osutil: add support for shell conditional syntax in envs

osutil/env.go

@@ -22,6 +22,7 @@ package osutil
 import (
 	"fmt"
 	"os"
+	"regexp"
 	"sort"
 	"strconv"
 	"strings"
@@ -209,6 +210,38 @@ func NewExpandableEnv(pairs ...string) ExpandableEnv {
 	return ExpandableEnv{OrderedMap: strutil.NewOrderedMap(pairs...)}
 }
 
+var envExpandRegExp = regexp.MustCompile("[a-zA-Z0-9_]+:[+-]")
+
+// Adds support for bash conditional syntax ${VARIABLE:+XXX} and ${VARIABLE:-XXX}
+func (env *Environment) expand(value string) string {
+	return os.Expand(value, func(varName string) string {
+		loc := envExpandRegExp.FindStringIndex(varName)
+		if loc == nil {
+			return (*env)[varName]
+		}
+		envVar := string(varName[loc[0]:(loc[1] - 2)])
+		operation := string(varName[loc[1]-1])
+		newVal := string(varName[loc[1]:])
+		envVarValue := (*env)[envVar]
+		switch operation {
+		case "-":
+			if envVarValue == "" {
+				return env.expand(newVal)
+			} else {
+				return envVarValue
+			}
+		case "+":
+			if envVarValue == "" {
+				return ""
+			} else {
+				return env.expand(newVal)
+			}
+		default: // never can really happen, but the compiler complains without it
+			return (*env)[varName]
+		}
+	})
+}
+
 // ExtendWithExpanded extends the environment with eenv.
 //
 // Environment is modified in place. Each variable defined by eenv is
@@ -220,9 +253,7 @@ func (env *Environment) ExtendWithExpanded(eenv ExpandableEnv) {
 	}
 
 	for _, key := range eenv.Keys() {
-		(*env)[key] = os.Expand(eenv.Get(key), func(varName string) string {
-			return (*env)[varName]
-		})
+		(*env)[key] = env.expand(eenv.Get(key))
 	}
 }
 

osutil/env_test.go

@@ -330,3 +330,34 @@ func (s *envSuite) TestForExecEscapeUnsafeNothingToEscape(c *C) {
 		"XDG_STUFF=xdg-stuff",
 	})
 }
+
+func (s *envSuite) TestExpandEnvVariable(c *C) {
+	env := osutil.Environment{
+		"LD_LIBRARY_PATH": "/usr/lib:/usr/local/lib",
+		"TMPDIR":          "/var/tmp",
+		"A":               "foo",
+		"B":               "bad-value",
+		"D_default":       "default",
+	}
+
+	env.ExtendWithExpanded(osutil.NewExpandableEnv(
+		"LD_LIBRARY_PATH", "${LD_LIBRARY_PATH:+$LD_LIBRARY_PATH:}/usr/lib/x86_64-linux-gnu",
+		"PATH", "${PATH:+$PATH:}/usr/local/bin",
+		"TMPDIR", "${TMPDIR:-this-wont-be}",
+		"NOEXISTS", "${NOEXISTS:-a-new-value}",
+		"A", "${A:+}",
+		"B", "${B:+goodvalue}",
+		"C", "${C:-}",
+		"D", "${D:-$D_default}",
+	))
+	c.Check(env, DeepEquals, osutil.Environment{
+		"LD_LIBRARY_PATH": "/usr/lib:/usr/local/lib:/usr/lib/x86_64-linux-gnu",
+		"NOEXISTS":        "a-new-value",
+		"TMPDIR":          "/var/tmp",
+		"PATH":            "/usr/local/bin",
+		"A":               "",
+		"B":               "goodvalue",
+		"C":               "",
+		"D":               "default",
+		"D_default":       "default"})
+}