grep1-4  Exercise 3: A Script That Counts, and Tells Silence from Failure
=========================================================================

(Run from an empty folder. The first block makes the two practice files from the chapter; your own files can skip it.)

cat > app.log <<'EOF'
09:00:01 INFO  service starting
09:00:02 INFO  loading config
09:00:03 WARN  config key 'timeout' missing, using default
09:00:04 INFO  connecting to database
09:00:09 ERROR database connection refused
09:00:10 INFO  retrying in 5s
09:00:15 ERROR database connection refused
09:00:16 ERROR giving up after 2 attempts
09:00:17 INFO  switching to read-only mode
09:00:20 INFO  listening on port 8080
09:05:00 INFO  health check ok
09:10:00 WARN  slow response: 2300 ms
09:10:30 INFO  health check ok
09:15:00 ERROR disk usage 95%, ERROR threshold 90%
09:15:01 INFO  alert sent
09:20:00 INFO  health check ok
EOF

cat > access.log <<'EOF'
10.0.0.5 GET /index.html 200
10.0.0.7 GET /about.html 200
10.0.0.5 POST /login 302
10.0.0.9 GET /missing 404
10.0.0.5 GET /index.html 200
10.0.0.7 GET /index.html 200
10.0.0.5 GET /dashboard 200
10.0.0.9 GET /missing 404
10.0.0.7 GET /logout 302
EOF

# --- the exercise ---
cat > errcount.sh <<'EOF'
#!/bin/bash
# usage: errcount.sh FILE
n=$(grep -sc ERROR "$1")
case $? in
  0) echo "$1: $n line(s) with ERROR" ;;
  1) echo "$1: no ERROR lines" ;;
  *) echo "errcount: cannot read $1" >&2; exit 2 ;;
esac
EOF

bash errcount.sh app.log;    echo "exit status: $?"
bash errcount.sh access.log; echo "exit status: $?"
bash errcount.sh nosuchfile; echo "exit status: $?"
echo
echo "two busiest clients in access.log:"
grep -oE '^[0-9.]+' access.log | sort | uniq -c | sort -rn | head -2

Output:

app.log: 4 line(s) with ERROR
exit status: 0
access.log: no ERROR lines
exit status: 0
errcount: cannot read nosuchfile
exit status: 2

two busiest clients in access.log:
      4 10.0.0.5
      3 10.0.0.7

WHY THIS WORKS AS AN ANSWER
---------------------------
-s hides grep's own message about a missing file so the script can print one of its own; -c prints the count. The exit status is what separates the cases: 0 (there were matching lines, and n holds the count), 1 (nothing matched: -c printed 0 and the script says so plainly), and 2 (the file could not be read, in which case -c printed nothing at all, and the script says so on standard error and exits with 2 itself).
The status has to be read straight after the command substitution: $? is the status of n=$(...), which is grep's own. Putting any command in between would overwrite it.
The last line is the chapter's counting recipe again, with head -2 to keep the two busiest.