By default, write patches to an output file (#214)

* By default, write patches to an output file

If -o filename is given, write to filename.
If no -o filename is given and stdout is not a pipe, write to patches.json.
If no -o filename is given and stdout is a pipe, write to pipe.

* Simplify pipe rules

cmd/src/actions_exec.go

@@ -7,6 +7,7 @@ import (
 	"encoding/json"
 	"flag"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -18,7 +19,7 @@ import (
 	"time"
 
 	"github.com/fatih/color"
-	multierror "github.com/hashicorp/go-multierror"
+	"github.com/hashicorp/go-multierror"
 	"github.com/mattn/go-isatty"
 	"github.com/pkg/errors"
 	"github.com/sourcegraph/go-diff/diff"
@@ -64,7 +65,7 @@ Execute an action on code in repositories. The output of an action is a set of p
 
 Examples:
 
-  Execute an action defined in ~/run-gofmt.json:
+  Execute an action defined in ~/run-gofmt.json and save the patches it produced to 'patches.json'
 
 	$ src actions exec -f ~/run-gofmt.json
 
@@ -80,6 +81,10 @@ Examples:
 
 	$ src actions exec -f ~/run-gofmt.json | src campaign patchset create-from-patches
 
+  Execute an action and save the patches it produced to 'patches.json'
+
+	$ src actions exec -f ~/run-gofmt.json -o patches.json 
+
   Read and execute an action definition from standard input:
 
 	$ cat ~/my-action.json | src actions exec -f -
@@ -137,6 +142,7 @@ Format of the action JSON files:
 
 	var (
 		fileFlag        = flagSet.String("f", "-", "The action file. If not given or '-' standard input is used. (Required)")
+		outputFlag      = flagSet.String("o", "patches.json", "The output file. Will be used as the destination for patches unless the command is being piped in which case patches are piped to stdout")
 		parallelismFlag = flagSet.Int("j", runtime.GOMAXPROCS(0), "The number of parallel jobs.")
 
 		cacheDirFlag   = flagSet.String("cache", displayUserCacheDir, "Directory for caching results.")
@@ -184,6 +190,28 @@ Format of the action JSON files:
 			return err
 		}
 
+		var outputWriter io.Writer
+		if !*createPatchSetFlag && !*forceCreatePatchSetFlag {
+			// If stdout is a pipe, write to pipe, otherwise
+			// write to output file
+			fi, err := os.Stdout.Stat()
+			if err != nil {
+				return err
+			}
+			isPipe := fi.Mode()&os.ModeCharDevice == 0
+
+			if isPipe {
+				outputWriter = os.Stdout
+			} else {
+				f, err := os.Create(*outputFlag)
+				if err != nil {
+					return errors.Wrap(err, "creating output file")
+				}
+				defer f.Close()
+				outputWriter = f
+			}
+		}
+
 		err = validateActionDefinition(actionFile)
 		if err != nil {
 			return err
@@ -269,7 +297,7 @@ Format of the action JSON files:
 
 			logger.ActionSuccess(patches, true)
 
-			return json.NewEncoder(os.Stdout).Encode(patches)
+			return json.NewEncoder(outputWriter).Encode(patches)
 		}
 
 		if err != nil {