Linux: grep for tab character

Two ways:

1. grep quote control-v tab quote
2. cat -T then pipe to grep '\^I'

First I created a file with tabs for testing.  I typed "Hi there" then the tab character, then return, then "this is a " then control-V, then the tab character then "real tab", and then the next line without tabs:

$ cat > foo
Hi there
this is a ^     real tab
isn't it.
$ cat foo
Hi there
this is a       real tab
isn't it.

1. You can grep for the tab character directly (sort of) by typing quote, then control-v, then another quote:
   

   $ grep "   " foo
   Hi there
   this is a       real tab

2. cat -T is an option to show the tabs in a file as "^I".
   It's tricky to grep for the "^I" though. grep \^I and grep "^I" don't work. You need grep '\^I'
   

   $ cat -T foo
   Hi there^I
   this is a ^Ireal tab
   isn't it.
   $ cat -T foo | grep "^I"
   $ cat -T foo | grep '\^I'
   Hi there^I
   this is a ^Ireal tab