Add head

README.md

@@ -36,7 +36,7 @@ Utilities:
  * [ ] `fold`
  * [ ] `getconf`
  * [ ] `grep`
- * [ ] `head`
+ * [x] `head`
  * [ ] `hostname`
  * [ ] `join`
  * [ ] `kill`

error.go

@@ -0,0 +1,13 @@
+package common
+
+import (
+	"fmt"
+	"os"
+)
+
+func ExitIfError(err error) {
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "%v: %v\n", os.Args[0], err)
+		os.Exit(1)
+	}
+}

go.mod

@@ -1,3 +1,6 @@
 module source.heropunch.io/tomo/go-coreutils
 
-require github.com/stretchr/testify v1.3.0
+require (
+	github.com/alexflint/go-arg v1.0.0
+	github.com/stretchr/testify v1.3.0
+)

go.sum

@@ -1,7 +1,16 @@
+github.com/alexflint/go-arg v1.0.0 h1:VWNnY3DyBHiq5lcwY2FlCE5t5qyHNV0o5i1bkCIHprU=
+github.com/alexflint/go-arg v1.0.0/go.mod h1:Cto8k5VtkP4pp0EXiWD4ZJMFOOinZ38ggVcQ/6CGuRI=
+github.com/alexflint/go-scalar v1.0.0 h1:NGupf1XV/Xb04wXskDFzS0KWOLH632W/EO4fAFi+A70=
+github.com/alexflint/go-scalar v1.0.0/go.mod h1:GpHzbCOZXEKMEcygYQ5n/aa4Aq84zbxjy3MxYW0gjYw=
 github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
+github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
+github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/stretchr/testify v1.3.0 h1:TivCn/peBQ7UY8ooIcPgZFpTNSz0Q2U6UrFlUfqbe0Q=
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=

head/head.go

@@ -1,10 +1,43 @@
 package main
 
 import (
+	"bufio"
 	"fmt"
+	"io"
 	"os"
+
+	"github.com/alexflint/go-arg"
+	common "source.heropunch.io/tomo/go-coreutils"
 )
 
+func head(num uint, reader io.Reader) {
+	scanner := bufio.NewScanner(reader)
+	for scanner.Scan() && num > 0 {
+		fmt.Println(scanner.Text())
+		num = num - 1
+	}
+	common.ExitIfError(scanner.Err())
+}
+
 func main() {
-	fmt.Println(os.Args)
+	var args struct {
+		Lines uint     `arg:"-n"`
+		Files []string `arg:"positional"`
+	}
+	args.Lines = 10
+	arg.MustParse(&args)
+
+	if len(args.Files) == 0 {
+		head(args.Lines, os.Stdin)
+		os.Exit(0)
+	}
+	for _, filename := range args.Files {
+		file, err := os.Open(filename)
+		common.ExitIfError(err)
+
+		head(args.Lines, file)
+
+		err = file.Close()
+		common.ExitIfError(err)
+	}
 }