Its always useful to have quick access to Cheat Sheets and reference guides so here are some based on Linux that I’ve useful.
Must know info for any sysadmin – LINUX Admin Quick Reference.
Index of commands for Linux – A-Z Index of the Bash command line for Linux.
Learn common Linux commands & short-cuts – Linux Shortcuts and Commands.
A collection of Unix/Linux/BSD commands – UNIX Toolbox.
Compare MS Windows command line with the Linux equivalent – UNIX For DOS Users.
Need to find a command? – Directory of Linux commands.
Get commands for rpm, with descriptions and examples – rpm Linux Cheat Sheet.
Common commands for SSH – SSH Cheat Sheet.
A good starting point for Linux security – Linux Security Quick Reference Guide.
Reference to Shell Scripting in Linux – Shell Scripting Cheat Sheet for Unix & Linux.