Does grep –color default to =auto or =always

colorsgrep

What is the default color behavior for grep --color if no [=WHEN] is specified?

The grep man page states:

--color[=WHEN], --colour[=WHEN]

    Surround the matched (non-empty) strings, matching lines, context lines,
    file names, line numbers, byte offsets, and separators (for fields and
    groups of context lines) with escape sequences to display them in color
    on the terminal. 
    … 
    WHEN is never, always, or auto.

Does the default [=WHEN] for grep --color become:

grep --color=auto

or

grep --color=always

or does it depend on implementation and platform?

In my tests on Ubuntu 14.04 with GNU grep 2.16:

echo "foo bar" | \grep --color foo

results in foobar, while

echo "foo bar" | \grep --color foo | \grep --color bar

results in foobar, so it appears that grep --color=auto is in effect here. I have not tested this on Windows or Mac, however; I don't know if this behavior is universal.

This default for [=WHEN] is a little different from ls --color where the man page for ls states:

--color[=WHEN]

    colorize the output. 
    WHEN defaults to always or can be never or auto.

Here, the behavior of the missing WHEN is explicit.

Best Answer

It defaults to auto.

grep --color is the same as writing --color=auto.

This seems to be deficiently documented, or they consider the simple --color deprecated, but that can be seen both from testing and from its source code:

  case COLOR_OPTION:
    if (optarg)
      {
        if (!strcasecmp (optarg, "always") || !strcasecmp (optarg, "yes")
            || !strcasecmp (optarg, "force"))
          color_option = 1;
        else if (!strcasecmp (optarg, "never") || !strcasecmp (optarg, "no")
                 || !strcasecmp (optarg, "none"))
          color_option = 0;
        else if (!strcasecmp (optarg, "auto") || !strcasecmp (optarg, "tty")
                 || !strcasecmp (optarg, "if-tty"))
          color_option = 2;
        else
          show_help = 1;
      }
    else
      color_option = 2;
    break;

Notice that the lack of an optarg results in the same as it being equal to auto or tty.

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