Generate an arbitrary number of process substitutions as tee arguments - bash

Question

I'm using this code in a bash script. I use it to transfer a source folder to multiple destinations:
cd /Volumes/ ; tar cf - SOURCE/ | tee \
>( cd /Volumes/dest1 ; tar xf - ) \
>( cd /Volumes/dest2 ; tar xf - ) \
> /dev/null
This command works well. I want to set the destinations at the beginning of the script. So the number of destinations can vary.
For example the destinations can be in a var or an array:
destinationList=/Volumes/dest1 /Volumes/dest2
cd /Volumes/Untitled/ ; tar cf - SOURCE/ | tee \
# for item in destinationList
# do
# add this code ">( cd $item ; tar xf - )"
# done
> /dev/null
Is there a nice way to do it?

Answer 1

This is a case where eval is one of the easier options, though it needs to be very used very carefully.
unpackInDestinations() {
local dest currArg='' evalStr=''
for dest; do
printf -v currArg '>(cd %q && exec tar xf -)' "$dest"
evalStr+=" $currArg"
done
eval "tee $evalStr >/dev/null"
}
tar cf - SOURCE/ | unpackInDestinations /Volumes/dest{1,2}
Less efficiently (but without, perhaps, causing anyone trying to audit the code's security as much consternation), one can also write a recursive function:
unpackInDestinations() {
local dest
if (( $# == 0 )); then
cat >/dev/null
elif (( $# == 1 )); then
cd "$1" && tar xf -
else
dest=$1; shift
tee >(cd "$dest" && exec tar xf -) | unpackInDestinations "$#"
fi
}
The number of tees this creates varies with the number of arguments, so it's substantially less efficient than the hand-written code or the eval-based equivalent to same.
If you only need to support new versions of bash (the below requires at least 4.1), there's some additional magic available that can provide the best of both worlds:
unpackInDestinations() {
local -a dest_fds=( ) args=( )
local arg fd_num retval
# open a file descriptor for each argument
for arg; do
exec {fd_num}> >(cd "$arg" && exec tar xf -)
dest_fds+=( "$fd_num" )
args+=( "/dev/fd/$fd_num" )
done
tee "${args[#]}" >/dev/null; retval=$?
# close the FDs
for fd_num in "${dest_fds[#]}"; do
exec {fd_num}>&-
done
# and return the exit status we got from tee
return "$retval"
}