diff --git a/private/git.rkt b/private/git.rkt
index e241013..6085733 100644
--- a/private/git.rkt
+++ b/private/git.rkt
@@ -21,6 +21,12 @@
          rebellion/collection/range-set)
 
 
+(module+ test
+  (require racket/file
+           rackunit
+           resyntax/private/run-command))
+
+
 ;@----------------------------------------------------------------------------------------------------
 
 
@@ -73,3 +79,99 @@
     (raise-arguments-error 'git-commit!
                            "committing files to Git failed"
                            "commit message" message)))
+
+
+(module+ test
+
+  (define (git-quietly! . args)
+    (parameterize ([current-output-port (open-output-nowhere)]
+                   [current-error-port (open-output-nowhere)])
+      (unless (apply system* (find-executable-path "git") args)
+        (error 'git-quietly! "git command failed: git ~a" (string-join args " ")))))
+
+  (define (call-with-temporary-git-repository body)
+    (define repo-dir (make-temporary-directory "resyntax-git-test-~a"))
+    (dynamic-wind
+     void
+     (λ ()
+       (parameterize ([current-directory repo-dir])
+         (git-quietly! "init")
+         (git-quietly! "config" "user.name" "Resyntax Tests")
+         (git-quietly! "config" "user.email" "resyntax-tests@example.com")
+         (git-quietly! "config" "commit.gpgsign" "false")
+         (display-to-file "line one\nline two\nline three\nline four\nline five\n" "example.txt")
+         (git-quietly! "add" "--all")
+         (git-quietly! "commit" "--message" "initial commit")
+         (body)))
+     (λ () (delete-directory/files repo-dir))))
+
+  (define (line-range start end)
+    (closed-open-range start end #:comparator natural<=>))
+
+  (test-case "lex-line"
+
+    (test-case "file name line"
+      (check-equal? (lex-line "+++ b/some/file.rkt") (present (build-path "some/file.rkt"))))
+
+    (test-case "single-line hunk"
+      (check-equal? (lex-line "@@ -2 +7 @@") (present (line-range 7 8))))
+
+    (test-case "multi-line hunk"
+      (check-equal? (lex-line "@@ -2,3 +7,4 @@") (present (line-range 7 11))))
+
+    (test-case "deletion-only hunk"
+      (check-equal? (lex-line "@@ -2,3 +7,0 @@") absent))
+
+    (test-case "unrecognized line"
+      (check-exn exn:fail:contract? (λ () (lex-line "not a diff line")))))
+
+  (test-case "git-diff-modified-lines"
+
+    (test-case "no changes"
+      (call-with-temporary-git-repository
+       (λ ()
+         (check-equal? (git-diff-modified-lines "HEAD") (hash)))))
+
+    (test-case "modified lines"
+      (call-with-temporary-git-repository
+       (λ ()
+         (display-to-file "line one\nCHANGED\nline three\nCHANGED\nCHANGED\n" "example.txt"
+                          #:exists 'replace)
+         (check-equal? (git-diff-modified-lines "HEAD")
+                       (hash (build-path "example.txt")
+                             (range-set (line-range 2 3) (line-range 4 6)))))))
+
+    (test-case "added file"
+      (call-with-temporary-git-repository
+       (λ ()
+         (display-to-file "line one\nline two\nline three\n" "added.txt")
+         (git-quietly! "add" "--all")
+         (check-equal? (git-diff-modified-lines "HEAD")
+                       (hash (build-path "added.txt") (range-set (line-range 1 4)))))))
+
+    (test-case "deleted lines are not modifications"
+      (call-with-temporary-git-repository
+       (λ ()
+         (display-to-file "line one\nline two\nline four\nline five\n" "example.txt"
+                          #:exists 'replace)
+         (check-equal? (git-diff-modified-lines "HEAD") (hash))))))
+
+  (test-case "git-commit!"
+
+    (test-case "commits all modified files"
+      (call-with-temporary-git-repository
+       (λ ()
+         (display-to-file "line one\nCHANGED\nline three\nline four\nline five\n" "example.txt"
+                          #:exists 'replace)
+         (git-commit! "add a line, don't break the 'quoting'")
+         (check-equal? (string-trim (run-command "git" "log" "-1" "--format=%s"))
+                       "add a line, don't break the 'quoting'")
+         (check-equal? (string-trim (run-command "git" "status" "--porcelain")) ""))))
+
+    (test-case "raises when committing fails"
+      (call-with-temporary-git-repository
+       (λ ()
+         (check-exn #rx"committing files to Git failed"
+                    (λ ()
+                      (parameterize ([current-output-port (open-output-nowhere)])
+                        (git-commit! "empty commit")))))))))