fix: set sh as default shell for containers (#853)

* fix: set default shell for containers to sh

This matches GitHub Actions behaviour where,
runners use bash since it's guaranteed to exist there
while containers use sh for compatibility

* tests: fixup for default shell

pkg/runner/runner_test.go

@@ -97,6 +97,7 @@ func TestRunEvent(t *testing.T) {
 		{"testdata", "fail", "push", "exit with `FAILURE`: 1", platforms, ""},
 		{"testdata", "runs-on", "push", "", platforms, ""},
 		{"testdata", "checkout", "push", "", platforms, ""},
+		{"testdata", "shells/defaults", "push", "", platforms, ""},
 		{"testdata", "shells/pwsh", "push", "", map[string]string{"ubuntu-latest": "ghcr.io/justingrote/act-pwsh:latest"}, ""}, // custom image with pwsh
 		{"testdata", "shells/bash", "push", "", platforms, ""},
 		{"testdata", "shells/python", "push", "", map[string]string{"ubuntu-latest": "node:12-buster"}, ""}, // slim doesn't have python

pkg/runner/step_context.go

@@ -183,6 +183,7 @@ func (sc *StepContext) setupEnv(ctx context.Context) (ExpressionEvaluator, error
 	return evaluator, nil
 }
 
+// nolint:gocyclo
 func (sc *StepContext) setupShellCommand() common.Executor {
 	rc := sc.RunContext
 	step := sc.Step
@@ -237,6 +238,11 @@ func (sc *StepContext) setupShellCommand() common.Executor {
 		if step.Shell == "" {
 			step.Shell = rc.Run.Workflow.Defaults.Run.Shell
 		}
+		if rc.Run.Job().Container() != nil {
+			if rc.Run.Job().Container().Image != "" && step.Shell == "" {
+				step.Shell = "sh"
+			}
+		}
 		scCmd := step.ShellCommand()
 
 		var finalCMD []string

pkg/runner/testdata/container-hostname/push.yml

@@ -1,6 +1,10 @@
 name: container-hostname
 on: push
 
+defaults:
+  run:
+    shell: bash
+
 jobs:
   with-hostname:
     runs-on: ubuntu-latest

pkg/runner/testdata/shells/bash/push.yml

@@ -5,6 +5,19 @@ jobs:
     steps:
       - shell: bash
         run: |
-          if [[ -n "$PATH" ]] ; then
-            echo "I'm Bash!"
+          if [[ -n "$BASH" ]]; then
+            echo "I'm $BASH!"
+          else
+            exit 1
+          fi
+  check-container:
+    runs-on: ubuntu-latest
+    container: node:12-buster-slim
+    steps:
+      - shell: bash
+        run: |
+          if [[ -n "$BASH" ]]; then
+            echo "I'm $BASH!"
+          else
+            exit 1
           fi

pkg/runner/testdata/shells/defaults/push.yml

@@ -0,0 +1,21 @@
+on: push
+jobs:
+  check: # GHA uses `bash` as default for runners
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          if [[ -n "$BASH" ]]; then
+            echo "I'm $BASH!"
+          else
+            exit 1
+          fi
+  check-container: # GHA uses `sh` as default for containers
+    runs-on: ubuntu-latest
+    container: alpine:latest
+    steps:
+      - run: |
+          if [ -z ${BASH+x} ]; then
+            echo "I'm sh!"
+          else
+            exit 1
+          fi

pkg/runner/testdata/shells/pwsh/push.yml

@@ -6,3 +6,10 @@ jobs:
       - shell: pwsh
         run: |
           $PSVersionTable
+  check-container:
+    runs-on: ubuntu-latest
+    container: ghcr.io/justingrote/act-pwsh:latest
+    steps:
+      - shell: pwsh
+        run: |
+          $PSVersionTable

pkg/runner/testdata/shells/python/push.yml

@@ -7,3 +7,11 @@ jobs:
         run: |
           import platform
           print(platform.python_version())
+  check-container:
+    runs-on: ubuntu-latest
+    container: node:12-buster
+    steps:
+      - shell: python
+        run: |
+          import platform
+          print(platform.python_version())

pkg/runner/testdata/shells/sh/push.yml

@@ -5,6 +5,19 @@ jobs:
     steps:
       - shell: sh
         run: |
-          if [ -n "$PATH" ] ; then
+          if [ -z ${BASH+x} ]; then
             echo "I'm sh!"
+          else
+            exit 1
+          fi
+  check-container:
+    runs-on: ubuntu-latest
+    container: alpine:latest
+    steps:
+      - shell: sh
+        run: |
+          if [ -z ${BASH+x} ]; then
+            echo "I'm sh!"
+          else
+            exit 1
           fi