Timestamping photos based on filename?

Question

(I'm using Ubuntu 16.04 but this isn't necessarily Ubuntu-specific which is why I'm asking here and not on askubuntu)

I have a set of photos that I would like to date/time-stamp but some of them have missing EXIF data which I would like to somehow repopulate using exiftool or whatever else is more appropriate. The photo filenames are already correct (e.g. IMG_20120921_210143.jpg)

Is there a simple way to pass the filename to exiftool and populate the EXIF data so that I can then use that with Imagemagick to timestamp the photo?

Answer 1

The third paragraph under Exiftool FAQ 5 has the command:
exiftool "-alldates<filename" <DirOrFile>

This command creates backup files. Add -overwrite_original to suppress the creation of backup files. Add -r to recurse into subdirectories. If this command is run under Unix/Mac, reverse any double/single quotes to avoid bash interpretation.