Add fs cat command for dbfs files (#430)

## Changes
TSIA

## Tests
Manually and integration tests

cmd/fs/cat.go

@@ -0,0 +1,41 @@
+package fs
+
+import (
+	"github.com/databricks/cli/cmd/root"
+	"github.com/databricks/cli/libs/cmdio"
+	"github.com/databricks/cli/libs/filer"
+	"github.com/spf13/cobra"
+)
+
+var catCmd = &cobra.Command{
+	Use:     "cat FILE_PATH",
+	Short:   "Show file content",
+	Long:    `Show the contents of a file.`,
+	Args:    cobra.ExactArgs(1),
+	PreRunE: root.MustWorkspaceClient,
+
+	RunE: func(cmd *cobra.Command, args []string) error {
+		ctx := cmd.Context()
+		w := root.WorkspaceClient(ctx)
+
+		path, err := resolveDbfsPath(args[0])
+		if err != nil {
+			return err
+		}
+
+		f, err := filer.NewDbfsClient(w, "/")
+		if err != nil {
+			return err
+		}
+
+		r, err := f.Read(ctx, path)
+		if err != nil {
+			return err
+		}
+		return cmdio.RenderReader(ctx, r)
+	},
+}
+
+func init() {
+	fsCmd.AddCommand(catCmd)
+}

internal/fs_cat_test.go

@@ -0,0 +1,70 @@
+package internal
+
+import (
+	"context"
+	"io/fs"
+	"path"
+	"strings"
+	"testing"
+
+	"github.com/databricks/cli/libs/filer"
+	"github.com/databricks/databricks-sdk-go"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestFsCatForDbfs(t *testing.T) {
+	t.Log(GetEnvOrSkipTest(t, "CLOUD_ENV"))
+
+	ctx := context.Background()
+	w, err := databricks.NewWorkspaceClient()
+	require.NoError(t, err)
+
+	tmpDir := temporaryDbfsDir(t, w)
+
+	f, err := filer.NewDbfsClient(w, tmpDir)
+	require.NoError(t, err)
+
+	err = f.Write(ctx, "a/hello.txt", strings.NewReader("abc"), filer.CreateParentDirectories)
+	require.NoError(t, err)
+
+	stdout, stderr := RequireSuccessfulRun(t, "fs", "cat", "dbfs:"+path.Join(tmpDir, "a", "hello.txt"))
+	assert.Equal(t, "", stderr.String())
+	assert.Equal(t, "abc", stdout.String())
+}
+
+func TestFsCatForDbfsOnNonExistentFile(t *testing.T) {
+	t.Log(GetEnvOrSkipTest(t, "CLOUD_ENV"))
+
+	_, _, err := RequireErrorRun(t, "fs", "cat", "dbfs:/non-existent-file")
+	assert.ErrorIs(t, err, fs.ErrNotExist)
+}
+
+func TestFsCatForDbfsInvalidScheme(t *testing.T) {
+	t.Log(GetEnvOrSkipTest(t, "CLOUD_ENV"))
+
+	_, _, err := RequireErrorRun(t, "fs", "cat", "dab:/non-existent-file")
+	assert.ErrorContains(t, err, "expected dbfs path (with the dbfs:/ prefix): dab:/non-existent-file")
+}
+
+func TestFsCatDoesNotSupportOutputModeJson(t *testing.T) {
+	t.Log(GetEnvOrSkipTest(t, "CLOUD_ENV"))
+
+	ctx := context.Background()
+	w, err := databricks.NewWorkspaceClient()
+	require.NoError(t, err)
+
+	tmpDir := temporaryDbfsDir(t, w)
+
+	f, err := filer.NewDbfsClient(w, tmpDir)
+	require.NoError(t, err)
+
+	err = f.Write(ctx, "hello.txt", strings.NewReader("abc"))
+	require.NoError(t, err)
+
+	_, _, err = RequireErrorRun(t, "fs", "cat", "dbfs:"+path.Join(tmpDir, "hello.txt"), "--output=json")
+	assert.ErrorContains(t, err, "json output not supported")
+}
+
+// TODO: Add test asserting an error when cat is called on an directory. Need this to be
+// fixed in the SDK first (https://github.com/databricks/databricks-sdk-go/issues/414)

libs/cmdio/io.go

@@ -87,6 +87,19 @@ func RenderWithTemplate(ctx context.Context, v any, template string) error {
 	}
 }
 
+func RenderReader(ctx context.Context, r io.Reader) error {
+	c := fromContext(ctx)
+	switch c.outputFormat {
+	case flags.OutputJSON:
+		return fmt.Errorf("json output not supported")
+	case flags.OutputText:
+		_, err := io.Copy(c.out, r)
+		return err
+	default:
+		return fmt.Errorf("invalid output format: %s", c.outputFormat)
+	}
+}
+
 type tuple struct{ Name, Id string }
 
 func (c *cmdIO) Select(names map[string]string, label string) (id string, err error) {