Convert to POSIX part 3

tmpmail

@@ -248,7 +248,7 @@ EOF
     if [ ! "$ATTACHMENTS" = "0" ] && [ ! "$ATTACHMENTS" = 0 ]
     then
         HTML_MAIL="$HTML_MAIL<hr><br>Attachments<br>"
-        MINUS1="$(($ATTACHMENTS-1))"
+        MINUS1="$((ATTACHMENTS-1))"
         for index in $(seq 0 "$MINUS1"); do
             FILENAME=$(printf %s "$DATA" | jq -r ".attachments | .[$index] | .filename")
             LINK="<a href=${TMPMAIL_API_URL}?action=download&login=$USERNAME&domain=$DOMAIN&id=$EMAIL_ID&file=$FILENAME download>$FILENAME</a><br>"
@@ -270,11 +270,11 @@ EOF
 list_attachments () {
     EMAIL_ID="$1"
     DATA=$(curl -sL "${TMPMAIL_API_URL}?action=readMessage&login=$USERNAME&domain=$DOMAIN&id=$EMAIL_ID")
-    if [ $DATA == "Message not found" ]; then
+    if [ $DATA = "Message not found" ]; then
         print_error "Message not found"
     fi
     ATTACHMENTSLEN=$(printf %s "$DATA" | jq -r ".attachments | length")
-    MINUS1="$(($ATTACHMENTSLEN-1))"
+    MINUS1="$((ATTACHMENTSLEN-1))"
     printf "%s\n" "[ Attachments in $EMAIL_ID ]"
     echo
     for index in $(seq 0 "$MINUS1"); do