add PREFER_APPEND_FILE_SYNC env

_tests/project_with_custom_append_file_sync_env/package.json

@@ -0,0 +1,10 @@
+{
+  "name": "simple-git-hooks-test-package",
+  "version": "1.0.0",
+  "simple-git-hooks": {
+    "pre-commit": "prettier --write"
+  },
+  "devDependencies": {
+    "simple-pre-commit": "1.0.0"
+  }
+}

simple-git-hooks.js

@@ -202,7 +202,13 @@ function _setHook(hook, command, projectRoot=process.cwd()) {
         fs.mkdirSync(normalizedHookDirectory)
     }
 
-    fs.writeFileSync(hookPath, hookCommand)
+    // use PREFER_APPEND_FILE_SYNC env to append hook command into file
+    if (['1', 'true'].includes(process.env.PREFER_APPEND_FILE_SYNC)) {
+        fs.appendFileSync(hookPath, `
+${hookCommand}`)
+    } else {
+        fs.writeFileSync(hookPath, hookCommand)
+    }
     fs.chmodSync(hookPath, 0o0755)
 
     console.info(`[INFO] Successfully set the ${hook} with command: ${command}`)

simple-git-hooks.test.js

@@ -173,6 +173,9 @@ describe("Simple Git Hooks tests", () => {
     const PROJECT_WITH_CUSTOM_CONF = path.normalize(
         path.join(testsFolder, "project_with_custom_configuration")
     );
+    const PROJECT_WITH_CUSTOM_APPEND_FILE_SYNC_ENV = path.normalize(
+      path.join(testsFolder, "project_with_custom_append_file_sync_env")
+    )
 
     // Incorrect configurations
     const PROJECT_WITH_BAD_CONF_IN_PACKAGE_JSON_ = path.normalize(
@@ -627,6 +630,66 @@ describe("Simple Git Hooks tests", () => {
       })
     });
 
+    describe("Env var PREFER_APPEND_FILE_SYNC tests", () => {
+      const GIT_USER_NAME = "github-actions";
+      const GIT_USER_EMAIL = "github-actions@github.com";
+      const EXISTING_PRE_COMMIT_FILE_CONTENT = "test pre commit";
+      const GIT_HOOKS_PATH = path.join(PROJECT_WITH_CUSTOM_APPEND_FILE_SYNC_ENV, ".git", "hooks");
+      const PRE_COMMIT_FILE_PATH = path.join(GIT_HOOKS_PATH, "pre-commit");
+      const ADD_PRE_COMMIT_FILE_CONTENT = "prettier --write";
+
+      const initializeGitRepository = (path) => {
+        execSync(
+          `git init \
+            && git config user.name ${GIT_USER_NAME} \
+            && git config user.email ${GIT_USER_EMAIL}`,
+          { cwd: path }
+        );
+      };
+
+      // replace pre commit content with 'test pre commit' 
+      const writePreCommitFile = () => {
+        fs.writeFileSync(PRE_COMMIT_FILE_PATH, EXISTING_PRE_COMMIT_FILE_CONTENT);
+      }
+
+      beforeEach(() => {
+        initializeGitRepository(PROJECT_WITH_CUSTOM_APPEND_FILE_SYNC_ENV);
+        createGitHooksFolder(PROJECT_WITH_CUSTOM_APPEND_FILE_SYNC_ENV);
+      });
+
+      describe("PREFER_APPEND_FILE_SYNC", () => {
+        afterEach(() => {
+          delete process.env.PREFER_APPEND_FILE_SYNC;
+        });
+
+        it("should append to the hook file when PREFER_APPEND_FILE_SYNC is 1", () => {
+          process.env.PREFER_APPEND_FILE_SYNC = 1;
+          writePreCommitFile();
+          simpleGitHooks.setHooksFromConfig(PROJECT_WITH_CUSTOM_APPEND_FILE_SYNC_ENV);
+          const installedHooks = getInstalledGitHooks(GIT_HOOKS_PATH);
+          expect(installedHooks["pre-commit"]).toBe(`${EXISTING_PRE_COMMIT_FILE_CONTENT}
+${simpleGitHooks.PREPEND_SCRIPT}${ADD_PRE_COMMIT_FILE_CONTENT}`);
+        })
+
+        it("should append to the hook file when PREFER_APPEND_FILE_SYNC is true", () => {
+          process.env.PREFER_APPEND_FILE_SYNC = "true";
+          writePreCommitFile();
+          simpleGitHooks.setHooksFromConfig(PROJECT_WITH_CUSTOM_APPEND_FILE_SYNC_ENV);
+          const installedHooks = getInstalledGitHooks(GIT_HOOKS_PATH);
+          expect(installedHooks["pre-commit"]).toBe(`${EXISTING_PRE_COMMIT_FILE_CONTENT}
+${simpleGitHooks.PREPEND_SCRIPT}${ADD_PRE_COMMIT_FILE_CONTENT}`);
+        })
+
+        it("should write to the hook file when PREFER_APPEND_FILE_SYNC is not true", () => {
+          writePreCommitFile();
+          simpleGitHooks.setHooksFromConfig(PROJECT_WITH_CUSTOM_APPEND_FILE_SYNC_ENV);
+          const installedHooks = getInstalledGitHooks(GIT_HOOKS_PATH);
+          expect(installedHooks["pre-commit"]).toBe(`${simpleGitHooks.PREPEND_SCRIPT}${ADD_PRE_COMMIT_FILE_CONTENT}`);
+        })
+      })
+
+    })
+
     afterEach(() => {
       [
         PROJECT_WITH_CONF_IN_SEPARATE_JS_ALT,