John,
I've never used it, although I really should :)
jhead supports the following options:
-dc Delete comment field (as left by progs like Photoshop & Compupic)
-ce Edit comment field. Uses environment variable 'editor' to
determine which editor to use. If editor not set, uses VI
under Unix and notepad with windows
-cs <name> Save comment section to a file
-ci <name> Insert comment section from a file. -cs and -ci use same naming
scheme as used by the -st option
-cl string Insert literal comment string
so in theory jhead -cl "this is my string" should work.
-Bart
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