Colorizing tail output with sed

Question

How would I go about colorizing the output of tail with sed?

echo "`tput setaf 1`foo`tput op`" works as expected

echo "foo" | sed -e 's/(foo)/`tput setaf 1`\0`tput op`/g' however, does not.

What am I missing here?

Answer 1

The backticks `` in the echo command spawn a process and it's output is substituted into the echo command line. You can see this with e.g.

echo "`tput setaf 1`" | hexdump -C
00000000  1b 5b 33 31 6d 0a                       |.[31m.|

This works because the contents of the "..." are expanded before being passed to echo.

The sed command you're using wraps everything in ' so it is passed to sed without expansion and sed sees it all as literal text.

You can fix this by putting "" around your sed command

echo "foo" | sed -e "s/\(foo\)/`tput setaf 1`\1`tput op`/g"

You also had an error in your sed command. The ( and ) need to be escaped \( and \) to denote the remembered pattern. I've also never use \0 I've always used \1 as the first remembered pattern.

Answer 2

Does it have to be sed?

echo "foo bar baz" | grep --color=auto "bar"

The above will highlight bar in red by default. The man page says you can choose what colour to use with the environment variable GREP_COLORS.

To make it print all lines and only highlight bar:

echo "foo bar baz" | grep -E --color=auto "^|bar"