Validate XML with XSD or DTD on Mac

Two levels of correct

An XML file can be wrong in two different ways.

• Not well-formed. A tag is never closed, tags overlap, or a quote is missing. Any XML parser rejects the file.
• Not valid. The file parses fine but breaks the rules of an XSD (XML Schema) or a DTD: a required attribute is missing, an element appears where it is not allowed, or a value is the wrong type.

Some tools that consume XML, such as build systems, feeds, and configuration loaders, fail only at the second level, and their error messages can be vague. Checking against the schema yourself, while you edit, is faster.

Option 1: xmllint in Terminal

macOS includes xmllint at /usr/bin/xmllint. These four commands cover the common cases:

xmllint --noout file.xml
xmllint --noout --schema order.xsd file.xml
xmllint --noout --dtdvalid order.dtd file.xml
xmllint --noout --valid file.xml

• The first checks that the file is well-formed.
• The second validates against an XSD.
• The third validates against a separate DTD file.
• The fourth validates against the DTD declared in the file's own DOCTYPE.

Each of these was run on macOS with sample files. A file with an unclosed tag reports:

bad.xml:5: parser error : Opening and ending tag mismatch: item line 4 and order

A well-formed file that does not match the XSD reports:

wrong.xml:2: element order: Schemas validity error : Element 'order': The attribute 'id' is required but missing.
wrong.xml:4: element product: Schemas validity error : Element 'product': This element is not expected. Expected is ( item ).
wrong.xml fails to validate

To check several files against the same schema:

for f in *.xml; do xmllint --noout --schema order.xsd "$f" 2>&1 | tail -1; done

Each file then prints one line, either validates or fails to validate. This is the right tool for a batch or a build script. It is less pleasant for fixing one file, because you read a line number in Terminal and then go find it in an editor.

Option 2: The XML panel in notePad++++

When you open an XML file, an XML button appears at the top right of the editor. If problems are found, the number of issues is shown next to it. The panel runs the same xmllint that macOS provides, so the messages come from that tool, shown with the line number and, when xmllint reports one, the column.

1. Syntax is checked by default. The panel starts in Check XML syntax mode. It re-checks shortly after you stop typing and does not modify your file.
2. Errors are marked in the editor. Each problem is listed as line:column · message. Click one and the cursor jumps to that spot.
3. Validate against the file's own DTD. Choose Validate using the XML's DTD declaration to check the file against the DOCTYPE it declares. If that declaration points to a separate .dtd file, the app may need permission to read the folder it sits in.
4. Validate against your own XSD or DTD. Click Choose XSD or DTD…, pick the rule file, and confirm the folder-access window that follows. The current editor text is then checked, including changes you have not saved. This step is part of Pro.

After you choose a rule, it is remembered. The next XML file you open, and the next launch of the app, use the same rule until you pick another one or click Clear rule, which returns to syntax checking. If the remembered rule can no longer be found, the panel tells you to choose it again instead of quietly showing a pass.

When the schema pulls in other files

XSD files often include or import other local files, and a DTD can reference files too. If one of them cannot be read, the panel shows a red notice with the full path, and explains that a file in another folder may need extra access. Allow Another Folder… grants it and the check retries automatically. Folder access you have already given is kept. Granting an extra folder is part of Pro.

What it does not do

• XSD 1.0 only. XML syntax, XSD 1.0, external DTDs, and DTDs declared in the file are supported.
• No schema download. The schema address written inside your XML is not followed, and external entities and XInclude are not expanded. Save a remote schema to disk and choose that file.
• Size limits. Automatic validation supports XML up to 16 MiB, rule files up to 5 MiB, and each check is limited to 10 seconds. If a limit is exceeded, the panel says why and the file can still be edited normally.
• One file at a time. For a folder of files, use the xmllint loop above.

Which route to use