python-update-frontmatter-link

1. Background

When editing Markdown blog posts, it’s common to set both og_image (for social media sharing) and thumbnail (for blog card display). Maintaining these two fields manually is time-consuming and error-prone—especially when image filenames are designed to match post slugs. With a simple Python script, you can batch-update the og_image and thumbnail fields in your posts’ Front Matter to ensure they correctly point to the corresponding image files, significantly improving maintenance efficiency.

2. Requirements

  • Blog posts are stored in the _posts folder, with filenames like: 2025-06-20-my-post.md
  • Corresponding images are located in assets/img/blog/, with filenames like: my-post.webp
  • Each post’s og_image and thumbnail fields should point to: /assets/img/blog/my-post.webp
  • If the fields do not exist, automatically insert og_image first, then thumbnail
  • Only modify Front Matter, not the main content
  • Preserve the existing path prefix and only replace the image filename

3. Solution

Use a Python script to loop through all posts, find the matching image file, and update the relevant Front Matter fields.

Steps:

  1. Strip the date prefix (e.g. 2025-06-20) from the post filename to extract the slug.
  2. Check whether a .webp image file named with that slug exists in the image folder.
  3. Parse the YAML Front Matter, detect or insert the og_image and thumbnail fields.
  4. Update or insert the fields, ensuring correct order and path format.
  5. Save the updated file without modifying the main content.

4. Final Script update_thumbnail.py 

import os
import re

# === Set directories ===
posts_dir = './_posts'
img_dir = './assets/img/blog'

# Get all Markdown files, sorted alphabetically
post_files = sorted([f for f in os.listdir(posts_dir) if f.endswith('.md')])

for post_file in post_files:
    post_name = os.path.splitext(post_file)[0]

    # Remove the date prefix to get the slug
    slug = re.sub(r'^\d{4}-\d{2}-\d{2}-', '', post_name)
    expected_img = slug + ".webp"
    img_path = os.path.join(img_dir, expected_img)

    # Skip if image doesn't exist
    if not os.path.exists(img_path):
        print(f"Missing image: {expected_img}, skipping {post_file}")
        continue

    img_web_path = f"/assets/img/blog/{expected_img}"

    # Read the post file
    post_path = os.path.join(posts_dir, post_file)
    with open(post_path, 'r', encoding='utf-8') as f:
        lines = f.readlines()

    # Detect YAML Front Matter block
    in_front_matter = False
    front_matter_end_idx = 0
    for idx, line in enumerate(lines):
        if line.strip() == '---':
            if not in_front_matter:
                in_front_matter = True
            else:
                front_matter_end_idx = idx
                break

    fm_lines = lines[1:front_matter_end_idx]
    new_fm_lines = []
    og_image_found = False
    thumbnail_found = False

    for line in fm_lines:
        m = re.match(r'^(thumbnail:|og_image:)\s*(.+)$', line)
        if m:
            field_name = m.group(1)
            old_path = m.group(2).strip()
            prefix_match = re.match(r'(.*/)', old_path)
            prefix = prefix_match.group(1) if prefix_match else "/assets/img/blog/"
            new_path = prefix + expected_img
            new_fm_lines.append(f"{field_name} {new_path}\n")
            if field_name == "og_image:":
                og_image_found = True
            elif field_name == "thumbnail:":
                thumbnail_found = True
        else:
            new_fm_lines.append(line)

    # Insert missing fields
    if not og_image_found:
        new_fm_lines.append(f"og_image: {img_web_path}\n")
    if not thumbnail_found:
        new_fm_lines.append(f"thumbnail: {img_web_path}\n")

    # Write back to file
    new_lines = ['---\n'] + new_fm_lines + ['---\n'] + lines[front_matter_end_idx + 1:]
    with open(post_path, 'w', encoding='utf-8') as f:
        f.writelines(new_lines)

    print(f"Updated Front Matter: {post_file} → image {expected_img}")

5. Usage

Save the script as update_thumbnail.py and place it in the root directory (e.g., xx).

Ensure the following folder structure:

xx/
├── update_thumbnail.py
├── _posts/
│   ├── 2025-06-20-my-post.md
│   └── ...
└── assets/
    └── img/
        └── blog/
            ├── my-post.webp
            └── ...

Run the script:

cd ~/Desktop/xx
python3 update_thumbnail.py

6. Example

Original Front Matter:

---
layout: post
title: Example
date: 2025-06-20
---

After running the script:

---
layout: post
title: Example
date: 2025-06-20
og_image: /assets/img/blog/my-post.webp
thumbnail: /assets/img/blog/my-post.webp
---