Find Command – Why -exec cp {} dir + Not Working

A function is local to a shell, so you'd need find -exec to spawn a shell and have that function defined in that shell before being able to use it. Something like:

find ... -exec ksh -c '
  function foo {
    echo blan: "$@"
  }
  foo "$@"' ksh {} +

bash allows one to export functions via the environment with export -f, so you can do (in bash):

foo() { ...; }
export -f foo
find ... -exec bash -c 'foo "$@"' bash {} +

ksh88 has typeset -fx to export function (not via the environment), but that can only be used by she-bang less scripts executed by ksh, so not with ksh -c.

Another option is to do:

find ... -exec ksh -c "
  $(typeset -f foo)"'
  foo "$@"' ksh {} +

That is, use typeset -f to dump the definition of the foo function inside the inline script. Note that if foo uses other functions, you'll also need to dump them as well.

There was several issues with your attempts, including backticks used instead of quotes (removed in later edits to the question), missing quotes where they are required, extra quotes where they are useless, missing parentheses to group -o clauses, and different implementations of findused (see the comments and chat for details).

Anyway, the command can be simplified like this:

find a/folder b/folder -name "*.[ch]" -exec grep -I foobar {} +

or, should you use an archaic GNU find version, this should always work:

find a/folder b/folder -name "*.[ch]" -exec grep -I foobar {} \;