I am having a hard time doing something quite simple. I have around 2000 .txt files which are composed of a single line of digits. I would like to fold the single line into one column with many rows, using the fold command and overwrite the original file with this new, folded version. My attempt:

```
#!/bin/bash
for i in *.txt ; do
  fold -w 1 $file > $file
done

```

Doesn't work. Help? Thanks.