Allow optional flags by asking if its been set/present on the command line

context.go

@@ -15,11 +15,12 @@ type Context struct {
 	App       *App
 	flagSet   *flag.FlagSet
 	globalSet *flag.FlagSet
+	setFlags  map[string]bool
 }
 
 // Creates a new context. For use in when invoking an App or Command action.
 func NewContext(app *App, set *flag.FlagSet, globalSet *flag.FlagSet) *Context {
-	return &Context{app, set, globalSet}
+	return &Context{app, set, globalSet, nil}
 }
 
 // Looks up the value of a local int flag, returns 0 if no int flag exists
@@ -77,6 +78,17 @@ func (c *Context) GlobalIntSlice(name string) []int {
 	return lookupIntSlice(name, c.globalSet)
 }
 
+// Determines if the flag was actually set exists
+func (c *Context) IsSet(name string) bool {
+	if c.setFlags == nil {
+		c.setFlags = make(map[string]bool)
+		c.flagSet.Visit(func(f *flag.Flag) {
+			c.setFlags[f.Name] = true
+		})
+	}
+	return c.setFlags[name] == true
+}
+
 type Args []string
 
 // Returns the command line arguments associated with the context.

context_test.go

@@ -45,3 +45,14 @@ func TestContext_Args(t *testing.T) {
 	expect(t, len(c.Args()), 2)
 	expect(t, c.Bool("myflag"), true)
 }
+
+func TestContext_IsSet(t *testing.T) {
+	set := flag.NewFlagSet("test", 0)
+	set.Bool("myflag", false, "doc")
+	set.String("otherflag", "hello world", "doc")
+	c := cli.NewContext(nil, set, set)
+	set.Parse([]string{"--myflag", "bat", "baz"})
+	expect(t, c.IsSet("myflag"), true)
+	expect(t, c.IsSet("otherflag"), false)
+	expect(t, c.IsSet("bogusflag"), false)
+}