Nano is a simple text editor preinstalled on most Linux distributions. Most useful commands:

• Ctrl+W - forward search
• Alt+W - forward search next
• Ctrl+Q - backward search
• Alt+Q - backward search next
• Alt+R - replace
• Ctrl+S - save file
• Ctrl+X - close file
• Alt+U - undo
• Alt+E - redo

Complete list is here.