Grep output with multiple Colors?

You can cascade greps with different colors by specifying --color=always and using the regular expression 'foo|$' to pass all lines.

For example:

tail -f myfwlog | GREP_COLOR='01;36' egrep --color=always 'ssh|$' | GREP_COLOR='01;31' egrep -i --color=always 'drop|deny|$'

If you want the entire line to be highlighted, update your regular expression accordingly:

.... GREP_COLOR='01;31' egrep -i --color=always '^.*drop.*$|^.*deny.*$|$'

grep is a regular expression matcher, not a syntax highlighter :). You'll have to use multiple invocations of grep, using a different value of GREP_COLOR for each.

GREP_COLOR="1;32" grep foo file.txt | GREP_COLOR="1;36" grep bar

That would highlight "foo" and "bar" in different colors in lines that match both. I don't think there is a (simple) way to handle all occurrences of either pattern, short of merging the output stream of two independent calls:

{ GREP_COLOR="1;32" grep foo file.txt
  GREP_COLOR="1;36" grep bar file.txt
} | ...

which will obviously look different than if there were a way to assign a separate color to each regular expression.

You can use awk to substitute each match with itself wrapped in the correct control code.

 echo "foo bar" | awk '{ gsub("bar", "\033[1;33m&\033[0m");
                         gsub("foo", "\033[1;36m&\033[0m"); print }'

In each line, you globally replace anything matching the given regular expression with itself (&) wrapped in the ANSI escape sequences for desired color (which grep --color does for you). After processing all of the possible matches, you need to explicitly print the line.

If you want something out of the box, you're probably looking for hhighlighter.

Here's an example:

Take a look. It's incredibly useful for coloring words in different colors automatically. It's an impressive project that's built on top of ack.