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How Man Communicates Its Colors With Less On Os X
Oct 19, 2010
2 minutes read

Today I was playing with the man utility and I discovered that it does not use the same strategy as ls for coloring its output.

If we type:

ls

Colors will be displayed and the files will be listed in multiple columns, but if we send ls stdout to cat:

ls | cat

Colors will be removed and there will be one file per line. Why?
Because when ls detects that its stdout is a tty, it outputs the list in multiple columns with colors. If not, it will output one file per line without any color.

If we type:

man ls | cat

We don’t get any colors.man behaves like ls. But if we type:

man ls | cat | less

less will output its colors to the terminal. It does not behave like ls.

So let’s see the invisible characters that man outputs with cat’s -v option.

man ls | cat -v

We will discover that each colored character is printed, followed by a backspace and the same character is printed again. To help you understand here is an example:

To send a “HELLO” colored string to the less utility type:

printf "H\bHE\bEL\bLL\bLO\bO" | less

The \b a backspace character.

Conclusion

On OS X, man communicates its colors by adding a backspace to the colored character and a copy it whether the stdout is a tty or not. less interprets those three characters as one colored character.

UPDATE: On Linux,man does not output backslashes when stdout is not the tty.


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