Proxy exec.Cmd Stdout / Stderr without losing TTY

I've got the following code that executes an arbitrary shell command and pipes the stdout and stderr to the terminal.:

c := exec.Command("/bin/sh", "-c", cmd)
c.Stdin = os.Stdin
c.Stdout = os.Stdout
c.Stderr = os.Stderr

However, I need to process the output before I print it, so I've wrapped it with a proxy io.Writer interface:

type ProxyWriter struct {
	file   *os.File
}

func NewProxyWriter(file *os.File) *ProxyWriter {
	return &ProxyWriter{
		file: file,
	}
}

func (w *ProxyWriter) Write(p []byte) (int, error) {
    // ... do something with bytes first
    fmt.Fprintf(w.file, "%s", string(p))
	return len(p), nil
}

So the original code is now:

c := exec.Command("/bin/sh", "-c", cmd)
c.Stdin = os.Stdin
c.Stdout = NewProxyWriter(os.Stdout)
c.Stderr = NewProxyWriter(os.Stderr)

This works for the most part, however, stdout and stderr no longer seem to qualify as a TTY. Any previously styled or colored output is no longer styled or colored.

I've confirmed that this is not an issue of my ProxyWriter simply messing with the formatting by setting the command to the following, which outputs the colored text correctly.

c := exec.Command("echo", "3[0;31mTEST3[0m")

A more explicit test is:

c := exec.Command("/bin/sh", "-c", "if [ -t 1 ] ; then echo \"terminal\"; else echo \"not a terminal\"; fi")

Which outputs:

not a terminal

Is there anyway I can wrap the commands stdout / stderr without losing the TTY status?

Replace

func (w *ProxyWriter) Write(p []byte) (int, error) {
    // ... do something with bytes first
    fmt.Fprintf(w.file, "%s", string(p))
    return len(p), nil
}

To

func (w *ProxyWriter) Write(p []byte) (int, error) {
    return w.Write(p)
}

fmt.Fprintf have some logic to avoid terminal breaking.

You will need a full tty implementation to make an artificial stdout qualify as a terminal. The library that mattn suggested seems like a good candidate.

However, another much simpler option may be available depending on the command you are trying to keep colorization for. Most GNU utilities with color output such as ls, grep, and dmesg have a --color flag which can override the TTY detection.

For example:

$ ls --color=always

$ ls -lsh | grep --color=always drwx

I use this frequently to pipe colored dmesg output through less so that I can paginate it:

$ sudo dmesg --color=always | less -R

You can also do the opposite and disable color output:

ls --color=never