Get notified if a file changes and display the changes.

feed-diff-ticker.sh

#!/bin/sh
# feed-diff-ticker.sh — print only the added lines since last snapshot
# Usage:
#   feed-diff-ticker.sh /tmp/someodd-feed.txt /tmp/someodd-feed.prev 10 | xrootconsole &
# Env:
#   MAX_LINES=100   # optional cap on burst output
# I put this in my crontab:
#   curl -s gopher://gopher.someodd.zip:70/0/gateway/status/feed > /tmp/someodd-feed.txt
# I added this to my GNUstep/Library/WindowMaker/autostart:
#   /home/tilde/bin/feed-diff-ticker.sh /tmp/someodd-feed.txt /tmp/someodd-feed.prev 10 | xrootconsole &

set -eu

FEED="${1:-/tmp/someodd-feed.txt}"
STATE="${2:-/tmp/someodd-feed.prev}"
INTERVAL="${3:-10}"
MAX_LINES="${MAX_LINES:-}"

WORK_DIR="$(dirname "$STATE")"
TMP_NEW="$WORK_DIR/.feed.new.$$"
TMP_DIF="$WORK_DIR/.feed.diff.$$"

cleanup() { rm -f "$TMP_NEW" "$TMP_DIF" 2>/dev/null || true; }
trap cleanup EXIT INT TERM

# Seed state file if missing
[ -f "$STATE" ] || : > "$STATE"

while :; do
  # Skip if feed missing or empty
  if [ ! -s "$FEED" ]; then
    sleep "$INTERVAL"; continue
  fi

  # Snapshot current feed (best with atomic mv in cron)
  cp -f -- "$FEED" "$TMP_NEW" 2>/dev/null || { sleep "$INTERVAL"; continue; }

  # Compute diff; exit codes: 0=same, 1=different, >1=error
  if diff -u --label old "$STATE" --label new "$TMP_NEW" >"$TMP_DIF" 2>/dev/null; then
    # No differences: do not touch STATE
    :
  else
    # We have differences (exit code 1). Extract ONLY added lines from “new”.
    # Strip headers (---/+++), hunks (@@), keep lines starting with '+', then drop the leading '+'.
    ADDED="$(sed -n '
      1,2d;                 # drop the first two header lines
      /^\+\+\+ /d;          # drop +++ header
      /^--- /d;             # drop --- header (in case)
      /^@@/d;               # drop hunk markers
      s/^\+//p              # print added lines with leading + removed
    ' "$TMP_DIF")"

    if [ -n "$ADDED" ]; then
      if [ -n "$MAX_LINES" ]; then
        printf "%s\n" "$ADDED" | tail -n "$MAX_LINES"
      else
        printf "%s\n" "$ADDED"
      fi
      # Update STATE only when we actually emitted new content
      mv -f -- "$TMP_NEW" "$STATE"
      : > "$TMP_NEW" 2>/dev/null || true
    fi
  fi

  sleep "$INTERVAL"
done