N option: Start from Nth last line

cmd/gotail/gotail.go

@@ -7,16 +7,18 @@ package main
 import (
 	"flag"
 	"fmt"
-    "io"
-	"os"
 	"github.com/nxadm/tail"
+	"io"
+	"os"
 )
 
 func args2config() (tail.Config, int64) {
 	config := tail.Config{Follow: true}
+	N := uint64(0)
 	n := int64(0)
 	maxlinesize := int(0)
 	flag.Int64Var(&n, "n", 0, "tail from the last Nth location")
+	flag.Uint64Var(&N, "N", 0, "tail from the last Nth line")
 	flag.IntVar(&maxlinesize, "max", 0, "max line size")
 	flag.BoolVar(&config.Follow, "f", false, "wait for additional data to be appended to the file")
 	flag.BoolVar(&config.ReOpen, "F", false, "follow, and track file rename/rotation")
@@ -25,6 +27,7 @@ func args2config() (tail.Config, int64) {
 	if config.ReOpen {
 		config.Follow = true
 	}
+	config.N = N
 	config.MaxLineSize = maxlinesize
 	return config, n
 }

tail.go

@@ -70,6 +70,7 @@ type logger interface {
 // Config is used to specify how a file must be tailed.
 type Config struct {
 	// File-specifc
+	N         uint64    // Start from n last lines (tail -n)
 	Location  *SeekInfo // Tail from this location. If nil, start at the beginning of the file
 	ReOpen    bool      // Reopen recreated files (tail -F)
 	MustExist bool      // Fail early if the file does not exist
@@ -282,6 +283,54 @@ func (tail *Tail) tailFileSync() {
 		}
 	}
 
+	if tail.Location == nil && tail.N > 0 {
+		var lines uint64 = 0
+		var cursor int64 = 0
+		stat, err := tail.file.Stat()
+		if err == nil {
+			filesize := stat.Size()
+			for {
+				if filesize == 0 {
+					break
+				}
+				cursor--
+				if cursor == -1 {
+					_, err = tail.file.Seek(-1, io.SeekEnd)
+				} else {
+					_, err = tail.file.Seek(-2, io.SeekCurrent)
+				}
+				if err != nil {
+					tail.Killf("Seek error on %s: %s", tail.Filename, err)
+					return
+				}
+
+				char := make([]byte, 1)
+				_, err = tail.file.Read(char)
+				if err != nil {
+					tail.Killf("Seek error on %s: %s", tail.Filename, err)
+					return
+				}
+
+				if cursor != -1 && (char[0] == 10 || char[0] == 13) { // line found
+					lines++
+
+					if lines == tail.N {
+						cursor++
+						break
+					}
+				}
+
+				if cursor == -filesize {
+					break
+				}
+			}
+
+			if cursor < 0 {
+				tail.Location = &SeekInfo{Offset: cursor, Whence: io.SeekEnd}
+			}
+		}
+	}
+
 	// Seek to requested location on first open of the file.
 	if tail.Location != nil {
 		_, err := tail.file.Seek(tail.Location.Offset, tail.Location.Whence)

tail_test.go

@@ -282,6 +282,78 @@ func TestLocationMiddle(t *testing.T) {
 	tailTest.Cleanup(tail, true)
 }
 
+func TestNLastLinesEmptyFile(t *testing.T) {
+	// Test reading from middle.
+	tailTest, cleanup := NewTailTest("n-last-lines-empty-file", t)
+	defer cleanup()
+	tailTest.CreateFile("test.txt", "")
+	tail := tailTest.StartTail("test.txt", Config{Follow: true, N: 5})
+	go tailTest.VerifyTailOutput(tail, []string{"1 line"}, false)
+
+	<-time.After(100 * time.Millisecond)
+	tailTest.AppendFile("test.txt", "1 line\n")
+
+	// Delete after a reasonable delay, to give tail sufficient time
+	// to read all lines.
+	<-time.After(100 * time.Millisecond)
+	tailTest.RemoveFile("test.txt")
+	tailTest.Cleanup(tail, true)
+}
+
+func TestNLastLinesOneEmpty(t *testing.T) {
+	// Test reading from middle.
+	tailTest, cleanup := NewTailTest("n-last-lines-one-empty", t)
+	defer cleanup()
+	tailTest.CreateFile("test.txt", "\n")
+	tail := tailTest.StartTail("test.txt", Config{Follow: true, N: 5})
+	go tailTest.VerifyTailOutput(tail, []string{"", "1 line"}, false)
+
+	<-time.After(100 * time.Millisecond)
+	tailTest.AppendFile("test.txt", "1 line\n")
+
+	// Delete after a reasonable delay, to give tail sufficient time
+	// to read all lines.
+	<-time.After(100 * time.Millisecond)
+	tailTest.RemoveFile("test.txt")
+	tailTest.Cleanup(tail, true)
+}
+
+func TestNLastLines(t *testing.T) {
+	// Test reading from middle.
+	tailTest, cleanup := NewTailTest("n-last-lines", t)
+	defer cleanup()
+	tailTest.CreateFile("test.txt", "1 line\n2 line\n3 line\n")
+	tail := tailTest.StartTail("test.txt", Config{Follow: true, N: 5})
+	go tailTest.VerifyTailOutput(tail, []string{"1 line", "2 line", "3 line", "4 line"}, false)
+
+	<-time.After(100 * time.Millisecond)
+	tailTest.AppendFile("test.txt", "4 line\n")
+
+	// Delete after a reasonable delay, to give tail sufficient time
+	// to read all lines.
+	<-time.After(100 * time.Millisecond)
+	tailTest.RemoveFile("test.txt")
+	tailTest.Cleanup(tail, true)
+}
+
+func TestN5LastLines(t *testing.T) {
+	// Test reading from middle.
+	tailTest, cleanup := NewTailTest("n5-last-lines", t)
+	defer cleanup()
+	tailTest.CreateFile("test.txt", "1 line\n2 line\n3 line\n4 line\n5 line\n6 line\n")
+	tail := tailTest.StartTail("test.txt", Config{Follow: true, N: 5})
+	go tailTest.VerifyTailOutput(tail, []string{"2 line", "3 line", "4 line", "5 line", "6 line", "7 line"}, false)
+
+	<-time.After(100 * time.Millisecond)
+	tailTest.AppendFile("test.txt", "7 line\n")
+
+	// Delete after a reasonable delay, to give tail sufficient time
+	// to read all lines.
+	<-time.After(100 * time.Millisecond)
+	tailTest.RemoveFile("test.txt")
+	tailTest.Cleanup(tail, true)
+}
+
 // The use of polling file watcher could affect file rotation
 // (detected via renames), so test these explicitly.