Added sudo's notes to the knowledge base

knowledge base/sudo.md

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+# Sudo
+
+> Avoid modifying the `sudoers` files manually and execute `visudo` instead; it will check the syntax on save, preventing you from screwing up the file.
+
+## TL;DR
+
+```shell
+# Make changes to a sudoers file.
+visudo
+visudo -f path/to/file
+
+# Check the syntax of a sudoers file.
+visudo -c path/to/file
+```
+
+## Drop privileges
+
+```shell
+# Invalidate the user's cached credentials.
+sudo -k
+
+# Ignore the user's cached credentials for the given command only.
+sudo -k ls
+```
+
+## Restrict permissions a little
+
+```shell
+# file /etc/sudoers.d/user
+Cmnd_Alias UPGRADE_CMND  = /usr/bin/apt update, /usr/bin/apt list --upgradable, /usr/bin/apt upgrade
+Cmnd_Alias SHUTDOWN_CMND = /sbin/shutdown
+user ALL=(ALL:ALL) NOPASSWD: SHUTDOWN_CMND, UPGRADE_CMND
+```
+
+## Avoid providing a password
+
+```shell
+# file /etc/sudoers.d/user
+user ALL=(ALL:ALL) NOPASSWD: ALL
+```
+
+## Execute commands as a specific user
+
+Invoke a login shell using the `-i, --login` option. When one does not specify a command a login shell prompt is returned, otherwise the output of the command is returned:
+
+```shell
+% whoami
+root
+% sudo -i -u user
+$ whoami
+user
+
+% sudo -i -u user whoami
+user
+```
+
+## Troubleshooting
+
+### I modified a sudoers file manually, messed it up, and now I cannot use sudo anymore
+
+Should you see something similar to this when using `sudo`:
+
+```sh
+$ sudo visudo
+>>> /etc/sudoers: syntax error near line 28 <<<
+sudo: parse error in /etc/sudoers near line 28
+sudo: no valid sudoers sources found, quitting
+```
+
+try using another access method like `PolicyKit` and fix the file up:
+
+```sh
+pkexec visudo -f /etc/sudoers.d/user
+```
+
+## Sources
+
+- [How to modify an invalid sudoers file]
+- [sudo as another user with their environment]
+- [sudo: Drop root privileges]
+
+[how to modify an invalid sudoers file]: https://askubuntu.com/questions/73864/how-to-modify-an-invalid-etc-sudoers-file
+[sudo as another user with their environment]: https://unix.stackexchange.com/questions/176997/sudo-as-another-user-with-their-environment
+[sudo: drop root privileges]: https://coderwall.com/p/x2oica/sudo-drop-root-privileges