Ant: Comparing files

Doing something like a unix 'diff -q' via Ant

There are three approaches you can use in Ant to compare two files:

• Use built-in features of Ant, with no add-ons, and no external dependencies
• Use third-party add-ons for Ant (which may require extra .jar files, and some tricky configuration)
• Rely on the system “diff” command (which might make your build.xml file non-portable across operating systems)

This file will focus on the first approach: a way of doing file comparison that uses ONLY features built into standard Apache Ant, with no extra add ons.

Example of how to implement something like diff -q

Below is an excerpt from an example build.xml that shows how to make an Ant file do different things depending on whether two files match or not. This example uses this to check whether the output of a program matches the expected output.

Note that this example ONLY depends on whether the files match or not; it does NOT do the full diff behavior of showing the lines that do or do not match. So, it reality, it is more like the unix diff -q than a true Unix diff.)

The diff-test target shown uses the java task to run a program from a jar file, taking standard input from testdata/test01.input.txt, and writing its standard output into a temporary directory called temp.d, in a file called test01.output.txt.

It then compares the files testdata/test01.expected.txt and temp.d/test01.output.txt to see if they match. If they do, then the property test01passed is set to "true".

Then, echo tasks with the if:set and unless:set attributes are used to print output conditionally based on whether the test01passed property has been set or not. Note that the following must appear in the opening <project> tag of the build.xml file if the if if:set and unless:set tags are to be used (more information at: https://ant.apache.org/manual/ifunless.html)

   xmlns:if="ant:if" xmlns:unless="ant:unless"                                              

Here’s a partial listing of the build.xml that implements the file comparison.

<project default="compile" xmlns:if="ant:if" xmlns:unless="ant:unless">
...
<target name="diff-test" depends="compile" description="run tests using CLI, diffing actual and expected output ">
    <delete dir="temp.d" quiet="true" />
    <mkdir dir="temp.d" />
    <java jar="${jar.file}" fork="true">
      <redirector 
         input="testdata/test01.input.txt" 
         output="temp.d/test01.output.txt" />
    </java>
    <condition property="test01passed" value="true">
       <filesmatch file1="testdata/test01.expected.txt" 
                   file2="temp.d/test01.output.txt" />
    </condition>
   <echo if:set="test01passed">test01 passed</echo>
   <echo unless:set="test01passed">test01 failed</echo>
</target>