How can I pipe from terminal in Perl without losing color?

Question

I'm trying to write a perl script which takes the output of colorgcc (or any other script that prints colored text to terminal), adds/removes parts of the string, and then prints the result in the same color as the input string.

The following code will print "Hello World" in front of each line produced by the color_producing_script. The output will be all black, while the input is multicolored. How can I modified this script to conserve the original colors?

open(CMD, "color_producing_script |");

while(<CMD>) {
    print 'Hello World' . $_;
}

I'm using bash terminal.

Edit

Per the excellent first comment, this is not a Perl issue per se. Just running color_producing_script | cat strips the color off. So the question can be rephrased to "How do you force color through the pipe?"

Edit 2

It looks like the latest version of gcc (1.3.2) includes the CGCC_FORCE_COLOR environment variable in the if clause, and if it's defined, colorgcc forces color:

export CGCC_FORCE_COLOR=true

Answer 1

Does color_producing_script change its behavior when it's used in a pipe? Try

color_producing_script | cat

at the command line. It may have an option to force color output even when it is.

The Perl script, colorgcc, is specifically checking to see if output is to a non-tty and skipping the colorization if that's the case.

# Get the terminal type. 
$terminal = $ENV{"TERM"} || "dumb";

# If it's in the list of terminal types not to color, or if
# we're writing to something that's not a tty, don't do color.
if (! -t STDOUT || $nocolor{$terminal})
{
   exec $compiler, @ARGV
      or die("Couldn't exec");
}

Edit:

You could modify the script in one or more of the following ways:

• comment out the test and make it always produce color output
• add functionality to support reading an environment variable that sets whether to colorize
• add functionality to support a color-always option in the ~/.colorgccrc configuration file
• add functionality to support a color-always command line option that you strip before passing the rest of the options to the compiler

You could also use the expect script unbuffer to create a pseudo-tty like this:

unbuffer gcc file.c | cat

(where cat is a standin recipient).

All of this is based on using colorgcc from the command line. There should be analogs for doing similar things within a Perl script.

Answer 2

Many programs that generate colored output detect if they're writing to a TTY, and switch off colors if they aren't. This is because color codes are annoying when you only want to capture the text, so they try to "do the right thing" automatically.

The simplest way to capture color output from a program like that is to tell it to write color even though it's not connected to a TTY. You'll have to read the program's documentation to find out if it has that option.

The other option is to use a Pty instead of a pipe. In Perl, you can do this with IO::Pty::HalfDuplex or IO::Pty::Easy, both of which are higher-level wrappers around the low-level module IO::Pty.

Answer 3

The source code of ColorGCC is quite clear about this topic!

#! /usr/bin/perl -w
# ...
# from: colorgcc-4.1.2/colorgcc-4.1.2
# downloaded from: http://www.console-colors.de/index.php?n=ConsColors.Downloads
#
# Note:
#
# colorgcc will only emit color codes if:
# 
#    (1) Its STDOUT is a tty and
#    (2) the value of $TERM is not listed in the "nocolor" option.
#
# If colorgcc colorizes the output, the compiler's STDERR will be
# combined with STDOUT. Otherwise, colorgcc just passes the output from
# the compiler through without modification.
# .....
# If it's in the list of terminal types not to color, or if
# we're writing to something that's not a tty, don't do color.
if (! -t STDOUT || $nocolor{$terminal})
{
   exec $compiler, @ARGV
      or die("Couldn't exec");
}

In addition to use a Pty instead of a pipe in Perl (as already pointed out by cjm) you can trick an application into thinking its stdin is interactive, not a pipe on the command line as well.

For example:

# Linux
script -c "[executable string]" /dev/null

# FreeBSD, Mac OS X
script -q /dev/null "[executable string]"

For further solutions see: bash: force exec'd process to have unbuffered stdout