There is a very small, but finite risk of the Pi being turned off in the middle of a file write
Ha ha, I have long experience contemplating this annoying and tricky problem! The straightforward solution I concocted back in 2003 (and am still happy with) is to simply write a new file on the same file system ... and then use (atomic) rename to clobber the original file - but only after the new file has been successfully written. This is described in detail at:

In reply to Re^2: Yet another config file editing programme : Tell me how to make it better ! by eyepopslikeamosquito
in thread Yet another config file editing programme : Tell me how to make it better ! by dazz

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