Enable PATH variable to be expanded as default value, if not set by prior ENV command

src/build/build.go

@@ -43,6 +43,10 @@ var (
 
 	// ExportsPath is the path within EXPORT volume containers
 	ExportsPath = "/.rocker_exports"
+
+	// DefaultPathEnv is a system PATH variable to be used in Env substitutions
+	// if path is not set by ENV command
+	DefaultPathEnv = "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
 )
 
 // Config used specify parameters for the builder in New()

src/build/commands.go

@@ -1406,8 +1406,13 @@ func (c *CommandOnbuildWrap) ReplaceEnv(env []string) error {
 ////////// Private stuff //////////
 
 func replaceEnv(args []string, env []string) (err error) {
+
+	defaultEnv := []string{"PATH=" + DefaultPathEnv}
+
+	env1 := replaceOrAppendEnvValues(defaultEnv, env)
+
 	for i, v := range args {
-		if args[i], err = shellparser.ProcessWord(v, env); err != nil {
+		if args[i], err = shellparser.ProcessWord(v, env1); err != nil {
 			return err
 		}
 	}

src/build/util.go

@@ -20,6 +20,7 @@ import (
 	"crypto/md5"
 	"fmt"
 	"io"
+	"strings"
 
 	"github.com/fsouza/go-dockerclient"
 )
@@ -85,3 +86,25 @@ func (r readerVoidCloser) Read(p []byte) (int, error) {
 func (r readerVoidCloser) Close() error {
 	return nil
 }
+
+func replaceOrAppendEnvValues(defaults, overrides []string) []string {
+
+	cache := make(map[string]int, len(defaults))
+	for i, e := range defaults {
+		parts := strings.SplitN(e, "=", 2)
+		cache[parts[0]] = i
+	}
+
+	for _, value := range overrides {
+		parts := strings.SplitN(value, "=", 2)
+
+		if i, exists := cache[parts[0]]; exists {
+			defaults[i] = value
+		} else {
+			cache[parts[0]] = len(defaults)
+			defaults = append(defaults, value)
+		}
+	}
+
+	return defaults
+}

test/env_test.go

@@ -30,3 +30,14 @@ RUN ls -l /replaced-onbuild-bar`, "--no-cache")
 
 	assert.Nil(t, err, "should expand variable in ONBUILD command")
 }
+
+func TestEnvExpansionPath(t *testing.T) {
+	err := runRockerBuildWithOptions(`
+FROM alpine
+ENV foo=/opt/foo/bin:$PATH
+RUN echo $foo
+RUN test $foo == /opt/foo/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+`, "--no-cache")
+	assert.Nil(t, err, "should use PATH from the default PATH setting, if PATH is not set "+
+		"with ENV command in any of the parent containers")
+}