Python 2.7+ compatible script for adding and modifying FITS header values. Can work on multiple files. This is a wrapper around pyfits.setval to make it accessible from the command line.

hedit.py

#!/usr/bin/env python
"""
Add or modify a header value. Type hedit -h for help.

Requires pyfits and Python 2.7+.

Author
------
@jiffyclub
git.io/jiffyclub

Examples
--------

Modify a single file:

> hedit jb1f98q1q_raw.fits EXPSTART 58000 -i
jb1f98q1q_raw.fits[0]: EXPSTART -> 58000

Modify multiple files:

> hedit *.fits FLATCORR PERFORM

Modify an extension header:

> hedit jb1f98q1q_raw.fits SOMEKEY SOMEVALUE --ext 1

"""

import argparse

import pyfits


def setval(fits, key, value, ext):
    print('{}[{}]: {} -> {}'.format(fits,ext,key,value))
    pyfits.setval(fits, key, value=value, ext=ext)


def parse_args():
    parser = argparse.ArgumentParser(description=
                                      'Add or modify a header value.')

    parser.add_argument('fits_files', nargs='+', type=str,
                         help='Name of fits files.')

    parser.add_argument('keyword', help='Keyword to update.')

    parser.add_argument('new_value', help='New value of keyword')

    parser.add_argument('-e', '--ext', type=int, default=0,
                        help='Extension number. Defaults to 0.')

    parser.add_argument('-s', '--str', action='store_const', const=str,
                        dest='type',
                        help='Value will be stored as string (default).')

    parser.add_argument('-f', '--float', action='store_const', const=float,
                        dest='type', help='Value will be stored as float.')

    parser.add_argument('-i', '--int', action='store_const', const=int,
                        dest='type', help='Value will be stored as integer.')

    parser.add_argument('-b', '--bool', action='store_const', const=True,
                        dest='type', help='Value will be stored as boolean.')

    return parser.parse_args()


def main():
    args = parse_args()

    if args.type is not None:
        value_type = args.type
    else:
        # if nothing specified, default to string
        value_type = str

    # convert args.new_value to value_type
    if value_type is True:
        # boolean type
        if args.new_value == 'True':
            new_value = True
        elif args.new_value == 'False':
            new_value = False
        else:
            raise ValueError("Boolean values must be either 'True' or 'False'.")
    else:
        new_value = value_type(args.new_value)

    for fits_file in args.fits_files:
        setval(fits_file, args.keyword, new_value, args.ext)


if __name__ == '__main__':
    raise SystemExit(main())

A Python 2.3+ version of this that uses optparse is also available: https://gist.github.com/1401666.