Merge pull request #779 from slash3b/slash3b

visudo and getent were added

pages/common/visudo.md

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+# visudo 
+
+> Safely edit the sudoers file.
+
+- Edit sudoers file:
+
+`sudo visudo`
+
+- Check sudoers file for errors:
+
+`sudo visudo -c`

pages/linux/getent.md

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+# getent  
+
+> Get entries from Name Service Switch libraries.
+
+- Get list of all groups:
+
+`getent group`
+
+- See the members of a group:
+
+`getenet group {{group_name}}`
+
+- Get list of all services:
+
+`getent services`
+
+- Find a username by UID:
+
+`getent passwd 1000`
+
+- Perform a reverse DNS lookup:
+
+`getent hosts {{host}}`