Exiv2 is a command line utility to access image metadata:

* print the Exif metadata of JPEG, TIFF and several RAW image
formats as summary info, interpreted values, or the plain data
for each tag (a sample is here)
* print the IPTC metadata of JPEG images
* print, set and delete the JPEG comment of JPEG images
* set, add and delete Exif and IPTC metadata of JPEG images
* adjust the Exif timestamp (that's how it all started...)
* rename Exif image files according to the Exif timestamp
* extract, insert and delete Exif metadata, IPTC metadata and JPEG
comments
* extract, insert and delete the thumbnail image embedded in the
Exif metadata
* fix the Exif ISO setting of picture taken with Nikon cameras