特定のディレクトリ配下にあるプレーンな.md,.markdownファイル全部にfrontmatterをつけるスクリプト

add_frontmatter.php

<?php

$targetDir = __DIR__.'/awos';
$dir = dir($targetDir);
$mdFiles = [];
while ($file = $dir->read()) {
    if (false !== strpos($file, '.md') || false !== strpos($file, '.markdown')) {
        $mdFiles[] = $file;
    }
}
$dir->close();

$frontmatterWithoutTitle = "---\nlayout: default\n---\n\n";
$frontmatterWithTitle = "---\nlayout: default\ntitle: \"%s\"\n---\n\n";

foreach ($mdFiles as $mdFile) {
    $content = file_get_contents($targetDir.'/'.$mdFile);
    if (false === strpos($content, '---')) {
        // 既にfrontmatterが入っていない場合のみ追加
        $title = detectTitle($content);
        $frontmatter = $title ? sprintf($frontmatterWithTitle, $title) : $frontmatterWithoutTitle;
        
        file_put_contents($targetDir.'/'.$mdFile, $frontmatter.$content);
    }
}

function detectTitle($content)
{
    $title = null;
    if (false !== strpos($content, '===')) {
        list($title,) = explode('===', $content);
        $title = str_replace(["\n", "\r"], '', $title);
    } elseif (false !== strpos($content, '# ')) {
        foreach (explode("\r", $content) as $line) {
            if ('#' === $line[0]) {
                $title = ltrim($title, '#');
                break;
            }
        }
    }

    return $title;
}