#!/usr/bin/env bash
# Full example: counting pattern matches in a file

set -o nounset
set -o pipefail

usage() {
    printf 'Usage: %s [-v] [-o OUTPUT] PATTERN FILE\n' "$0"
}

verbose=false
output_file=''

while getopts ':vo:' opt; do
    case $opt in
        v) verbose=true ;;
        o) output_file=$OPTARG ;;
        :)
            printf 'Error: -%s requires an argument\n' "$OPTARG" >&2
            usage >&2
            exit 2
            ;;
        \?)
            printf 'Error: unknown option -%s\n' "$OPTARG" >&2
            usage >&2
            exit 2
            ;;
    esac
done

shift $((OPTIND - 1))

if [ $# -ne 2 ]; then
    usage >&2
    exit 2
fi

pattern=$1
input_file=$2

if [ ! -f "$input_file" ] || [ ! -r "$input_file" ]; then
    printf 'Error: input is not a readable file: %s\n' "$input_file" >&2
    exit 1
fi

tmp_file=$(mktemp)

cleanup() {
    rm -f -- "$tmp_file"
}

trap cleanup EXIT
trap 'exit 130' INT
trap 'exit 143' TERM

if [ "$verbose" = "true" ]; then
    printf 'Searching for <%s> in %s\n' "$pattern" "$input_file" >&2
fi

if grep -F -- "$pattern" "$input_file" > "$tmp_file"; then
    match_status=0
else
    match_status=$?
fi

# grep status 1 means no matches; treat that as a valid result.
if [ $match_status -gt 1 ]; then
    printf 'Error: grep failed with status %d\n' "$match_status" >&2
    exit "$match_status"
fi

if [ -n "$output_file" ]; then
    cp -- "$tmp_file" "$output_file"
    printf 'Wrote matches to %s\n' "$output_file"
else
    cat "$tmp_file"
fi

printf 'Matching lines: %s\n' "$(wc -l < "$tmp_file")" >&2

# Exit with grep's status: 0 for matches, 1 if there were none.
exit "$match_status"