Nano has experimental undo/redo feature. As you’ll see from the nano manual (type “man nano” in a Terminal to read that), you’ll need to start nano with the -u option
$ nano -u somefile.txt
… and then you can use Alt-U to undo and Alt-E to redo.
It’s little bit frustrating that you have to type the “-u” option every time when you want to edit a file, but hopefully there is a solution and its name is “alias”.
Aliases are a way for you to customize the commands by giving them aliases (nicknames). You can use them to remember commands with a lot of options or make their names shorter and easier to type. To setup aliases, all you need is to open the ~/.bash_aliases file and type in:
alias nano='nano -u'
If you want to add more aliases, enter each of them on separate line.
Now, typing “nano” is equal as typing “nano -u”. Tricky, yeah?