Started collection of Linux tips.

Linux/admin.md

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+# Linux admin tips
+
+## General
+* Show processes `$ ps -eF`
+
+## Networking
+* Find open ports: `$ netstat -tulpn`
+* Scan for open ports: `$ nmap -sT [addr]`
+
+## User management
+* Create regular user `$ `
+* Create system user `$ `