> File.open(file, 'a+') won't work when the file is not readable.
> 
> However, in a shell I can append a string using
>  echo string >> file
> 
> Am I forced to use exec + echo to append content, or is there a way to
> append a string to a file using ruby code.. without opening the file?
> 

Use mode 'a' instead of 'a+' in File.open

Yoann