bash - color escapes codes

Question

I have a script that uses escape codes to highlight text matches in its output. All is good except when the output is piped to less, which prints the escape codes

e.g.

$ echo -e "\033[31m -- Hello World! -- \033[m"
 -- Hello World! --

Piped to less:

$ echo -e "\033[31m -- Hello World! -- \033[m" | less
ESC[31m -- Hello World! -- ESC[m

I was wondering how other tools like ls, grep, etc are able to output in color yet it looks fine when piped to less?

Answer 1

Use less -R or add LESS=-R to the environment. This requests that less pass some escape sequences (such as color) to the terminal instead of printing them as normal characters.

Answer 2

grep and friends detect whether output is to a terminal. When piped to less, it isn't, so they disable colouring.

Look at isatty to check whether the output is a terminal.

Note that I sometimes find this quite annoying because I want less to display the colors:

alias less='less -SR'
alias grep='grep --color=always'

---

Also look at ANSIFilter for the reverse: to filter ANSI escapes out of existing streams (you can also use it to produce HTML, RTF, and possibly other formats from them)