Implement step scripts

docs/tasks.md

@@ -129,7 +129,7 @@ The `steps` field is required. You define one or more `steps` fields to define
 the body of a `Task`.
 
 If multiple `steps` are defined, they will be executed in the same order as they
-are defined, if the `Task` is invoked by a [TaskRun](taskruns.md).
+are defined, if the `Task` is invoked by a [`TaskRun`](taskruns.md).
 
 Each `steps` in a `Task` must specify a container image that adheres to the
 [container contract](./container-contract.md). For each of the `steps` fields,
@@ -146,6 +146,67 @@ or container images that you define:
   image in the Task, rather than requesting the sum of all of the container
   image's resource requests.
 
+#### Step Script
+
+To simplify executing scripts inside a container, a step can specify a `script`.
+If this field is present, the step cannot specify `command` or `args`.
+
+When specified, a `script` gets invoked as if it were the contents of a file in
+the container. Scripts should start with a
+[shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)) line to declare what
+tool should be used to interpret the script. That tool must then also be
+available within the step's container.
+
+This allows you to execute a Bash script, if the image includes `bash`:
+
+```yaml
+steps:
+- image: ubuntu  # contains bash
+  script: |
+    #!/usr/bin/env bash
+    echo "Hello from Bash!"
+```
+
+...or to execute a Python script, if the image includes `python`:
+
+```yaml
+steps:
+- image: python  # contains python
+  script: |
+    #!/usr/bin/env python3
+    print("Hello from Python!")
+```
+
+...or to execute a Node script, if the image includes `node`:
+
+```yaml
+steps:
+- image: node  # contains node
+  script: |
+    #!/usr/bin/env node
+    console.log("Hello from Node!")
+```
+
+This also simplifies executing script files in the workspace:
+
+```yaml
+steps:
+- image: ubuntu
+  script: |
+    #!/usr/bin/env bash
+    /workspace/my-script.sh  # provided by an input resource
+```
+
+...or in the container image:
+
+```yaml
+steps:
+- image: my-image  # contains /bin/my-binary
+  script: |
+    #!/usr/bin/env bash
+    /bin/my-binary
+```
+
 ### Inputs
 
 A `Task` can declare the inputs it needs, which can be either or both of:

examples/taskruns/step-script.yaml

@@ -0,0 +1,51 @@
+apiVersion: tekton.dev/v1alpha1
+kind: TaskRun
+metadata:
+  generateName: step-script-
+spec:
+  taskSpec:
+    steps:
+    - name: bash
+      image: ubuntu
+      env:
+      - name: FOO
+        value: foooooooo
+      script: |
+        #!/usr/bin/env bash
+        set -euxo pipefail
+        echo "Hello from Bash!"
+        echo FOO is ${FOO}
+        echo substring is ${FOO:2:4}
+        for i in {1..10}; do
+          echo line $i
+        done
+
+    - name: place-file
+      image: ubuntu
+      script: |
+        #!/usr/bin/env bash
+        echo "echo Hello from script file" > /workspace/hello
+        chmod +x /workspace/hello
+    - name: run-file
+      image: ubuntu
+      script: |
+        #!/usr/bin/env bash
+        /workspace/hello
+
+    - name: node
+      image: node
+      script: |
+        #!/usr/bin/env node
+        console.log("Hello from Node!")
+
+    - name: python
+      image: python
+      script: |
+        #!/usr/bin/env python3
+        print("Hello from Python!")
+
+    - name: perl
+      image: perl
+      script: |
+        #!/usr/bin/perl
+        print "Hello from Perl!"

pkg/apis/pipeline/images.go

@@ -16,7 +16,8 @@ limitations under the License.
 
 package pipeline
 
-// Images holds the images reference for a number of container images used accross tektoncd pipelines
+// Images holds the images reference for a number of container images used
+// across tektoncd pipelines.
 type Images struct {
 	// EntryPointImage is container image containing our entrypoint binary.
 	EntryPointImage string

pkg/apis/pipeline/v1alpha1/task_types.go

@@ -66,6 +66,11 @@ type TaskSpec struct {
 // provided by Container.
 type Step struct {
 	corev1.Container
+
+	// Script is the contents of an executable file to execute.
+	//
+	// If Script is not empty, the Step cannot have an Command or Args.
+	Script string `json:"script,omitempty"`
 }
 
 // Check that Task may be validated and defaulted.

pkg/apis/pipeline/v1alpha1/task_validation.go

@@ -127,6 +127,21 @@ func validateSteps(steps []Step) *apis.FieldError {
 			return apis.ErrMissingField("Image")
 		}
 
+		if s.Script != "" {
+			if len(s.Args) > 0 || len(s.Command) > 0 {
+				return &apis.FieldError{
+					Message: "script cannot be used with args or command",
+					Paths:   []string{"script"},
+				}
+			}
+			if !strings.HasPrefix(strings.TrimSpace(s.Script), "#!") {
+				return &apis.FieldError{
+					Message: "script must start with a shebang (#!)",
+					Paths:   []string{"script"},
+				}
+			}
+		}
+
 		if s.Name == "" {
 			continue
 		}

pkg/apis/pipeline/v1alpha1/task_validation_test.go

@@ -192,6 +192,18 @@ func TestTaskSpecValidate(t *testing.T) {
 				Image: "some-image",
 			},
 		},
+	}, {
+		name: "valid step with script",
+		fields: fields{
+			Steps: []v1alpha1.Step{{
+				Container: corev1.Container{
+					Image: "my-image",
+				},
+				Script: `
+				#!/usr/bin/env bash
+				hello world`,
+			}},
+		},
 	}}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -263,19 +275,17 @@ func TestTaskSpecValidateError(t *testing.T) {
 		fields: fields{
 			Inputs: &v1alpha1.Inputs{
 				Resources: []v1alpha1.TaskResource{validResource},
-				Params: []v1alpha1.ParamSpec{
-					{
-						Name:        "validparam",
-						Type:        v1alpha1.ParamTypeString,
-						Description: "parameter",
-						Default:     builder.ArrayOrString("default"),
-					}, {
-						Name:        "param-with-invalid-type",
-						Type:        "invalidtype",
-						Description: "invalidtypedesc",
-						Default:     builder.ArrayOrString("default"),
-					},
-				},
+				Params: []v1alpha1.ParamSpec{{
+					Name:        "validparam",
+					Type:        v1alpha1.ParamTypeString,
+					Description: "parameter",
+					Default:     builder.ArrayOrString("default"),
+				}, {
+					Name:        "param-with-invalid-type",
+					Type:        "invalidtype",
+					Description: "invalidtypedesc",
+					Default:     builder.ArrayOrString("default"),
+				}},
 			},
 			Steps: validSteps,
 		},
@@ -288,14 +298,12 @@ func TestTaskSpecValidateError(t *testing.T) {
 		fields: fields{
 			Inputs: &v1alpha1.Inputs{
 				Resources: []v1alpha1.TaskResource{validResource},
-				Params: []v1alpha1.ParamSpec{
-					{
-						Name:        "task",
-						Type:        v1alpha1.ParamTypeArray,
-						Description: "param",
-						Default:     builder.ArrayOrString("default"),
-					},
-				},
+				Params: []v1alpha1.ParamSpec{{
+					Name:        "task",
+					Type:        v1alpha1.ParamTypeArray,
+					Description: "param",
+					Default:     builder.ArrayOrString("default"),
+				}},
 			},
 			Steps: validSteps,
 		},
@@ -308,14 +316,12 @@ func TestTaskSpecValidateError(t *testing.T) {
 		fields: fields{
 			Inputs: &v1alpha1.Inputs{
 				Resources: []v1alpha1.TaskResource{validResource},
-				Params: []v1alpha1.ParamSpec{
-					{
-						Name:        "task",
-						Type:        v1alpha1.ParamTypeString,
-						Description: "param",
-						Default:     builder.ArrayOrString("default", "array"),
-					},
-				},
+				Params: []v1alpha1.ParamSpec{{
+					Name:        "task",
+					Type:        v1alpha1.ParamTypeString,
+					Description: "param",
+					Default:     builder.ArrayOrString("default", "array"),
+				}},
 			},
 			Steps: validSteps,
 		},
@@ -598,22 +604,65 @@ func TestTaskSpecValidateError(t *testing.T) {
 			Message: `non-existent variable in "$(inputs.params.foo) && $(inputs.params.inexistent)" for step arg[0]`,
 			Paths:   []string{"taskspec.steps.arg[0]"},
 		},
-	},
-		{
-			name: "Multiple volumes with same name",
-			fields: fields{
-				Steps: validSteps,
-				Volumes: []corev1.Volume{{
-					Name: "workspace",
-				}, {
-					Name: "workspace",
-				}},
-			},
-			expectedError: apis.FieldError{
-				Message: `multiple volumes with same name "workspace"`,
-				Paths:   []string{"volumes.name"},
-			},
-		}}
+	}, {
+		name: "Multiple volumes with same name",
+		fields: fields{
+			Steps: validSteps,
+			Volumes: []corev1.Volume{{
+				Name: "workspace",
+			}, {
+				Name: "workspace",
+			}},
+		},
+		expectedError: apis.FieldError{
+			Message: `multiple volumes with same name "workspace"`,
+			Paths:   []string{"volumes.name"},
+		},
+	}, {
+		name: "step with script and args",
+		fields: fields{
+			Steps: []v1alpha1.Step{{
+				Container: corev1.Container{
+					Image: "myimage",
+					Args:  []string{"arg"},
+				},
+				Script: "script",
+			}},
+		},
+		expectedError: apis.FieldError{
+			Message: "script cannot be used with args or command",
+			Paths:   []string{"steps.script"},
+		},
+	}, {
+		name: "step with script without shebang",
+		fields: fields{
+			Steps: []v1alpha1.Step{{
+				Container: corev1.Container{
+					Image: "my-image",
+				},
+				Script: "does not begin with shebang",
+			}},
+		},
+		expectedError: apis.FieldError{
+			Message: "script must start with a shebang (#!)",
+			Paths:   []string{"steps.script"},
+		},
+	}, {
+		name: "step with script and command",
+		fields: fields{
+			Steps: []v1alpha1.Step{{
+				Container: corev1.Container{
+					Image:   "myimage",
+					Command: []string{"command"},
+				},
+				Script: "script",
+			}},
+		},
+		expectedError: apis.FieldError{
+			Message: "script cannot be used with args or command",
+			Paths:   []string{"steps.script"},
+		},
+	}}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			ts := &v1alpha1.TaskSpec{