Add status option to prevent task from running

Closes #27

task.go

@@ -34,6 +34,7 @@ type Task struct {
 	Desc      string
 	Sources   []string
 	Generates []string
+	Status    []string
 	Dir       string
 	Vars      map[string]string
 	Set       string
@@ -140,6 +141,25 @@ func (t *Task) runDeps(ctx context.Context) error {
 }
 
 func (t *Task) isUpToDate() (bool, error) {
+	if len(t.Status) > 0 {
+		environ, err := t.getEnviron()
+		if err != nil {
+			return false, err
+		}
+
+		for _, s := range t.Status {
+			err = execext.RunCommand(&execext.RunCommandOptions{
+				Command: s,
+				Dir:     t.Dir,
+				Env:     environ,
+			})
+			if err != nil {
+				return false, nil
+			}
+		}
+		return true, nil
+	}
+
 	if len(t.Sources) == 0 || len(t.Generates) == 0 {
 		return false, nil
 	}

task_test.go

@@ -1,6 +1,7 @@
 package task_test
 
 import (
+	"bytes"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -112,3 +113,34 @@ func TestTaskCall(t *testing.T) {
 		}
 	}
 }
+
+func TestStatus(t *testing.T) {
+	const dir = "testdata/status"
+	var file = filepath.Join(dir, "foo.txt")
+
+	_ = os.Remove(file)
+
+	if _, err := os.Stat(file); err == nil {
+		t.Errorf("File should not exists: %v", err)
+	}
+	c := exec.Command("task", "gen-foo")
+	c.Dir = dir
+	if err := c.Run(); err != nil {
+		t.Error(err)
+	}
+	if _, err := os.Stat(file); err != nil {
+		t.Errorf("File should exists: %v", err)
+	}
+
+	buff := bytes.NewBuffer(nil)
+	c = exec.Command("task", "gen-foo")
+	c.Dir = dir
+	c.Stderr = buff
+	c.Stdout = buff
+	if err := c.Run(); err != nil {
+		t.Error(err)
+	}
+	if buff.String() != `task: Task "gen-foo" is up to date`+"\n" {
+		t.Errorf("Wrong output message: %s", buff.String())
+	}
+}

testdata/status/.gitignore

@@ -0,0 +1 @@
+*.txt

testdata/status/Taskfile.yml

@@ -0,0 +1,5 @@
+gen-foo:
+  cmds:
+    - touch foo.txt
+  status:
+    - test -f foo.txt