Add -previous and example

Author: sgreben

Makefile

@@ -1,4 +1,4 @@
-VERSION = 1.0.0
+VERSION = 1.1.0
 
 PACKAGES := $(shell go list -f {{.Dir}} ./...)
 GOFILES  := $(addsuffix /*.go,$(PACKAGES))

README.md

@@ -18,6 +18,8 @@ Or [download the binary](https://github.com/sgreben/ts/releases) from the releas
 
 ```text
 Usage of ts:  
+  -previous
+        include previous line
   -plain
         -template='{{.Time}} +{{.DeltaNanos}} {{.Text}}'
   -template string
@@ -85,4 +87,27 @@ $ (echo Hello; echo World) | ts -template '{{ .I }} {{.TimeSecs}} {{.Text}}'
 1 1516649679 World
 ```
 
-The fields available to the template are specified in the [`line` struct](cmd/ts/main.go#L14).
+The fields available to the template are specified in the [`line` struct](cmd/ts/main.go#L14).
+
+## Example
+
+Finding the slowest step in a `docker build` (using `jq`):
+
+```bash
+$ cat Dockerfile
+FROM alpine
+RUN echo About to be slow...
+RUN sleep 10
+RUN echo Done being slow
+```
+
+```bash
+docker build . |
+    grep --line-buffered -E -e '^(Step|Successfully built)' |
+    ts -previous |
+    jq -s 'max_by(.deltaNanos) | {step:.previous, duration:.delta}'
+```
+
+```json
+{"step":"Step 3/4 : RUN sleep 10","duration":"10.602026127s"}
+```

cmd/ts/main.go

@@ -25,13 +25,15 @@ type line struct {
 	TotalNanos  int64         `json:"totalNanos"`
 	Total       string        `json:"total,omitempty"`
 	Text        string        `json:"text,omitempty"`
+	Previous    string        `json:"previous,omitempty"`
 }
 
 type configuration struct {
 	timeFormat string // -timeformat="..."
 	template   string // -template="..."
 	plain      bool   // -plain
 	version    string
+	previous   bool
 }
 
 var config = configuration{}
@@ -88,6 +90,7 @@ func init() {
 	flag.StringVar(&config.template, "template", "", "go template (https://golang.org/pkg/text/template)")
 	flag.StringVar(&config.timeFormat, "timeformat", "RFC3339", timeFormatsHelp())
 	flag.BoolVar(&config.plain, "plain", false, "-template='{{.Time}} +{{.DeltaNanos}} {{.Text}}'")
+	flag.BoolVar(&config.previous, "previous", false, "include previous line")
 	flag.Parse()
 	if knownFormat, ok := timeFormats[config.timeFormat]; ok {
 		config.timeFormat = knownFormat
@@ -108,6 +111,7 @@ func main() {
 	line := line{}
 	last := now
 	first := now
+	previous := ""
 	i := uint64(0)
 	for scanner.Scan() {
 		now = time.Now()
@@ -126,6 +130,10 @@ func main() {
 		line.TimeString = now.Format(config.timeFormat)
 		line.Text = scanner.Text()
 		line.I = i
+		if config.previous {
+			line.Previous = previous
+			previous = line.Text
+		}
 		if err := printer(&line); err != nil {
 			fmt.Fprintln(os.Stderr, "output error:", err)
 		}