Our bash scripts use color for log-messages, but only, when stderr is a tty:

if [ -t 2 ]
then
    function log {
        # Use blue color for our messages, when stderr is a terminal
        printf "%(%F %T)T %s e[1;34m%se[1;0mn" -1 ${0##*/} "$*" >&2
    }

else
    function log {
        printf "%(%F %T)T %s %sn" -1 ${0##*/} "$*" >&2
    }
fi

We’d like for the logs produced by the Java-programs to also be colored — but only, when written to a tty. I tried adding the %highlight{} to the pattern of our existing appender:

    <Appenders>
        <Console name="stderr" target="SYSTEM_ERR">
            <PatternLayout
                pattern="%d{yyyy-MM-dd HH:mm:ss} %highlight{%-5p} %c{1}:%L - %m%n"/>
        </Console>
    </Appenders>

but that turns coloring on unconditionally, even when stderr is redirected to a file — or into a pager.

Can it be told to automatically detect, whether the output is a TTY?