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add preface #73

	name: PR Check

	on:
	pull_request:

	permissions:
	contents: read

	jobs:
	link-check:
	name: link-check
	runs-on: ubuntu-latest
	steps:
	- uses: actions/checkout@v4
	with:
	fetch-depth: 0

	- name: Extract links from changed lines
	run: \|
	BASE="${{ github.event.pull_request.base.sha }}"
	HEAD="${{ github.event.pull_request.head.sha }}"

	git diff -U0 "$BASE...$HEAD" -- '*.md' \
	\| grep '^+' \| grep -v '^+++' \
	\| grep -oE 'https?://[^ )"'"'"'>]+' \
	\| sed 's/[.,;:!?）)\]]*$//' \
	\| sort -u > links.txt \|\| true

	echo "===== 本次 PR 新增的链接 ====="
	cat links.txt
	echo "==============================="

	- name: Install lychee
	uses: taiki-e/install-action@v2
	with:
	tool: lychee

	- name: Check links
	run: \|
	if [ ! -s links.txt ]; then
	echo "本次改动中没有新增链接，跳过检查。"
	exit 0
	fi
	lychee --no-progress --timeout 20 --max-retries 2 - < links.txt