grep1-2  Exercise 2: What -I Hides, and Which Files Lack a TODO
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(Run from an empty folder. The first block builds the practice project from the chapter; your own tree can skip it.)

mkdir -p project/src project/notes project/build project/logs project/data project/.git "project/my notes"
cd project

printf '# Demo project\nTODO write the install guide\nRun app.py to start\n' > README.md
printf 'import os\n# TODO handle errors\ndef main():\n    print("hello")\n' > app.py
printf 'def helper():\n    # TODO remove this\n    return 1\n' > src/util.py
printf '// TODO rename\nconsole.log("hi");\n' > src/main.js
printf 'buy milk\nTODO call the plumber\n' > notes/todo.txt
printf '/* generated */ var a = 1; // TODO generated noise\n' > build/bundle.js
printf 'INFO started\nERROR something broke\n' > logs/app.log
printf 'data\0TODO inside a binary file\n' > data/image.bin
printf 'API_KEY=abc123\npassword=hunter2\n' > .env
printf '[core]\n\trepositoryformatversion = 0\n# TODO placeholder inside .git\n' > .git/config
printf 'TODO book the room\npassword reminder: change it\n' > "my notes/meeting notes.txt"

# --- the exercise ---
echo "Files that contain TODO but are hidden by -I:"
comm -13 <(grep -rIl TODO . | sort) <(grep -rl TODO . | sort)
echo
echo "Files with no TODO at all (the .git folder skipped):"
grep -rL TODO --exclude-dir=.git .
echo
echo "The same with -I: binary files now count as 'no match', so the binary file is listed as well:"
grep -rIL TODO --exclude-dir=.git .

Output:

Files that contain TODO but are hidden by -I:
./data/image.bin

Files with no TODO at all (the .git folder skipped):
./.env
./logs/app.log

The same with -I: binary files now count as 'no match', so the binary file is listed as well:
./.env
./data/image.bin
./logs/app.log

WHY THIS WORKS AS AN ANSWER
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comm -13 prints the lines that are only in the second list. The second list is every file with a match, the first is every text file with a match (-I), so what is left is the files that match but are binary: data/image.bin.
Both lists go through sort because comm needs sorted input, and the <( ... ) form feeds a command's output to comm as if it were a file (bash, not plain sh).
-L lists files without the word. Without -I, the binary file DOES contain TODO (inside its bytes), so it is correctly not listed: the answer is .env and logs/app.log. With -I the binary file is skipped as 'no match', and -L lists it, which here is the wrong answer to 'which files lack a TODO'. So -I and -L together need care: -I means 'pretend binary files never match', not 'ignore binary files'.
Do not test the exit status of -L (Chapter 2): it is 0 when nothing is listed and 1 when something is.