Merge pull request #530 from urfave/consider-empty-env-vars

Consider empty environment variables as set

altsrc/flag.go

@@ -2,9 +2,9 @@ package altsrc
 
 import (
 	"fmt"
-	"os"
 	"strconv"
 	"strings"
+	"syscall"
 
 	"gopkg.in/urfave/cli.v1"
 )
@@ -237,13 +237,11 @@ func (f *Float64Flag) ApplyInputSourceValue(context *cli.Context, isc InputSourc
 func isEnvVarSet(envVars string) bool {
 	for _, envVar := range strings.Split(envVars, ",") {
 		envVar = strings.TrimSpace(envVar)
-		if envVal := os.Getenv(envVar); envVal != "" {
+		if _, ok := syscall.Getenv(envVar); ok {
 			// TODO: Can't use this for bools as
 			// set means that it was true or false based on
 			// Bool flag type, should work for other types
-			if len(envVal) > 0 {
+			return true
-				return true
-			}
 		}
 	}
 

app.go

@@ -174,7 +174,11 @@ func (a *App) Run(arguments []string) (err error) {
 	a.Setup()
 
 	// parse flags
-	set := flagSet(a.Name, a.Flags)
+	set, err := flagSet(a.Name, a.Flags)
+	if err != nil {
+		return err
+	}
+
 	set.SetOutput(ioutil.Discard)
 	err = set.Parse(arguments[1:])
 	nerr := normalizeFlags(a.Flags, set)
@@ -293,7 +297,11 @@ func (a *App) RunAsSubcommand(ctx *Context) (err error) {
 	}
 
 	// parse flags
-	set := flagSet(a.Name, a.Flags)
+	set, err := flagSet(a.Name, a.Flags)
+	if err != nil {
+		return err
+	}
+
 	set.SetOutput(ioutil.Discard)
 	err = set.Parse(ctx.Args().Tail())
 	nerr := normalizeFlags(a.Flags, set)

app_test.go

@@ -1523,7 +1523,11 @@ func TestApp_OnUsageError_WithWrongFlagValue_ForSubcommand(t *testing.T) {
 func TestHandleAction_WithNonFuncAction(t *testing.T) {
 	app := NewApp()
 	app.Action = 42
-	err := HandleAction(app.Action, NewContext(app, flagSet(app.Name, app.Flags), nil))
+	fs, err := flagSet(app.Name, app.Flags)
+	if err != nil {
+		t.Errorf("error creating FlagSet: %s", err)
+	}
+	err = HandleAction(app.Action, NewContext(app, fs, nil))
 
 	if err == nil {
 		t.Fatalf("expected to receive error from Run, got none")
@@ -1547,7 +1551,11 @@ func TestHandleAction_WithNonFuncAction(t *testing.T) {
 func TestHandleAction_WithInvalidFuncSignature(t *testing.T) {
 	app := NewApp()
 	app.Action = func() string { return "" }
-	err := HandleAction(app.Action, NewContext(app, flagSet(app.Name, app.Flags), nil))
+	fs, err := flagSet(app.Name, app.Flags)
+	if err != nil {
+		t.Errorf("error creating FlagSet: %s", err)
+	}
+	err = HandleAction(app.Action, NewContext(app, fs, nil))
 
 	if err == nil {
 		t.Fatalf("expected to receive error from Run, got none")
@@ -1571,7 +1579,11 @@ func TestHandleAction_WithInvalidFuncSignature(t *testing.T) {
 func TestHandleAction_WithInvalidFuncReturnSignature(t *testing.T) {
 	app := NewApp()
 	app.Action = func(_ *Context) (int, error) { return 0, nil }
-	err := HandleAction(app.Action, NewContext(app, flagSet(app.Name, app.Flags), nil))
+	fs, err := flagSet(app.Name, app.Flags)
+	if err != nil {
+		t.Errorf("error creating FlagSet: %s", err)
+	}
+	err = HandleAction(app.Action, NewContext(app, fs, nil))
 
 	if err == nil {
 		t.Fatalf("expected to receive error from Run, got none")
@@ -1602,5 +1614,9 @@ func TestHandleAction_WithUnknownPanic(t *testing.T) {
 		fn(ctx)
 		return nil
 	}
-	HandleAction(app.Action, NewContext(app, flagSet(app.Name, app.Flags), nil))
+	fs, err := flagSet(app.Name, app.Flags)
+	if err != nil {
+		t.Errorf("error creating FlagSet: %s", err)
+	}
+	HandleAction(app.Action, NewContext(app, fs, nil))
 }

command.go

@@ -91,7 +91,10 @@ func (c Command) Run(ctx *Context) (err error) {
 		c.Flags = append(c.Flags, BashCompletionFlag)
 	}
 
-	set := flagSet(c.Name, c.Flags)
+	set, err := flagSet(c.Name, c.Flags)
+	if err != nil {
+		return err
+	}
 	set.SetOutput(ioutil.Discard)
 
 	if c.SkipFlagParsing {

context.go

@@ -3,9 +3,9 @@ package cli
 import (
 	"errors"
 	"flag"
-	"os"
 	"reflect"
 	"strings"
+	"syscall"
 )
 
 // Context is a type that is passed through to
@@ -91,7 +91,7 @@ func (c *Context) IsSet(name string) bool {
 
 				eachName(envVarValue.String(), func(envVar string) {
 					envVar = strings.TrimSpace(envVar)
-					if envVal := os.Getenv(envVar); envVal != "" {
+					if _, ok := syscall.Getenv(envVar); ok {
 						c.setFlags[name] = true
 						return
 					}
@@ -147,6 +147,11 @@ func (c *Context) Parent() *Context {
 	return c.parentContext
 }
 
+// value returns the value of the flag coressponding to `name`
+func (c *Context) value(name string) interface{} {
+	return c.flagSet.Lookup(name).Value.(flag.Getter).Get()
+}
+
 // Args contains apps console arguments
 type Args []string
 

context_test.go

@@ -184,18 +184,30 @@ func TestContext_IsSet(t *testing.T) {
 // XXX Corresponds to hack in context.IsSet for flags with EnvVar field
 // Should be moved to `flag_test` in v2
 func TestContext_IsSet_fromEnv(t *testing.T) {
-	var timeoutIsSet, tIsSet, noEnvVarIsSet, nIsSet bool
+	var (
+		timeoutIsSet, tIsSet    bool
+		noEnvVarIsSet, nIsSet   bool
+		passwordIsSet, pIsSet   bool
+		unparsableIsSet, uIsSet bool
+	)
 
-	os.Clearenv()
+	clearenv()
 	os.Setenv("APP_TIMEOUT_SECONDS", "15.5")
+	os.Setenv("APP_PASSWORD", "")
 	a := App{
 		Flags: []Flag{
 			Float64Flag{Name: "timeout, t", EnvVar: "APP_TIMEOUT_SECONDS"},
+			StringFlag{Name: "password, p", EnvVar: "APP_PASSWORD"},
+			Float64Flag{Name: "unparsable, u", EnvVar: "APP_UNPARSABLE"},
 			Float64Flag{Name: "no-env-var, n"},
 		},
 		Action: func(ctx *Context) error {
 			timeoutIsSet = ctx.IsSet("timeout")
 			tIsSet = ctx.IsSet("t")
+			passwordIsSet = ctx.IsSet("password")
+			pIsSet = ctx.IsSet("p")
+			unparsableIsSet = ctx.IsSet("unparsable")
+			uIsSet = ctx.IsSet("u")
 			noEnvVarIsSet = ctx.IsSet("no-env-var")
 			nIsSet = ctx.IsSet("n")
 			return nil
@@ -204,8 +216,15 @@ func TestContext_IsSet_fromEnv(t *testing.T) {
 	a.Run([]string{"run"})
 	expect(t, timeoutIsSet, true)
 	expect(t, tIsSet, true)
+	expect(t, passwordIsSet, true)
+	expect(t, pIsSet, true)
 	expect(t, noEnvVarIsSet, false)
 	expect(t, nIsSet, false)
+
+	os.Setenv("APP_UNPARSABLE", "foobar")
+	a.Run([]string{"run"})
+	expect(t, unparsableIsSet, false)
+	expect(t, uIsSet, false)
 }
 
 func TestContext_GlobalIsSet(t *testing.T) {
@@ -230,14 +249,22 @@ func TestContext_GlobalIsSet(t *testing.T) {
 // XXX Corresponds to hack in context.IsSet for flags with EnvVar field
 // Should be moved to `flag_test` in v2
 func TestContext_GlobalIsSet_fromEnv(t *testing.T) {
-	var timeoutIsSet, tIsSet, noEnvVarIsSet, nIsSet bool
+	var (
+		timeoutIsSet, tIsSet    bool
+		noEnvVarIsSet, nIsSet   bool
+		passwordIsSet, pIsSet   bool
+		unparsableIsSet, uIsSet bool
+	)
 
-	os.Clearenv()
+	clearenv()
 	os.Setenv("APP_TIMEOUT_SECONDS", "15.5")
+	os.Setenv("APP_PASSWORD", "")
 	a := App{
 		Flags: []Flag{
 			Float64Flag{Name: "timeout, t", EnvVar: "APP_TIMEOUT_SECONDS"},
+			StringFlag{Name: "password, p", EnvVar: "APP_PASSWORD"},
 			Float64Flag{Name: "no-env-var, n"},
+			Float64Flag{Name: "unparsable, u", EnvVar: "APP_UNPARSABLE"},
 		},
 		Commands: []Command{
 			{
@@ -245,6 +272,10 @@ func TestContext_GlobalIsSet_fromEnv(t *testing.T) {
 				Action: func(ctx *Context) error {
 					timeoutIsSet = ctx.GlobalIsSet("timeout")
 					tIsSet = ctx.GlobalIsSet("t")
+					passwordIsSet = ctx.GlobalIsSet("password")
+					pIsSet = ctx.GlobalIsSet("p")
+					unparsableIsSet = ctx.GlobalIsSet("unparsable")
+					uIsSet = ctx.GlobalIsSet("u")
 					noEnvVarIsSet = ctx.GlobalIsSet("no-env-var")
 					nIsSet = ctx.GlobalIsSet("n")
 					return nil
@@ -252,11 +283,22 @@ func TestContext_GlobalIsSet_fromEnv(t *testing.T) {
 			},
 		},
 	}
-	a.Run([]string{"run", "hello"})
+	if err := a.Run([]string{"run", "hello"}); err != nil {
+		t.Logf("error running Run(): %+v", err)
+	}
 	expect(t, timeoutIsSet, true)
 	expect(t, tIsSet, true)
+	expect(t, passwordIsSet, true)
+	expect(t, pIsSet, true)
 	expect(t, noEnvVarIsSet, false)
 	expect(t, nIsSet, false)
+
+	os.Setenv("APP_UNPARSABLE", "foobar")
+	if err := a.Run([]string{"run"}); err != nil {
+		t.Logf("error running Run(): %+v", err)
+	}
+	expect(t, unparsableIsSet, false)
+	expect(t, uIsSet, false)
 }
 
 func TestContext_NumFlags(t *testing.T) {

flag.go

@@ -3,11 +3,11 @@ package cli
 import (
 	"flag"
 	"fmt"
-	"os"
 	"reflect"
 	"runtime"
 	"strconv"
 	"strings"
+	"syscall"
 	"time"
 )
 
@@ -62,13 +62,29 @@ type Flag interface {
 	GetName() string
 }
 
-func flagSet(name string, flags []Flag) *flag.FlagSet {
+// errorableFlag is an interface that allows us to return errors during apply
+// it allows flags defined in this library to return errors in a fashion backwards compatible
+// TODO remove in v2 and modify the existing Flag interface to return errors
+type errorableFlag interface {
+	Flag
+
+	applyWithError(*flag.FlagSet) error
+}
+
+func flagSet(name string, flags []Flag) (*flag.FlagSet, error) {
 	set := flag.NewFlagSet(name, flag.ContinueOnError)
 
 	for _, f := range flags {
-		f.Apply(set)
+		//TODO remove in v2 when errorableFlag is removed
+		if f, ok := f.(errorableFlag); ok {
+			if err := f.applyWithError(set); err != nil {
+				return nil, err
+			}
+		} else {
+			f.Apply(set)
+		}
 	}
-	return set
+	return set, nil
 }
 
 func eachName(longName string, fn func(string)) {
@@ -87,13 +103,22 @@ type Generic interface {
 
 // Apply takes the flagset and calls Set on the generic flag with the value
 // provided by the user for parsing by the flag
+// Ignores parsing errors
 func (f GenericFlag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError takes the flagset and calls Set on the generic flag with the value
+// provided by the user for parsing by the flag
+func (f GenericFlag) applyWithError(set *flag.FlagSet) error {
 	val := f.Value
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
-				val.Set(envVal)
+				if err := val.Set(envVal); err != nil {
+					return fmt.Errorf("could not parse %s as value for flag %s: %s", envVal, f.Name, err)
+				}
 				break
 			}
 		}
@@ -102,9 +127,11 @@ func (f GenericFlag) Apply(set *flag.FlagSet) {
 	eachName(f.Name, func(name string) {
 		set.Var(f.Value, name, f.Usage)
 	})
+
+	return nil
 }
 
-// StringSlice is an opaque type for []string to satisfy flag.Value
+// StringSlice is an opaque type for []string to satisfy flag.Value and flag.Getter
 type StringSlice []string
 
 // Set appends the string value to the list of values
@@ -123,16 +150,29 @@ func (f *StringSlice) Value() []string {
 	return *f
 }
 
+// Get returns the slice of strings set by this flag
+func (f *StringSlice) Get() interface{} {
+	return *f
+}
+
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f StringSliceFlag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f StringSliceFlag) applyWithError(set *flag.FlagSet) error {
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
 				newVal := &StringSlice{}
 				for _, s := range strings.Split(envVal, ",") {
 					s = strings.TrimSpace(s)
-					newVal.Set(s)
+					if err := newVal.Set(s); err != nil {
+						return fmt.Errorf("could not parse %s as string value for flag %s: %s", envVal, f.Name, err)
+					}
 				}
 				f.Value = newVal
 				break
@@ -146,9 +186,11 @@ func (f StringSliceFlag) Apply(set *flag.FlagSet) {
 		}
 		set.Var(f.Value, name, f.Usage)
 	})
+
+	return nil
 }
 
-// IntSlice is an opaque type for []int to satisfy flag.Value
+// IntSlice is an opaque type for []int to satisfy flag.Value and flag.Getter
 type IntSlice []int
 
 // Set parses the value into an integer and appends it to the list of values
@@ -171,18 +213,28 @@ func (f *IntSlice) Value() []int {
 	return *f
 }
 
+// Get returns the slice of ints set by this flag
+func (f *IntSlice) Get() interface{} {
+	return *f
+}
+
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f IntSliceFlag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f IntSliceFlag) applyWithError(set *flag.FlagSet) error {
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
 				newVal := &IntSlice{}
 				for _, s := range strings.Split(envVal, ",") {
 					s = strings.TrimSpace(s)
-					err := newVal.Set(s)
+					if err := newVal.Set(s); err != nil {
-					if err != nil {
+						return fmt.Errorf("could not parse %s as int slice value for flag %s: %s", envVal, f.Name, err)
-						fmt.Fprintf(ErrWriter, err.Error())
 					}
 				}
 				f.Value = newVal
@@ -197,9 +249,11 @@ func (f IntSliceFlag) Apply(set *flag.FlagSet) {
 		}
 		set.Var(f.Value, name, f.Usage)
 	})
+
+	return nil
 }
 
-// Int64Slice is an opaque type for []int to satisfy flag.Value
+// Int64Slice is an opaque type for []int to satisfy flag.Value and flag.Getter
 type Int64Slice []int64
 
 // Set parses the value into an integer and appends it to the list of values
@@ -222,18 +276,28 @@ func (f *Int64Slice) Value() []int64 {
 	return *f
 }
 
+// Get returns the slice of ints set by this flag
+func (f *Int64Slice) Get() interface{} {
+	return *f
+}
+
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f Int64SliceFlag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f Int64SliceFlag) applyWithError(set *flag.FlagSet) error {
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
 				newVal := &Int64Slice{}
 				for _, s := range strings.Split(envVal, ",") {
 					s = strings.TrimSpace(s)
-					err := newVal.Set(s)
+					if err := newVal.Set(s); err != nil {
-					if err != nil {
+						return fmt.Errorf("could not parse %s as int64 slice value for flag %s: %s", envVal, f.Name, err)
-						fmt.Fprintf(ErrWriter, err.Error())
 					}
 				}
 				f.Value = newVal
@@ -248,19 +312,33 @@ func (f Int64SliceFlag) Apply(set *flag.FlagSet) {
 		}
 		set.Var(f.Value, name, f.Usage)
 	})
+	return nil
 }
 
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f BoolFlag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f BoolFlag) applyWithError(set *flag.FlagSet) error {
 	val := false
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
-				envValBool, err := strconv.ParseBool(envVal)
+				if envVal == "" {
-				if err == nil {
+					val = false
-					val = envValBool
+					break
 				}
+
+				envValBool, err := strconv.ParseBool(envVal)
+				if err != nil {
+					return fmt.Errorf("could not parse %s as bool value for flag %s: %s", envVal, f.Name, err)
+				}
+
+				val = envValBool
 				break
 			}
 		}
@@ -273,20 +351,35 @@ func (f BoolFlag) Apply(set *flag.FlagSet) {
 		}
 		set.Bool(name, val, f.Usage)
 	})
+
+	return nil
 }
 
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f BoolTFlag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f BoolTFlag) applyWithError(set *flag.FlagSet) error {
 	val := true
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
-				envValBool, err := strconv.ParseBool(envVal)
+				if envVal == "" {
-				if err == nil {
+					val = false
-					val = envValBool
 					break
 				}
+
+				envValBool, err := strconv.ParseBool(envVal)
+				if err != nil {
+					return fmt.Errorf("could not parse %s as bool value for flag %s: %s", envVal, f.Name, err)
+				}
+
+				val = envValBool
+				break
 			}
 		}
 	}
@@ -298,14 +391,22 @@ func (f BoolTFlag) Apply(set *flag.FlagSet) {
 		}
 		set.Bool(name, val, f.Usage)
 	})
+
+	return nil
 }
 
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f StringFlag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f StringFlag) applyWithError(set *flag.FlagSet) error {
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
 				f.Value = envVal
 				break
 			}
@@ -319,19 +420,28 @@ func (f StringFlag) Apply(set *flag.FlagSet) {
 		}
 		set.String(name, f.Value, f.Usage)
 	})
+
+	return nil
 }
 
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f IntFlag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f IntFlag) applyWithError(set *flag.FlagSet) error {
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
 				envValInt, err := strconv.ParseInt(envVal, 0, 64)
-				if err == nil {
+				if err != nil {
-					f.Value = int(envValInt)
+					return fmt.Errorf("could not parse %s as int value for flag %s: %s", envVal, f.Name, err)
-					break
 				}
+				f.Value = int(envValInt)
+				break
 			}
 		}
 	}
@@ -343,19 +453,29 @@ func (f IntFlag) Apply(set *flag.FlagSet) {
 		}
 		set.Int(name, f.Value, f.Usage)
 	})
+
+	return nil
 }
 
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f Int64Flag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f Int64Flag) applyWithError(set *flag.FlagSet) error {
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
 				envValInt, err := strconv.ParseInt(envVal, 0, 64)
-				if err == nil {
+				if err != nil {
-					f.Value = envValInt
+					return fmt.Errorf("could not parse %s as int value for flag %s: %s", envVal, f.Name, err)
-					break
 				}
+
+				f.Value = envValInt
+				break
 			}
 		}
 	}
@@ -367,19 +487,29 @@ func (f Int64Flag) Apply(set *flag.FlagSet) {
 		}
 		set.Int64(name, f.Value, f.Usage)
 	})
+
+	return nil
 }
 
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f UintFlag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f UintFlag) applyWithError(set *flag.FlagSet) error {
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
 				envValInt, err := strconv.ParseUint(envVal, 0, 64)
-				if err == nil {
+				if err != nil {
-					f.Value = uint(envValInt)
+					return fmt.Errorf("could not parse %s as uint value for flag %s: %s", envVal, f.Name, err)
-					break
 				}
+
+				f.Value = uint(envValInt)
+				break
 			}
 		}
 	}
@@ -391,19 +521,29 @@ func (f UintFlag) Apply(set *flag.FlagSet) {
 		}
 		set.Uint(name, f.Value, f.Usage)
 	})
+
+	return nil
 }
 
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f Uint64Flag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f Uint64Flag) applyWithError(set *flag.FlagSet) error {
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
 				envValInt, err := strconv.ParseUint(envVal, 0, 64)
-				if err == nil {
+				if err != nil {
-					f.Value = uint64(envValInt)
+					return fmt.Errorf("could not parse %s as uint64 value for flag %s: %s", envVal, f.Name, err)
-					break
 				}
+
+				f.Value = uint64(envValInt)
+				break
 			}
 		}
 	}
@@ -415,19 +555,29 @@ func (f Uint64Flag) Apply(set *flag.FlagSet) {
 		}
 		set.Uint64(name, f.Value, f.Usage)
 	})
+
+	return nil
 }
 
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f DurationFlag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f DurationFlag) applyWithError(set *flag.FlagSet) error {
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
 				envValDuration, err := time.ParseDuration(envVal)
-				if err == nil {
+				if err != nil {
-					f.Value = envValDuration
+					return fmt.Errorf("could not parse %s as duration for flag %s: %s", envVal, f.Name, err)
-					break
 				}
+
+				f.Value = envValDuration
+				break
 			}
 		}
 	}
@@ -439,18 +589,29 @@ func (f DurationFlag) Apply(set *flag.FlagSet) {
 		}
 		set.Duration(name, f.Value, f.Usage)
 	})
+
+	return nil
 }
 
 // Apply populates the flag given the flag set and environment
+// Ignores errors
 func (f Float64Flag) Apply(set *flag.FlagSet) {
+	f.applyWithError(set)
+}
+
+// applyWithError populates the flag given the flag set and environment
+func (f Float64Flag) applyWithError(set *flag.FlagSet) error {
 	if f.EnvVar != "" {
 		for _, envVar := range strings.Split(f.EnvVar, ",") {
 			envVar = strings.TrimSpace(envVar)
-			if envVal := os.Getenv(envVar); envVal != "" {
+			if envVal, ok := syscall.Getenv(envVar); ok {
 				envValFloat, err := strconv.ParseFloat(envVal, 10)
-				if err == nil {
+				if err != nil {
-					f.Value = float64(envValFloat)
+					return fmt.Errorf("could not parse %s as float64 value for flag %s: %s", envVal, f.Name, err)
 				}
+
+				f.Value = float64(envValFloat)
+				break
 			}
 		}
 	}
@@ -462,6 +623,8 @@ func (f Float64Flag) Apply(set *flag.FlagSet) {
 		}
 		set.Float64(name, f.Value, f.Usage)
 	})
+
+	return nil
 }
 
 func visibleFlags(fl []Flag) []Flag {

flag_test.go

@@ -29,6 +29,81 @@ func TestBoolFlagHelpOutput(t *testing.T) {
 	}
 }
 
+func TestFlagsFromEnv(t *testing.T) {
+	var flagTests = []struct {
+		input  string
+		output interface{}
+		flag   Flag
+		err    error
+	}{
+		{"", false, BoolFlag{Name: "debug", EnvVar: "DEBUG"}, nil},
+		{"1", true, BoolFlag{Name: "debug", EnvVar: "DEBUG"}, nil},
+		{"false", false, BoolFlag{Name: "debug", EnvVar: "DEBUG"}, nil},
+		{"foobar", true, BoolFlag{Name: "debug", EnvVar: "DEBUG"}, fmt.Errorf(`could not parse foobar as bool value for flag debug: strconv.ParseBool: parsing "foobar": invalid syntax`)},
+
+		{"", false, BoolTFlag{Name: "debug", EnvVar: "DEBUG"}, nil},
+		{"1", true, BoolTFlag{Name: "debug", EnvVar: "DEBUG"}, nil},
+		{"false", false, BoolTFlag{Name: "debug", EnvVar: "DEBUG"}, nil},
+		{"foobar", true, BoolTFlag{Name: "debug", EnvVar: "DEBUG"}, fmt.Errorf(`could not parse foobar as bool value for flag debug: strconv.ParseBool: parsing "foobar": invalid syntax`)},
+
+		{"1s", 1 * time.Second, DurationFlag{Name: "time", EnvVar: "TIME"}, nil},
+		{"foobar", false, DurationFlag{Name: "time", EnvVar: "TIME"}, fmt.Errorf(`could not parse foobar as duration for flag time: time: invalid duration foobar`)},
+
+		{"1.2", 1.2, Float64Flag{Name: "seconds", EnvVar: "SECONDS"}, nil},
+		{"1", 1.0, Float64Flag{Name: "seconds", EnvVar: "SECONDS"}, nil},
+		{"foobar", 0, Float64Flag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse foobar as float64 value for flag seconds: strconv.ParseFloat: parsing "foobar": invalid syntax`)},
+
+		{"1", int64(1), Int64Flag{Name: "seconds", EnvVar: "SECONDS"}, nil},
+		{"1.2", 0, Int64Flag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse 1.2 as int value for flag seconds: strconv.ParseInt: parsing "1.2": invalid syntax`)},
+		{"foobar", 0, Int64Flag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse foobar as int value for flag seconds: strconv.ParseInt: parsing "foobar": invalid syntax`)},
+
+		{"1", 1, IntFlag{Name: "seconds", EnvVar: "SECONDS"}, nil},
+		{"1.2", 0, IntFlag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse 1.2 as int value for flag seconds: strconv.ParseInt: parsing "1.2": invalid syntax`)},
+		{"foobar", 0, IntFlag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse foobar as int value for flag seconds: strconv.ParseInt: parsing "foobar": invalid syntax`)},
+
+		{"1,2", IntSlice{1, 2}, IntSliceFlag{Name: "seconds", EnvVar: "SECONDS"}, nil},
+		{"1.2,2", IntSlice{}, IntSliceFlag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse 1.2,2 as int slice value for flag seconds: strconv.ParseInt: parsing "1.2": invalid syntax`)},
+		{"foobar", IntSlice{}, IntSliceFlag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse foobar as int slice value for flag seconds: strconv.ParseInt: parsing "foobar": invalid syntax`)},
+
+		{"1,2", Int64Slice{1, 2}, Int64SliceFlag{Name: "seconds", EnvVar: "SECONDS"}, nil},
+		{"1.2,2", Int64Slice{}, Int64SliceFlag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse 1.2,2 as int64 slice value for flag seconds: strconv.ParseInt: parsing "1.2": invalid syntax`)},
+		{"foobar", Int64Slice{}, Int64SliceFlag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse foobar as int64 slice value for flag seconds: strconv.ParseInt: parsing "foobar": invalid syntax`)},
+
+		{"foo", "foo", StringFlag{Name: "name", EnvVar: "NAME"}, nil},
+
+		{"foo,bar", StringSlice{"foo", "bar"}, StringSliceFlag{Name: "names", EnvVar: "NAMES"}, nil},
+
+		{"1", uint(1), UintFlag{Name: "seconds", EnvVar: "SECONDS"}, nil},
+		{"1.2", 0, UintFlag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse 1.2 as uint value for flag seconds: strconv.ParseUint: parsing "1.2": invalid syntax`)},
+		{"foobar", 0, UintFlag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse foobar as uint value for flag seconds: strconv.ParseUint: parsing "foobar": invalid syntax`)},
+
+		{"1", uint64(1), Uint64Flag{Name: "seconds", EnvVar: "SECONDS"}, nil},
+		{"1.2", 0, Uint64Flag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse 1.2 as uint64 value for flag seconds: strconv.ParseUint: parsing "1.2": invalid syntax`)},
+		{"foobar", 0, Uint64Flag{Name: "seconds", EnvVar: "SECONDS"}, fmt.Errorf(`could not parse foobar as uint64 value for flag seconds: strconv.ParseUint: parsing "foobar": invalid syntax`)},
+
+		{"foo,bar", &Parser{"foo", "bar"}, GenericFlag{Name: "names", Value: &Parser{}, EnvVar: "NAMES"}, nil},
+	}
+
+	for _, test := range flagTests {
+		os.Clearenv()
+		os.Setenv(reflect.ValueOf(test.flag).FieldByName("EnvVar").String(), test.input)
+		a := App{
+			Flags: []Flag{test.flag},
+			Action: func(ctx *Context) error {
+				if !reflect.DeepEqual(ctx.value(test.flag.GetName()), test.output) {
+					t.Errorf("expected %+v to be parsed as %+v, instead was %+v", test.input, test.output, ctx.value(test.flag.GetName()))
+				}
+				return nil
+			},
+		}
+
+		err := a.Run([]string{"run"})
+		if !reflect.DeepEqual(test.err, err) {
+			t.Errorf("expected error %s, got error %s", test.err, err)
+		}
+	}
+}
+
 var stringFlagTests = []struct {
 	name     string
 	usage    string
@@ -941,6 +1016,38 @@ func TestParseMultiBoolFromEnvCascade(t *testing.T) {
 	a.Run([]string{"run"})
 }
 
+func TestParseBoolTFromEnv(t *testing.T) {
+	var boolTFlagTests = []struct {
+		input  string
+		output bool
+	}{
+		{"", false},
+		{"1", true},
+		{"false", false},
+		{"true", true},
+	}
+
+	for _, test := range boolTFlagTests {
+		os.Clearenv()
+		os.Setenv("DEBUG", test.input)
+		a := App{
+			Flags: []Flag{
+				BoolTFlag{Name: "debug, d", EnvVar: "DEBUG"},
+			},
+			Action: func(ctx *Context) error {
+				if ctx.Bool("debug") != test.output {
+					t.Errorf("expected %+v to be parsed as %+v, instead was %+v", test.input, test.output, ctx.Bool("debug"))
+				}
+				if ctx.Bool("d") != test.output {
+					t.Errorf("expected %+v to be parsed as %+v, instead was %+v", test.input, test.output, ctx.Bool("d"))
+				}
+				return nil
+			},
+		}
+		a.Run([]string{"run"})
+	}
+}
+
 func TestParseMultiBoolT(t *testing.T) {
 	a := App{
 		Flags: []Flag{
@@ -1036,6 +1143,10 @@ func (p *Parser) String() string {
 	return fmt.Sprintf("%s,%s", p[0], p[1])
 }
 
+func (p *Parser) Get() interface{} {
+	return p
+}
+
 func TestParseGeneric(t *testing.T) {
 	a := App{
 		Flags: []Flag{

helpers_unix_test.go

@@ -0,0 +1,9 @@
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package cli
+
+import "os"
+
+func clearenv() {
+	os.Clearenv()
+}

helpers_windows_test.go

@@ -0,0 +1,20 @@
+package cli
+
+import (
+	"os"
+	"syscall"
+)
+
+// os.Clearenv() doesn't actually unset variables on Windows
+// See: https://github.com/golang/go/issues/17902
+func clearenv() {
+	for _, s := range os.Environ() {
+		for j := 1; j < len(s); j++ {
+			if s[j] == '=' {
+				keyp, _ := syscall.UTF16PtrFromString(s[0:j])
+				syscall.SetEnvironmentVariable(keyp, nil)
+				break
+			}
+		}
+	}
+}