Why does grep lose colored output when executed from a bash script? [duplicate]

Question

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grep loses coloring when run from bash script

I have a simple bash script to print a header on top of my grep results:

#!/bin/bash

for var in "$@"
do
    if [[ $var != -* ]];
    then
        break 
    fi
done

echo
echo -en "\e[1;31m     ====== GREP $var ======\e[0m\n"
echo

grep $@

But the final command is somehow not the same as actually running grep from the prompt directly, because the colors are missing from the results. When executing grep directly, the results show filenames in purple and matches in red, but now all the output is the normal terminal text color. Can someone tell me how to get the colored version from my script?

Answer 1

Looks like grep does not produce colors when not in interactive mode. You can force it to produce colored output:

grep --color=always $@

Answer 2

Grep has 3 color modes, Auto, Always, and Off.

Auto strips out the codes when it's connected to a non interactive output, such as a pipe (if you want to see why, try redirecting the output of grep --color=always into a file and then look at the file.. control codes everywhere)