diff --git a/.env b/.env
deleted file mode 100644
index 6923d3796..000000000
--- a/.env
+++ /dev/null
@@ -1,4 +0,0 @@
-MDBOOK_VERSION=0.4.7
-MDBOOK_LINKCHECK_VERSION=0.7.2
-MDBOOK_TOC_VERSION=0.6.1
-DEPLOY_DIR=book/html
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 2edc33e16..b7a33dd4e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -12,20 +12,18 @@ on:
 jobs:
   ci:
     runs-on: ubuntu-latest
+    env:
+      MDBOOK_VERSION: 0.4.7
+      MDBOOK_LINKCHECK_VERSION: 0.7.2
+      MDBOOK_TOC_VERSION: 0.6.1
+      DEPLOY_DIR: book/html
+      BASE_SHA: ${{ github.event.pull_request.base.sha }}
     steps:
       - uses: actions/checkout@v2
         with:
           # Set 0 to rebase later.
           fetch-depth: 0
 
-      - name: Read .env
-        run: |
-          . ./.env
-          echo "MDBOOK_VERSION=${MDBOOK_VERSION}" >> $GITHUB_ENV
-          echo "MDBOOK_LINKCHECK_VERSION=${MDBOOK_LINKCHECK_VERSION}" >> $GITHUB_ENV
-          echo "MDBOOK_TOC_VERSION=${MDBOOK_TOC_VERSION}" >> $GITHUB_ENV
-          echo "DEPLOY_DIR=${DEPLOY_DIR}" >> $GITHUB_ENV
-
       - name: Cache binaries
         id: mdbook-cache
         uses: actions/cache@v2
@@ -42,7 +40,7 @@ jobs:
           key: ${{ runner.os }}-${{ hashFiles('./book/linkcheck') }}
 
       - name: Check line lengths
-        if: github.event_name != 'schedule'
+        if: github.event_name != 'push'
         run: |
           shopt -s globstar
           MAX_LINE_LENGTH=100 bash ci/check_line_lengths.sh src/**/*.md
@@ -62,7 +60,7 @@ jobs:
           cargo install mdbook-toc --version ${{ env.MDBOOK_TOC_VERSION }}
 
       - name: Check build
-        if: github.event_name != 'schedule'
+        if: github.event_name != 'push'
         run: |
           git switch -c ci
           git fetch origin master
@@ -71,7 +69,7 @@ jobs:
           mdbook build
 
       - name: Deploy to gh-pages
-        if: github.event_name == 'schedule' && github.repository == 'rust-lang/rustc-dev-guide'
+        if: github.event_name == 'push' && github.repository == 'rust-lang/rustc-dev-guide'
         run: |
           mdbook build
           touch "${{ env.DEPLOY_DIR }}/.nojekyll"
diff --git a/ci/linkcheck.sh b/ci/linkcheck.sh
index 86ee000cb..abf1feb51 100755
--- a/ci/linkcheck.sh
+++ b/ci/linkcheck.sh
@@ -10,11 +10,15 @@ if [ "$GITHUB_EVENT_NAME" = "schedule" ] ; then # running in scheduled job
   echo "Doing full link check."
   set -x
 elif [ "$CI" = "true" ] ; then # running in PR CI build
+  if [ -z "$BASE_SHA" ]; then
+    echo "error: unexpected state: BASE_SHA must be non-empty in CI"
+    exit 1
+  fi
 
-  CHANGED_FILES=$(git diff --name-only origin/master... | tr '\n' ' ')
+  CHANGED_FILES=$(git diff --name-only $BASE_SHA...$GITHUB_SHA | tr '\n' ' ')
   FLAGS="--no-cache -f $CHANGED_FILES"
 
-  echo "Checking files changed in origin/master...: $CHANGED_FILES"
+  echo "Checking files changed in $BASE_SHA...$GITHUB_SHA: $CHANGED_FILES"
   set -x
 else # running locally
   COMMIT_RANGE=master...