# intersection (-i) of key fields:
# (default delimiter is \s+, so use "-d ," here)

$ cmpcol -i -d , left right
1
4
5

# print those records with the combined line
# content from both files:

$ cmpcol -i -d , -lb left right
1,a:<>:1,a
4,f:<>:4,g
5,h:<>:5,h

# keys only in "left" file, which is "#1" on the
# command line (-x1), showing full line (-l1):

$ cmpcol -x1 -l1 -d ,  left right
2,b

# lines with identical content (don't use
# the comma delimiter in this case):

$ cmpcol -i  left right
1,a
5,h

# all lines that are not identical (this
# always shows which file contained each line):

$ cmpcol -x left right
2,b <1
3,c <2
4,f <1
4,g <2

# and so on.  I think the "usage" message is
# the best part (at least I hope so):

$ cmpcol

Usage: cmpcol {-i|-u|-us|-x|-x1|-x2} [options] file1[:col#] file2[:col#]

 Comparison modes:
  -i        : produce intersection of file1[:col#] and file2[:col#]
  -u(s)     : produce union (and identify sources)
  -x        : produce exclusive-or, identifying sources
  -x1 (-x2) : produce items unique to file1 (or file2)

 Options:
  -l{1,2,b} : print whole lines from file1,file2 or both (default: print column)
  -g(v) ptn : grep (-v) -- only compare lines that (don't) contain /ptn/
  -c cchar  : ignore material following cchar
  -d delim  : use /delim/ as column separator (default is white-space)
              use "-d tab" for tab-delimited, "-d dot" for period-delimited

 file1 or file2 may be 'stdin'
 default ':col#' for comparison is first column from each file, which is ':1'