On 06/17/2015 08:08 AM, Paul J. Lucas wrote: > Hello - > > I have my own C program where I also use SGR to produce color just like grep > does. I pulled the grep source (2.21) to see how —color=auto was > implemented. In grep.c, lines 2440-2441, there is the code: > > if (color_option == 2) > color_option = isatty (STDOUT_FILENO) && should_colorize (); > > The thing about using isatty() is that it doesn’t work for the common case of > piping the output of grep to less. What about using fstat(STDOUT_FILENO) > instead? From testing (where “C” is an C program that outputs color using > SGR), I’ve gotten:
But you don't always want color when piping. For example, if you run a chain of commands 'grep | grep', if the first grep adds color, the second grep will NOT be grepping the original input, but the altered colorized output. > > COMMAND Should? isatty ISCHR ISFIFO ISREG > ======== ======= ====== ===== ====== ===== > C T T T F F > C > file F F F F >>T<<---- The interesting case. > C | less T F F T F > > Hence, it would seem that we want to output color except when ISREG on the > stat struct on stdout is T. Wouldn’t this make better logic for —color=auto ? No, we want --color=auto to be tied SPECIFICALLY to a tty, and if you want color to a pipe, you should use --color=always rather than --color=auto. This is true not only for grep --color, but also for coreutils' ls --color, and any other program that supports --color. [Side discussion - I wish we could have consistency in GNU projects about whether plain --color is shorthand for --color=auto or --color=always; different programs have picked different defaults, but both sides have an argument for their choice and an issue with potential back-compat breakage to clients if they change the default] -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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