如何更改bash脚本的`echo`输出？

Question

My problem consists that I need to change a bash script's output, printed with echo , using a tool (like sed or awk ) that works on most of all the GNU/Linux distributions and, if it is possible, on MacOS X too.

The script is available on the CS50 IDE, it is called apache50 and it's usage output is:

Usage: apache50 [start VHOST_DIR|restart|reload|stop|status]

The output I want to print is:

Usage: cli webserver [start VHOST_DIR|restart|reload|stop|status]

To execute apache50 I pass the instruction as argument to a function and then I try to change the output produced:

optional_args(){
    ${@:2} | sed 's/apache50/cli webserver/g'
}

case $opt in:
    webserver)
        optional_args $opt "apache50" $@
        break;
        ;;
esac

## output:
## Usage: apache50 [start VHOST_DIR|restart|reload|stop|status]

My script is not printing the wanted output.

Here is a part of apache50 :

usage() {
    echo "Usage: `basename $0` [start VHOST_DIR|restart|reload|stop|status]" 1>&2
    exit 1
}

case "$1" in
    *)
        usage
    ;;
esac

If you know how to make it in a better way, please make me a suggestion!

Answer 1

The usage information might be printed to standard error, while sed processes only standard input (on the other side of the pipe, the command's standard output ).

You might redirect standard error to standard output:

${@:2} 2>&1 | sed 's/apache50/cli webserver/g'

But that would conflict with the original intention (showing an error message along with usage info, for instance.)