CODEWITHSUNDEEP
linuxbashhttp-redirectcolorsgrep
Libin Wen
Libin Wen

Reputation: 444

How does grep make the color?

I know in bash I can print a colorful string, like: 

echo -e "\033[33;1mhello\033[0m"

The output in a shell will be hello with golden color. But when I redirect the output to a file test.txt, the \033[33; will be in the text file too. However the grep --color=auto command won't redirect these characters into the text file. How can it do this?

Upvotes: 0

Views: 678

Answers (3)

konsolebox
konsolebox

Reputation: 75478

Use the the GREP_COLORS variable with an export flag. Tested this and it works:

export GREP_COLORS='ms=01;33'
grep --color=auto -e hello

Upvotes: 0

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

It probably uses the isatty(3) library function on stdout file descriptor (i.e. 1). So use

if (isatty(STDOUT_FILENO)) {
   // enable auto colorization
}

in your C code.

In a shell script, use the tty(1) command:

if tty -s ; then
  # enable auto colorization
fi

or simply the -t test(1) 

if [ -t 1 ]; then
  # enable auto colorization
fi

Upvotes: 1

wooghie
wooghie

Reputation: 437

How about this? 

#!/bin/bash

if [ -t 1 ]; then
    echo -e "\033[33;1mhello\033[0m"
else
    echo hello
fi

Here the explanation:

test -t <fd>, whose short form is [ -t <fd> ], checks if the descriptor <fd> is a terminal or not. Source: help test

Upvotes: 3

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