update_modified_files.sh

#!/usr/bin/env bash

set -x

error() {
    echo "$@" >&2
}

usage() {
    error "usage: $0 <src> <dest>"
    exit 1
}

if ! [ "$#" = 2 ]; then
    usage
fi

src_dir=$1
dst_dir=$2

# $1 must be the absolute path of the source file.
dest_path() {
    # https://stackoverflow.com/a/16623897/7669110
    relative_src=${1#"$src_dir"}
    echo "$dst_dir/$relative_src"
}

IFS=':' read -ra MODIFIED <<< "$WATCHEXEC_WRITTEN_PATH:$WATCHEXEC_CREATED_PATH"
for f in ${MODIFIED[@]}; do
    abs_path=$WATCHEXEC_COMMON_PATH/$f
    cp "$abs_path" "$(dest_path "$abs_path")"
done

IFS=':' read -ra REMOVED <<< "$WATCHEXEC_REMOVED_PATH"
for f in ${REMOVED[@]}; do
    rm -f "$(dest_path "$WATCHEXEC_COMMON_PATH/$f")"
done

IFS=':' read -ra RENAMED <<< "$WATCHEXEC_RENAMED_PATH"
    abs_path=$WATCHEXEC_COMMON_PATH/$f
    dst_path=$(dest_path "$abs_path")
    # If a file is renamed, that will show up in "metadata changed". Avoid creating a file that's been deleted in the source dir.
    if [ -e "$abs_path" ]; then
        cp "$abs_path" "$dst_path"
    else
        rm -f "$dst_path"
    fi
done

IFS=':' read -ra CHANGED <<< "$WATCHEXEC_META_CHANGED_PATH"
for f in ${CHANGED[@]}; do
    abs_path=$WATCHEXEC_COMMON_PATH/$f
    # If a file is renamed, that will show up in "metadata changed". Avoid creating a file that's been deleted in the source dir.
    if [ -e "$abs_path" ]; then
        touch "$(dest_path "$WATCHEXEC_COMMON_PATH/$f")"
    fi
done

you don't want this. use rsync instead.