How can you prevent ANSI escape sequences from being written to a debug file in a Bash script?

[LangLangBart]

In the script below, I attempt to collect everything into a debug file when DEBUG_MODE is enabled.

#!/usr/bin/env bash
set -o allexport -o errexit -o nounset -o pipefail

unset FZF_DEFAULT_OPTS FZF_DEFAULT_COMMAND
DEBUG_MODE=${DEBUG_MODE:-0}
if ((DEBUG_MODE)); then
  debug_file=$(mktemp)
  trap 'echo "[DEBUG] $debug_file"' EXIT
  exec 2> >(tee -a "$debug_file")

  # Customized trace prompt
  PS4='+ $(date +%Y-%m-%d:%H:%M:%S) [${BASH_SOURCE[0]:+${BASH_SOURCE[0]##*/}}:${FUNCNAME[0]:+${FUNCNAME[0]}():}${LINENO}]:  '
  # BASH_XTRACEFD requires Bash 4.1+
  exec 3>>"$debug_file"; BASH_XTRACEFD=3; set -o xtrace
  # Ensure that xtrace and other optional options are enabled in all child processes started by 'fzf'.
  execution_shell="$(which bash) -o xtrace -o errexit -o nounset -o pipefail -c"
fi

check_internet() {
  if command ping -c 1 8.8.8.8 &>/dev/null; then
    failure_command
  else
    echo "Internet is down."
  fi
}

fzf --bind "start:become:check_internet" --with-shell "${execution_shell:-"$(which bash) -c"}"

DEBUG_MODE=1 ./script.sh
# environment: line 1: failure_command: command not found
# [DEBUG] /var/folders/5f/ws8xbgdx55l69f6y7v20jd300000gn/T/tmp.pvAwlUJd

The output file contains the encountered error from the failure_command, but also all the ANSI escape sequences.

I believe the UI goes to stderr¹, and errors encountered would also go there, so it's not possible to distinguish them, right? Or is there a way to not write ANSI escape sequences to the debug log file?

There are tools to remove the ANSI escape codes from the file, for example, strip-ansi², but I opened this question to ask if someone knows a tweak to prevent them from being written to the debug file in the first place.

Footnotes

1. How fzf operate with stdin/stdout/stderr/tty/pipe? · Issue #3741 · junegunn/fzf · GitHub ↩

2. GitHub - chalk/strip-ansi: Strip ANSI escape codes from a string ↩

[junegunn]

It's currently not possible. However, come to think of it, we can choose to write directly to /dev/tty instead of to stderr like so

diff --git a/src/tui/light.go b/src/tui/light.go
index 3ef8b60..4d3f58a 100644
--- a/src/tui/light.go
+++ b/src/tui/light.go
@@ -73,7 +73,7 @@ func (r *LightRenderer) csi(code string) string {
 
 func (r *LightRenderer) flush() {
 	if r.queued.Len() > 0 {
-		fmt.Fprint(os.Stderr, "\x1b[?7l\x1b[?25l"+r.queued.String()+"\x1b[?25h\x1b[?7h")
+		fmt.Fprint(r.ttyout, "\x1b[?7l\x1b[?25l"+r.queued.String()+"\x1b[?25h\x1b[?7h")
 		r.queued.Reset()
 	}
 }
@@ -88,6 +88,7 @@ type LightRenderer struct {
 	prevDownTime  time.Time
 	clicks        [][2]int
 	ttyin         *os.File
+	ttyout        *os.File
 	buffer        []byte
 	origState     *term.State
 	width         int
@@ -131,6 +132,10 @@ func NewLightRenderer(theme *ColorTheme, forceBlack bool, mouse bool, tabstop in
 	if err != nil {
 		return nil, err
 	}
+	out, err := openTtyOut()
+	if err != nil {
+		out = os.Stderr
+	}
 	r := LightRenderer{
 		closed:        util.NewAtomicBool(false),
 		theme:         theme,
@@ -138,6 +143,7 @@ func NewLightRenderer(theme *ColorTheme, forceBlack bool, mouse bool, tabstop in
 		mouse:         mouse,
 		clearOnExit:   clearOnExit,
 		ttyin:         in,
+		ttyout:        out,
 		yoffset:       0,
 		tabstop:       tabstop,
 		fullscreen:    fullscreen,
diff --git a/src/tui/light_unix.go b/src/tui/light_unix.go
index a5499a0..da675aa 100644
--- a/src/tui/light_unix.go
+++ b/src/tui/light_unix.go
@@ -14,6 +14,8 @@ import (
 	"golang.org/x/term"
 )
 
+var tty string
+
 func IsLightRendererSupported() bool {
 	return true
 }
@@ -48,12 +50,14 @@ func (r *LightRenderer) closePlatform() {
 	// NOOP
 }
 
-func openTtyIn() (*os.File, error) {
-	in, err := os.OpenFile(consoleDevice, syscall.O_RDONLY, 0)
+func openTty(mode int) (*os.File, error) {
+	in, err := os.OpenFile(consoleDevice, mode, 0)
 	if err != nil {
-		tty := ttyname()
+		if len(tty) == 0 {
+			tty = ttyname()
+		}
 		if len(tty) > 0 {
-			if in, err := os.OpenFile(tty, syscall.O_RDONLY, 0); err == nil {
+			if in, err := os.OpenFile(tty, mode, 0); err == nil {
 				return in, nil
 			}
 		}
@@ -62,6 +66,14 @@ func openTtyIn() (*os.File, error) {
 	return in, nil
 }
 
+func openTtyIn() (*os.File, error) {
+	return openTty(syscall.O_RDONLY)
+}
+
+func openTtyOut() (*os.File, error) {
+	return openTty(syscall.O_WRONLY)
+}
+
 func (r *LightRenderer) setupTerminal() {
 	term.MakeRaw(r.fd())
 }
diff --git a/src/tui/light_windows.go b/src/tui/light_windows.go
index 635b892..8c459d6 100644
--- a/src/tui/light_windows.go
+++ b/src/tui/light_windows.go
@@ -96,6 +96,11 @@ func openTtyIn() (*os.File, error) {
 	return nil, nil
 }
 
+func openTtyOut() (*os.File, error) {
+	// not used
+	return nil, nil
+}
+
 func (r *LightRenderer) setupTerminal() error {
 	if err := windows.SetConsoleMode(windows.Handle(r.outHandle), consoleFlagsOutput); err != nil {
 		return err

And then your code works perfectly. And I actually like this approach better. Less chance of user errors of redirectly stderr, and it allows you to capture stderr from fzf and its child processes.

fzf --bind 'enter:become:echo foo; echo bar >&2' > /tmp/foo 2> /tmp/bar

Let's do this. Any concerns about the change?

[LangLangBart]

Thank you for the patch! The debug log now looks as desired. Great work – your expertise is invaluable!