In a folder, I have multiple files (about 60K) with random names like this and I want to rename all of them in alphabetically order. 

```
jlg6aej.gif  
owu46tljff.JPG  
28LKG.JPG
uoeoo87967.jpg
....

```

I know how to rename them numeritically, like

`ls | cat -n | while read n f; do mv "f" "$n.txt"; done` 

right now I want to rename them alphabetically rather than numeritically, so any alphabetical order is fine, but please use only letters, no numbers in file name, for example: 

```
aaaa.txt  
aaab.txt  
aaac.txt
aaad.txt  
....

```

Thanks!