env: add support for escaping environment substitutions

initialize/env.go

@@ -3,6 +3,7 @@ package initialize
 import (
 	"os"
 	"path"
+	"regexp"
 	"strings"
 
 	"github.com/coreos/coreos-cloudinit/system"
@@ -62,9 +63,18 @@ func (e *Environment) SetSSHKeyName(name string) {
 	e.sshKeyName = name
 }
 
+// Apply goes through the map of substitutions and replaces all instances of
+// the keys with their respective values. It supports escaping substitutions
+// with a leading '\'.
 func (e *Environment) Apply(data string) string {
 	for key, val := range e.substitutions {
-		data = strings.Replace(data, key, val, -1)
+		matchKey := strings.Replace(key, `$`, `\$`, -1)
+		replKey := strings.Replace(key, `$`, `$$`, -1)
+
+		// "key" -> "val"
+		data = regexp.MustCompile(`([^\\]|^)`+matchKey).ReplaceAllString(data, `${1}`+val)
+		// "\key" -> "key"
+		data = regexp.MustCompile(`\\`+matchKey).ReplaceAllString(data, replKey)
 	}
 	return data
 }

initialize/env_test.go

@@ -55,6 +55,24 @@ ExecStop=/usr/bin/echo $unknown`,
 			"$private_ipv4\nfoobar",
 			"5.6.7.8\nfoobar",
 		},
+		{
+			// Escaping substitutions
+			map[string]string{"$private_ipv4": "127.0.0.1"},
+			`\$private_ipv4
+$private_ipv4
+addr: \$private_ipv4
+\\$private_ipv4`,
+			`$private_ipv4
+127.0.0.1
+addr: $private_ipv4
+\$private_ipv4`,
+		},
+		{
+			// No substitutions with escaping
+			nil,
+			"\\$test\n$test",
+			"\\$test\n$test",
+		},
 	} {
 
 		env := NewEnvironment("./", "./", "./", "", "", tt.subs)