Not one of my own, rather a colleague came up with the following to improve the speed of my slow `while` loop.  It takes advantage of the `parallel` command by turning the task for extracting the paths and making the actual copy into a function which can then be farmed out to multiple processes, taking input from the same file.

<code bash>
#!/bin/bash
#
# Description : This script takes a single file that lists loadable sources on s3 and exports
#               the compiled text files to the directory in which it is invoked, preserving
#               the scope/user_id/imei directory structure.
#               The copy is parallelised with the GNU command "parallel". The number of processors
#               used for the copy is 16 by default and can be modified with the -j option.
#               See https://www.gnu.org/software/parallel/ for more information.
#               Note that parallel can be installed in a conda environment with:
#               conda install parallel
#
# 
set -e
set -o pipefail

usage()
{
    echo "usage: aws-export.sh [ -f LOADABLE_SOURCE_FILE ] [ -o OUTPUT_DIR ] [ -j JOBS ]"
}

while getopts 'f:o:j:?h' option
do
    case $option in
	f)
	    LOADABLE_SRC=${OPTARG};;
	o)
	    OUTPUT_DIR=${OPTARG};;
    j)
        JOBS=${OPTARG};;
    h|?) 
        usage
        exit 0;;
    esac
done

echo "########################################"
echo "Exporting compiled text files from AWS"
echo "########################################"
echo "Date : $(date)"
echo "Host : $(hostname)"
echo "########################################"
echo ""
echo ""

if [ -z "$LOADABLE_SRC" ]; then
    echo "You have not specified a file that lists compiled text with the '-f' flag."
    echo "Please enter the path to such a file : "
    read -r LOADABLE_SRC
fi

if [ -z "$OUTPUT_DIR" ]; then
    OUTPUT_DIR='.'
    echo "########################################"
    echo "Exporting files to current directory"
    echo "########################################"
    echo ""
fi

if [ -z "$JOBS" ]; then
    JOBS=16
fi
echo "*** Parallel execution using $JOBS CPU threads ***"
echo ""

aws_copy()
{
    echo "Copying file $1"
    SUB_DIR=$(echo "$1" | awk '{split($0, path, "/"); print path[4]"/"path[5]"/"path[6]"/"}')
    echo "Target is : $2/$SUB_DIR"
    aws s3 cp "$1" "$2/$SUB_DIR" 2>/dev/null ; \
    echo "########################################"
}

# Set timer to zero
SECONDS=0

export -f aws_copy
parallel -j $JOBS aws_copy {} $OUTPUT_DIR < "$LOADABLE_SRC"

DURATION=$SECONDS
echo ""
echo ""
echo "Export took       : $DURATION seconds"
echo "Files exported to : $OUTPUT_DIR"
echo "########################################"
</code>