Check whether rpm is installed (fanal#39)

* Check whether rpm is installed

* Apply the review

Author: knqyf263

analyzer/pkg/rpmcmd/rpm.go

@@ -13,6 +13,8 @@ import (
 
 	"github.com/aquasecurity/fanal/analyzer"
 	"github.com/aquasecurity/fanal/extractor"
+	"github.com/aquasecurity/fanal/types"
+	"github.com/aquasecurity/fanal/utils"
 )
 
 func init() {
@@ -22,6 +24,9 @@ func init() {
 type rpmCmdPkgAnalyzer struct{}
 
 func (a rpmCmdPkgAnalyzer) Analyze(fileMap extractor.FileMap) (pkgs []analyzer.Package, err error) {
+	if !utils.IsCommandAvailable("rpm") {
+		return nil, types.ErrNoRpmCmd
+	}
 	var parsedPkgs []analyzer.Package
 	detected := false
 	for _, filename := range a.RequiredFiles() {

types/error.go

@@ -2,4 +2,7 @@ package types
 
 import "golang.org/x/xerrors"
 
-var InvalidURLPattern = xerrors.New("Invalid url pattern")
+var (
+	InvalidURLPattern = xerrors.New("invalid url pattern")
+	ErrNoRpmCmd       = xerrors.New("no rpm command")
+)

utils/utils.go

@@ -3,6 +3,7 @@ package utils
 import (
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
 )
 
@@ -29,3 +30,10 @@ func StringInSlice(a string, list []string) bool {
 	}
 	return false
 }
+
+func IsCommandAvailable(name string) bool {
+	if _, err := exec.LookPath(name); err != nil {
+		return false
+	}
+	return true
+}