is set and is a pathname, it too is ignored. Additionally, if the LC_*
variables contain the /
characters, they are ignored. If sudo has been compiled with SecurID support, the VAR_ACE
variables are cleared as well. The list of environment variables that sudo clears is contained in the output of sudo -V
when run as root.
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.
For security reasons, if your OS supports shared libraries and does not disable user-defined library search paths for setuid programs (most do), you should either use a linker option that disables this behavior or link sudo statically.
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 and only writable by 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 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 inadvertantly give the user an effective root shell.
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 filesystem 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"