The jpeg-xmp package contains the C source of four programs:

wrjpgxmp
insert XMP metadata into a JPEG file
wrjpgapp
insert application data into a JPEG file
rdjpgxmp
extract XMP metadata from a JPEG file
rdjpgapp
extract application data from a JPEG file

The xmptool package contains one program:

xmptool
query, insert or delete a property in an XMP file

jpegxmp is a Bash shell script:

jpegxmp
Set or modify common metadata properties in JPEG files

Use rdjpgxmp to extract the XMP data from a JPEG/JFIF file, use xmptool to query or update that XMP data, then use wrjpgxmp to put the XMP data back, e.g.: rdjpgxmp file.jpg | xmptool -w 'http...property' 'value' | wrjpgxmp file.jpg >newfile.jpg. The jpegxmp script is a wrapper around these to make the most common operations easier.

For more info on XMP/RDF, see: