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

choroba's solution to another question shows how to use exec to open a new file descriptor.

Translating that solution to this question gives something like:

# Open a new file descriptor that redirects to stdout:
exec 3>&1

log ()
{
    echo "Log message: $1" 1>&3
}

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

animals=`get_animals`
echo Animals: $animals

Executing the above produces:

Log message: Fetching animals
Animals: cat dog mouse

More information about using I/O redirection and file descriptors in Bash can be found at:

• Bash Guide for Beginners, section 8.2.3, Redirection and file descriptors
• Advanced Bash-Scripting Guide, Chapter 20, I/O Redirection