Allow to add/set tags without a value with the command line tool.

src/exiv2.1

@@ -384,6 +384,10 @@ is determined based on \fIkey\fP.
 The remaining text on the line is the value. It can optionally be
 enclosed in single quotes ('\fIvalue\fP') or double quotes ("\fIvalue\fP").
 .sp 1
+The value is optional. Not providing any value is equivalent to an
+empty value ("") and is mainly useful to create an XMP array property,
+e.g., a bag.
+.sp 1
 The format of Exif \fBComment\fP values includes an optional charset
 specification at the beginning:
 .sp 1

src/exiv2.cpp

@@ -1098,21 +1098,24 @@ namespace {
         bool explicitType = false;
         if (cmdId != del) {
             // Get type and value
-            std::string::size_type typeStart
-                = line.find_first_not_of(delim, keyEnd+1);
-            std::string::size_type typeEnd
-                = line.find_first_of(delim, typeStart+1);
+            std::string::size_type typeStart = std::string::npos;
+            if (keyEnd != std::string::npos) typeStart = line.find_first_not_of(delim, keyEnd+1);
+            std::string::size_type typeEnd = std::string::npos;
+            if (typeStart != std::string::npos) typeEnd = line.find_first_of(delim, typeStart+1);
             std::string::size_type valStart = typeStart;
-            std::string::size_type valEnd = line.find_last_not_of(delim);
+            std::string::size_type valEnd = std::string::npos;
+            if (valStart != std::string::npos) valEnd = line.find_last_not_of(delim);
 
-            if (   keyEnd == std::string::npos
-                || typeStart == std::string::npos
-                || valStart == std::string::npos) {
+            if (   cmdId == reg
+                && (   keyEnd == std::string::npos
+                    || valStart == std::string::npos)) {
                 throw Exiv2::Error(1, Exiv2::toString(num)
                                    + ": " + _("Invalid command line") + " " );
             }
 
-            if (cmdId != reg && typeEnd != std::string::npos) {
+            if (   cmdId != reg
+                && typeStart != std::string::npos
+                && typeEnd != std::string::npos) {
                 std::string typeStr(line.substr(typeStart, typeEnd-typeStart));
                 Exiv2::TypeId tmpType = Exiv2::TypeInfo::typeId(typeStr);
                 if (tmpType != Exiv2::invalidTypeId) {
@@ -1126,6 +1129,7 @@ namespace {
                 }
             }
 
+            if (valStart != std::string::npos) {
                 value = parseEscapes(line.substr(valStart, valEnd+1-valStart));
                 std::string::size_type last = value.length()-1;
                 if (   (value[0] == '"' && value[last] == '"')
@@ -1133,6 +1137,7 @@ namespace {
                     value = value.substr(1, value.length()-2);
                 }
             }
+        }
 
         modifyCmd.cmdId_ = cmdId;
         modifyCmd.key_ = key;