This is an extremely useful task from Doug Mink's wcstools package. Below are some simple examples. Note that the "-u" will insure that something is printed to standard out (i.e. "___") even if a non-present keyword is requested.

% gethead Rsco5368.fits OBJECT
N6951

% gethead -u aa aa
GETHEAD: no files specified
 
  This message appears when the image file does NOT exist. 
   

% ls
out.hccs  phaseI_begin.sh*  README  Rsco5368.fits
% gethead Rsco5368.fits dd   
 
  Nothing appears with a BOGUS keyword 
   

% gethead -u /home/sco/sco/Red_PhaseI_Data/jun_2007_p1/RREU2100.fits OBJECT
SA112
 
  We can use a fullpath name for the image file. 
   

% gethead -u /home/sco/sco/Red_PhaseI_Data/jun_2007_p1/RREU2100.fits OBJ   
___
 
  We at least get a "___" for a BOGUS keyword if the "-u" flag is used. 
   

% gethead -u /home/sco/sco/Red_PhaseI_Data/jun_2007_p1/RREU2100.fits OBJ >>aaa
% cat aaa
___

A fairly common thing I would want to do is find a list of images, and then determine what targets (objects) were in those images. HERE IS AN EXAMPLE.