text_formatter: detect tty based on fd

terminal_appengine.go

@@ -3,6 +3,6 @@
 package logrus
 
 // IsTerminal returns true if stderr's file descriptor is a terminal.
-func IsTerminal() bool {
+func IsTerminal(f io.Writer) bool {
 	return true
 }

terminal_notwindows.go

@@ -9,14 +9,20 @@
 package logrus
 
 import (
+	"io"
+	"os"
 	"syscall"
 	"unsafe"
 )
 
 // IsTerminal returns true if stderr's file descriptor is a terminal.
-func IsTerminal() bool {
+func IsTerminal(f io.Writer) bool {
-	fd := syscall.Stderr
 	var termios Termios
-	_, _, err := syscall.Syscall6(syscall.SYS_IOCTL, uintptr(fd), ioctlReadTermios, uintptr(unsafe.Pointer(&termios)), 0, 0, 0)
+	switch v := f.(type) {
+	case *os.File:
+		_, _, err := syscall.Syscall6(syscall.SYS_IOCTL, uintptr(v.Fd()), ioctlReadTermios, uintptr(unsafe.Pointer(&termios)), 0, 0, 0)
 		return err == 0
+	default:
+		return false
+	}
 }

terminal_solaris.go

@@ -9,7 +9,13 @@ import (
 )
 
 // IsTerminal returns true if the given file descriptor is a terminal.
-func IsTerminal() bool {
+func IsTerminal(f io.Writer) bool {
-	_, err := unix.IoctlGetTermios(int(os.Stdout.Fd()), unix.TCGETA)
+	var termios Termios
+	switch v := f.(type) {
+	case *os.File:
+		_, err := unix.IoctlGetTermios(int(f.Fd()), unix.TCGETA)
 		return err == nil
+	default:
+		return false
+	}
 }

terminal_windows.go

@@ -19,9 +19,13 @@ var (
 )
 
 // IsTerminal returns true if stderr's file descriptor is a terminal.
-func IsTerminal() bool {
+func IsTerminal(f io.Writer) bool {
-	fd := syscall.Stderr
+	switch v := f.(type) {
+	case *os.File:
 		var st uint32
-	r, _, e := syscall.Syscall(procGetConsoleMode.Addr(), 2, uintptr(fd), uintptr(unsafe.Pointer(&st)), 0)
+		r, _, e := syscall.Syscall(procGetConsoleMode.Addr(), 2, uintptr(v.Fd()), uintptr(unsafe.Pointer(&st)), 0)
 		return r != 0 && e == 0
+	default:
+		return false
+	}
 }

text_formatter.go

@@ -3,7 +3,6 @@ package logrus
 import (
 	"bytes"
 	"fmt"
-	"runtime"
 	"sort"
 	"strings"
 	"time"
@@ -20,12 +19,10 @@ const (
 
 var (
 	baseTimestamp time.Time
-	isTerminal    bool
 )
 
 func init() {
 	baseTimestamp = time.Now()
-	isTerminal = IsTerminal()
 }
 
 type TextFormatter struct {
@@ -50,6 +47,10 @@ type TextFormatter struct {
 	// that log extremely frequently and don't use the JSON formatter this may not
 	// be desired.
 	DisableSorting bool
+
+	// Whether the logger's out is to a terminal
+	isTerminal         bool
+	terminalDetermined bool
 }
 
 func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
@@ -70,8 +71,12 @@ func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
 
 	prefixFieldClashes(entry.Data)
 
-	isColorTerminal := isTerminal && (runtime.GOOS != "windows")
+	if !f.terminalDetermined {
-	isColored := (f.ForceColors || isColorTerminal) && !f.DisableColors
+		f.isTerminal = IsTerminal(entry.Logger.Out)
+		f.terminalDetermined = true
+	}
+
+	isColored := (f.ForceColors || f.isTerminal) && !f.DisableColors
 
 	timestampFormat := f.TimestampFormat
 	if timestampFormat == "" {