support monochrome mode

Use NO_COLOR environment variable to enable monochrome mode in which foreground and background
colors will be stripped.

Signed-off-by: Tw <tw19881113@gmail.com>

pkg/shell/shell.go

@@ -19,6 +19,7 @@ import (
 	"src.elv.sh/pkg/parse"
 	"src.elv.sh/pkg/prog"
 	"src.elv.sh/pkg/sys"
+	"src.elv.sh/pkg/ui"
 )
 
 var logger = logutil.GetLogger("[shell] ")
@@ -61,6 +62,7 @@ func (p *Program) Run(fds [3]*os.File, args []string) error {
 	cleanup2 := initSignal(fds)
 	defer cleanup2()
 
+	ui.NoColor = os.Getenv("NO_COLOR") != ""
 	interactive := len(args) == 0
 	ev := p.makeEvaler(fds[2], interactive)
 	defer ev.PreExit()

pkg/ui/style.go

@@ -5,6 +5,8 @@ import (
 	"strings"
 )
 
+var NoColor bool = false
+
 // Style specifies how something (mostly a string) shall be displayed.
 type Style struct {
 	Fg         Color
@@ -32,10 +34,10 @@ func (s Style) SGRValues() []string {
 	addIf(s.Underlined, "4")
 	addIf(s.Blink, "5")
 	addIf(s.Inverse, "7")
-	if s.Fg != nil {
+	if s.Fg != nil && !NoColor {
 		sgr = append(sgr, s.Fg.fgSGR())
 	}
-	if s.Bg != nil {
+	if s.Bg != nil && !NoColor {
 		sgr = append(sgr, s.Bg.bgSGR())
 	}
 	return sgr