Colorized grep — viewing the entire file with highlighted matches

Question

I find grep\'s --color=always flag to be tremendously useful. However, grep only prints lines with matches (unless you ask for context lines). Give

Answer 1

Is there some way I can tell grep to print every line being read regardless of whether there's a match?

Option -C999 will do the trick in the absence of an option to display all context lines. Most other grep variants support this too. However: 1) no output is produced when no match is found and 2) this option has a negative impact on grep's efficiency: when the -C value is large this many lines may have to be temporarily stored in memory for grep to determine which lines of context to display when a match occurs. Note that grep implementations do not load input files but rather reads a few lines or use a sliding window over the input. The "before part" of the context has to be kept in a window (memory) to output the "before" context lines later when a match is found.

A pattern such as ^|PATTERN or PATTERN|$ or any empty-matching sub-pattern for that matter such as [^ -~]?|PATTERN is a nice trick. However, 1) these patterns don't show non-matching lines highlighted as context and 2) this can't be used in combination with some other grep options, such as -F and -w for example.

So none of these approaches are satisfying to me. I'm using ugrep, and enhanced grep with option -y to efficiently display all non-matching output as color-highlighted context lines. Other grep-like tools such as ag and ripgrep also offer a pass-through option. But ugrep is compatible with GNU/BSD grep and offers a superset of grep options like -y and -Q. For example, here is what option -y shows when combined with -Q (interactive query UI to enter patterns):

ugrep -Q -y FILE ...