Adds more portable bash invocation.
Adds more severe bash evaluation.
Added h flag and made improper use print the usage function.
Added checks that make sure the parameters are okay.
Proper variable quoting, tests and $() use.
Setting / unsetting IFS variables can be removed by setting it only
in the loop context.
The for cat can be replaced by a simpler while read loop.
Proper variable quoting was added.
Changed the way modif was optionally passed, this removes the
extraneous ':' at the end.
echo(1) the lines as we go instead of building an array, this removes
the possibility of sub-shell screwups.
Benoît S.