grep alias to safely ignore multiple directories

I'd use find:

mygrep() (
  pattern=$1; shift
  find "$@" -type d -path '*/app/lib/bower' -exec sh -c '
    printf >&2 "Warning: ignoring \"%s\" directory\n" "$@"
    ' sh {} + -prune -o -type f -exec grep -IHne "$pattern" {} +
)

(-I and -H being GNU extensions).

Use as:

mygrep pattern dir1 dir2

(making sure dir1 and dir2 don't look like find predicates).

Remove the -exec sh...+ if you don't care about those warning messages.

It's a bad idea to use grep as your alias name when you change its behaviour so dramatically.

If you need to pass options to grep, you could use the GREP_OPTIONS variable like:

GREP_OPTIONS=-i mygrep foo dir

But note that support for that option will be removed in future versions of grep, so a better option might be to give a way in mygrep to pass options to grep like with a dedicated array if your shell supports them:

mygrep() (
  pattern=$1; shift
  find "$@" -type d -path '*/app/lib/bower' -exec sh -c '
    printf >&2 "Warning: ignoring \"%s\" directory\n" "$@"
    ' sh {} + -prune -o -type f -exec \
    grep -IHne "${g[@]}" "$pattern" {} +
)

g=(-i --exclude-dir=.git); mygrep foo dir1 dir2

Or use -- to tell mygrep where the options stop:

mygrep() (
  grep_options=()
  for i do
    grep_options+=("$i")
    shift
    [ "$i" != "--" ] || break
  done
  pattern=${1?need a pattern}; shift
  find "$@" -type d -path '*/app/lib/bower' -exec sh -c '
    printf >&2 "Warning: ignoring \"%s\" directory\n" "$@"
    ' sh {} + -prune -o -type f -exec \
    grep -H "${grep_options[@]}" "$pattern" {} +
)

mygrep -nI --exclude-dir=.git -- pattern dir1 dir2

(and make sure you don't pass -- as an option arguments, as in if you want to exclude the -- files, use --exclude=--, not --exclude -- for instance. That also precludes usages like mygrep -e pattern1 -e pattern2 -- dir or mygrep -f patternfile -- dir).