bug#24547: 24.4; dired should colour in sockets and pipes

[Trent W. Buck]

TL;DR: explanation of dircolors(1) and terminfo(5).

Juri Linkov wrote:
> >> With GNU ls --color in a terminal, different colors are used for
> >>
> >>     regular files,
> >>     directories,
> >>     symlinks,
> >>     sockets,
> >>     pipes,
> >>     (other stuff - dircolors?)
> >>
> >> In dired, the first three get different faces,
> >> but sockets and pipes look like regular files.
> >>
> >> I think it's probably easy to fix sockets and pipes,
> >> because the first character is "s" or "p",
> >> at least in GNU ls output.
> >
> > Sounds reasonable.  I've now done this in Emacs 27, but I've used just a
> > single face for both sockets and pipes.

Thanks! :-)

> What are the remaining special file types possible in ls output?
> 
> For example, in /run I see only "s" and "p" files.  But in /dev there are
> also "b" and "c".  These "block" and "character" device files could be
> highlighted using the same face with sockets and pipes.

As I hinted earlier, GNU ls's --color output is governed by the dircolors 
program.
"dircolors --print-database" gives a bunch of stuff.  The part you asked about 
is:

    DIR 01;34 # directory
    LINK 01;36 # symbolic link. (If you set this to 'target' instead of a
     # numerical value, the color is as for the file pointed to.)
    MULTIHARDLINK 00 # regular file with more than one link
    FIFO 40;33 # pipe
    SOCK 01;35 # socket
    DOOR 01;35 # door
    BLK 40;33;01 # block device driver
    CHR 40;33;01 # character device driver
    ORPHAN 40;31;01 # symlink to nonexistent file, or non-stat'able file ...
    MISSING 00 # ... and the files they point to

Note that GNU ls/dircolors also uses different colors for different
kinds of regular files e.g. "video", "audio", "pictures", "compressed
files", "executables".  This is done by filename extension rather than
MIME type, because GNU/* filesystems generally don't store MIME type
metadata, and running libmagic on every ls would be slow :-(

Those codes are SGR escape sequences, also called "ANSI color".
See also "man 5 terminfo".

01 above is "tput bold" (I think).
4x above is "tput setab x" (set background color).
3x above is "tput setaf x" (set foreground color).
There are 8 colors (0 through 7), which you can see at the top of
"emacs -nw -f list-colors-display":

    black red green yellow blue magenta cyan white

The "bright" variants are selected automatically when bold is also set
(if the terminal supports that).

You might also want to consider what "man 2 stat" has to say, viz:

           switch (sb.st_mode & S_IFMT) {
           case S_IFBLK:  printf("block device\n");            break;
           case S_IFCHR:  printf("character device\n");        break;
           case S_IFDIR:  printf("directory\n");               break;
           case S_IFIFO:  printf("FIFO/pipe\n");               break;
           case S_IFLNK:  printf("symlink\n");                 break;
           case S_IFREG:  printf("regular file\n");            break;
           case S_IFSOCK: printf("socket\n");                  break;
           default:       printf("unknown?\n");                break;
           }

dired (as at 26.1) doesn't seem to have faces for file types (or MIME
types), except for dired-directory and dired-symlink.

I suppose you could just add more faces.
The only downside to that, AFAICT, is that it will annoy people who write emacs 
color themes.

For my personal needs, I don't *REALLY* care about the details, so
long as "special" inodes stand out from "regular" inodes.  Doing so
acts as a warning to me to e.g. add "--devices=skip" to "grep -r".