Ttgjyuk

The man page for grep reads

 -i, --ignore-case 
 Ignore case distinctions in both the PATTERN and the input 
 files. (-i is specified by POSIX.)

However, if I change case on a filename, it won't work.

$ touch WHATEVER
$ grep -i pattern whatever
grep: whatever: No such file or directory

Am I missing something?

That confusing snippet was changed in newer versions of GNU grep to:

-i, -ignore-case
Ignore case distinctions, so that characters that differ only in case match each other.

See this commit: http://git.savannah.gnu.org/cgit/grep.git/commit/?id=e1ca01be48cb64e5eaa6b5b29910e7eea1719f91

 .BR -i ", " -^-ignore-case
-Ignore case distinctions in both the
-.I PATTERN
-and the input files.
+Ignore case distinctions, so that characters that differ only in case
+match each other.

"It ignores case in the file (contents) but also in the pattern",
this suggests (although it doesn't necessarily say it),
that it is possible to ignore case in the pattern, but not in the contents. 
I'd like to understand how this would work
(ignoring pattern, but not contents -- or the other way around).

Well, for example, it could be written
so that a pattern of “hello” would match “Hello” in the file,
but not vice versa. 
While this sounds hypothetical, it is the way spell-check works. 
If your dictionary contains “stack” and “exchange”,
and your document contains “Stack Exchange”,
spell-check will succeed without error. 
But if your dictionary contains “Unix” and your document contains “unix”,
that will be flagged as an error.

Apparently I have a different manpage.

 -i, --ignore-case
 Ignore case distinctions, so that characters that differ only in
 case match each other.

In any case, it's not about the filenames.

It ignores case in the file (contents) but also in the pattern.

Test file:

___________
Hello World
^^^^^^^^^^^

Grep results (ignore case of file contents):

$ grep hello test.txt 

$ grep Hello test.txt 
Hello World
$ grep -i HELLO test.txt 
Hello World
$ grep -i hello test.txt 
Hello World

Grep results (ignore case of pattern):

$ grep [a-Z] test.txt 
grep: Invalid range end
$ grep -i [a-Z] test.txt
Hello World
$ grep -i [A-z] test.txt
Hello World
$ grep [A-z] test.txt
___________
Hello World
^^^^^^^^^^^

As you can see the results can sometimes be a little unexpected.

Not sure if there is an example where this actually matters more.