Logging functions in bash and stdout

Question

I'd like to be able to put log messages in the middle of bash functions, without affecting the output of those very functions. For example, consider the following functions log() and get_animals():

# print a log a message
log ()
{
    echo "Log message: $1"
}

get_animals()
{
    log "Fetching animals"
    echo "cat dog mouse"
}

values=`get_animals`
echo $values

After which $values contains the string "Log message: Fetching animals cat dog mouse".

How should I modify this script so that "Log message: Fetching animals" is outputted to the terminal, and $values contains "cat dog mouse"?

Answer 1

You can redirect the output to the sdterr error file on file handle 2 using >&2

example :

# print a log a message
log ()
{
echo "Log message: $1" >&2
}

get_animals()
{
log "Fetching animals"
echo "cat dog mouse"
}

values=`get_animals`
echo $values

the `` only take the output on stdout, not on stderr. The console on the other hand displays both.

If you really want the Log message on the stdout you can redirect error back to stdout after assigning to the variable :

# print a log a message
log ()
{
    echo "Log message: $1" >&2
}

get_animals()
{
    log "Fetching animals"
    echo "cat dog mouse"
}

values=`get_animals` 2>&1
echo $values

Answer 2

You could redirect log output to the standard error stream:

log()
{
    echo 1>&2 "Log message: $1"
}