I've seen a zillion permutations on this question but I think none is an exact duplicate.
Can I copy only stderr (or eventually stderr and stdin) to a file, while keeping the colorized output on the screen (if the files are colorized also, that's a bonus)
Each command can have an a special command to always send color escape code as show in other answers. But a more generic approach is to use to following tools.
unbuffer ls -l --color=auto | tee output.log
unbuffer is a command from the expect package.
Ref. SuperUser - reserve colors while piping to tee
script --flush --quiet --command "ls -l --color=auto" | tee output.log
script is a command from the util-linux package.
Ref. StackOverflow - Can colorized output be captured via shell redirect? [duplicate]
A pipe or redirection won't remove any colors!
The initial command that you're using removes the colors when it notices that the output is not a terminal! So cannot be a general answer to that question.
See e.g. man ls:
lsemits color codes only when standard output is connected to a terminal.
You can change the behavior using --color=always.
Same for grep:
--color[=WHEN], --colour[=WHEN]
Surround the matched (non-empty) strings, matching lines, context lines, file names, line numbers, byte offsets, and separators (for fields
and groups of context lines) with escape sequences to display them in color on the terminal. The colors are defined by the environment
variable GREP_COLORS. The deprecated environment variable GREP_COLOR is still supported, but its setting does not have priority. WHEN is
never, always, or auto.
For example:
echo 123 | grep --color=always 2 | tee file
... will have the 2 colored for the output in the terminal and also in the file.
To have the colors in the output, but not in the file, you can check answers to this question and then run e.g. this:
echo 123 | grep --color=always 2 | tee /dev/tty | ansi2txt > file
