Is it possible to make stdout and stderr output be of different colors in XTerm or Konsole?

Is it even achievable?

I would like the output from a command’s stderr to be rendered in a different color than stdout (for example, in red).

I need such a modification to work with the Bash shell in the Konsole, XTerm, or GNOME Terminal terminal emulators on Linux.

Answer

Here’s a solution that combines some of the good ideas already presented.

Create a function in a bash script:

color() ( set -o pipefail; "$@" 2>&1>&3 | sed $'s,.*,e[31m&e[m,' >&2 ) 3>&1

Use it like this:

$ color command -program -args

It will show the command’s stderr in red.

Keep reading for an explanation of how it works. There are some interesting features demonstrated by this command.

• color()... — Creates a bash function called color.
• set -o pipefail — This is a shell option that preserves the error return code of a command whose output is piped into another command. This is done in a subshell, which is created by the parentheses, so as not to change the pipefail option in the outer shell.
• "$@" — Executes the arguments to the function as a new command. "$@" is equivalent to "$1" "$2" ...
• 2>&1 — Redirects the stderr of the command to stdout so that it becomes sed‘s stdin.
• >&3 — Shorthand for 1>&3, this redirects stdout to a new temporary file descriptor 3. 3 gets routed back into stdout later.
• sed ... — Because of the redirects above, sed‘s stdin is the stderr of the executed command. Its function is to surround each line with color codes.
• $'...' A bash construct that causes it to understand backslash-escaped characters
• .* — Matches the entire line.
• e[31m — The ANSI escape sequence that causes the following characters to be red
• & — The sed replace character that expands to the entire matched string (the entire line in this case).
• e[m — The ANSI escape sequence that resets the color.
• >&2 — Shorthand for 1>&2, this redirects sed‘s stdout to stderr.
• 3>&1 — Redirects the temporary file descriptor 3 back into stdout.

Here’s an extension of the same concept that also makes STDOUT green:

function stdred() (
    set -o pipefail;
    (
        "$@" 2>&1>&3 |
        sed $'s,.*,e[31m&e[m,' >&2
    ) 3>&1 |
    sed $'s,.*,e[32m&e[m,'
)