Download in parallel and use remote timestamp

There's something weird going on with --progress-bar, it was working
 before but I could not figure out which change broke it.

wcurl

@@ -62,12 +62,28 @@ do
     fi
 done
 
-# Download URLs one by one.
-# It's not possible to download them in parallel due to the way "--continue-at-"
-# works.
-# Paralelism can be achieved by the way the user invokes wcurl.
-for url in "${urls_to_download[@]}"; do
-    # shellcheck disable=SC2086
-    curl --location --remote-name --retry 10 --retry-max-time 10 $curl_opts \
-    --continue-at - "$url"
-done
+# The init read-only variable is used below in the for loop to check if the url
+# being appended to the command is the first one or not. That's because once we
+# are appending any URLs after the first one, we need to also make use of the
+# "--next" parameter.
+declare -r command_to_eval_init="curl --parallel"
+command_to_eval="$command_to_eval_init"
+declare -r per_url_parameters="--location --remote-name --remote-time --retry 10 --retry-max-time 10 $curl_opts --continue-at -"
+
+# If we have URLs to download.
+if (( ${#urls_to_download[@]} != 0 )); then
+    for url in "${urls_to_download[@]}"; do
+        # If this is the first command we've added, don't prepend "--next" to it.
+        if [[ "$command_to_eval" == "$command_to_eval_init" ]]; then
+            command_to_eval="$command_to_eval $per_url_parameters $url"
+        else
+            command_to_eval="$command_to_eval --next $per_url_parameters $url"
+        fi
+    done
+fi
+
+# Print command_to_eval if the debug environment variable is set.
+[[ -z "${WCURL_DEBUG}" ]] || echo "$command_to_eval"
+
+# Call curl with the generated parameters.
+eval "$command_to_eval"

wcurl.1

@@ -19,7 +19,9 @@ parameters via the \fB\-o/\-\-opts\fR option.
 .TP
 By default, \fBwcurl\fR will:
 .br
-\[bu]  Encode whitespaces;
+\[bu]  Encode whitespaces in URLs;
+.br
+\[bu]  Download multiple URLs in parallel;
 .br
 \[bu]  Follow redirects;
 .br
@@ -28,6 +30,8 @@ By default, \fBwcurl\fR will:
 \[bu]  Perform retries;
 .br
 \[bu]  Resume from broken/interrupted downloads.
+.br
+\[bu]  Set the downloaded file timestamp to the value provided by the server, if available;
 .SH OPTIONS
 .TP
 \fB\-o, \-\-opts=\fI<CURL_OPTIONS>\fR...\fR
@@ -46,7 +50,7 @@ Download a single file:
 .br
 \fBwcurl example.com/filename.txt\fR
 .PP
-Download two files:
+Download two files in parallel:
 .br
 \fBwcurl example.com/filename1.txt example.com/filename2.txt\fR
 .PP