grep cares about .dat extension?

Question

GNU grep 2.6.3
Linux version 2.6.32-220.el6.i686 (Red Hat 4.4.6-3)

I have a .dat file that grep will not function on. It's plain text and can be read by all standard output commands, e.g. "less", "cat", etc, so I know what it contains.

If I grep a phrase I absolutely know is in there it does not return, as if it found nothing.

The oddity is if I simply rename it with a different extension, like file.dat.txt, now grep will work just fine. If I rename it back to file.dat, once again grep will not find anything.

I also created a .dat file simply with the echo command:
echo "grep this" > test.dat
And still, grep "grep" test.dat returned nothing.
But after mv test.dat test.dat.txt, now grep "grep" test.dat.txt worked just fine.

Easy enough workaround (rename the file) but just thought this was odd behavior.

Answer 1

It is not normal behavior and I cannot reproduce the issue on my ubuntu nor centos machines.

It is very likely that GREP_OPTIONS is set in your bashrc (or bash_profile, profile or so on) to ignore *.dat.

To verify this, simply print the env variable:

 echo $GREP_OPTIONS

Somewhere in the output you should see -exclude=*.dat

EDIT: As suggested by Fox in the comments it could also be an alias, then command -v grep should show -exclude=*.dat somewhere in its output