How can I keep color codes with grep input?

Question

If I use two consecutive grep commands, e.g.:

echo "foo bar" | grep foo | grep bar

Then the first pattern ("foo") is not highlighted. In fact, it seems that grep removes color codes from its input. Is there any way to prevent this?

Answer 1

Use --color=always.

grep detects if output is to a pipe (or file). You most never want colors when output is to file – as that is escape sequences for the terminal. Typically:

foo ^[[01;31m^[[Kbar^[[m^[[K

from e.g.:

grep pattern file > result

To override use --color=always.

grep --color=always pattern file | ...

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Example:

Having file:

ID=111;Year=2013;foo=bar
ID=222;Year=2013;foo=baz

Then

grep --color=always ID file | grep --color=always 2013 | grep foo

would color ID, 2013 and foo.

grep --color=always ID file | grep 2013 | grep foo

would color ID and foo, but not 2013.

¹Important: You also have to remember that the added clutter from colors is passed to next command in the chain. Once ID is colored, you can't (with ease), match e.g. ID=111.

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On some occasion one would perhaps want terminal colors in file. Try e.g.

 grep --color=always foo file > result
 cat result

Though the resulting file would have very limited portability.

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The coloring itself is also an extension.

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As mentioned by the good @slm, you could add:

export GREP_OPTIONS="--color=always"

to your .bashrc etc, but don't unless you for some reason really understand the implications and still want to do it. It would in many ways break grep due to the fact mentioned above ¹.

Add an alias if you use it often.

 alias cgrep='grep --color=always'

GREP_OPTIONS and --color take three options: never, auto and always. The two first should be the only ones considered for GREP_OPTIONS.

You could also check out GREP_COLORS in the man pages or at gnu grep.