[FIXED JENKINS-9968] added an option to do polling before a build.

changelog.html

@@ -77,6 +77,9 @@ Upcoming changes</a>
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-10414">issue 10414</a>)
   <li class=rfe>
     Added a dignosis CLI command to report the current granted authorities.
+  <li class=rfe>
+    Added an option in CLI build command to check for SCM changes before carrying out a build
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-9968">issue 9968</a>)
   <li class=rfe>
     If CLI fails to connect via a JNLP Slave port, fall back to HTTP full-duplex.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-10611">issue 10611</a>)

core/src/main/java/hudson/cli/BuildCommand.java

@@ -30,13 +30,17 @@ import hudson.model.ParametersAction;
 import hudson.model.ParameterValue;
 import hudson.model.ParametersDefinitionProperty;
 import hudson.model.ParameterDefinition;
+import hudson.model.TaskListener;
 import hudson.Extension;
 import hudson.AbortException;
 import hudson.model.Item;
 import hudson.util.EditDistance;
+import hudson.scm.PollingResult;
+import hudson.util.StreamTaskListener;
 import org.kohsuke.args4j.Argument;
 import org.kohsuke.args4j.Option;
 
+import java.nio.charset.Charset;
 import java.util.concurrent.Future;
 import java.util.Map;
 import java.util.HashMap;
@@ -63,6 +67,9 @@ public class BuildCommand extends CLICommand {
     @Option(name="-s",usage="Wait until the completion/abortion of the command")
     public boolean sync = false;
 
+    @Option(name="-c",usage="Check for SCM changes before starting the build, and if there's no change, exit without doing a build")
+    public boolean checkSCM = false;
+
     @Option(name="-p",usage="Specify the build parameters in the key=value format.")
     public Map<String,String> parameters = new HashMap<String, String>();
 
@@ -89,6 +96,12 @@ public class BuildCommand extends CLICommand {
             a = new ParametersAction(values);
         }
 
+        if (checkSCM) {
+            if (job.poll(new StreamTaskListener(stdout, getClientCharset())) == PollingResult.NO_CHANGES) {
+                return 0;
+            }
+        }
+
         Future<? extends AbstractBuild> f = job.scheduleBuild2(0, new CLICause(), a);
         if (!sync)  return 0;
 
@@ -104,7 +117,9 @@ public class BuildCommand extends CLICommand {
             "Aside from general scripting use, this command can be\n" +
             "used to invoke another job from within a build of one job.\n" +
             "With the -s option, this command changes the exit code based on\n" +
-            "the outcome of the build (exit code 0 indicates a success.)\n"
+            "the outcome of the build (exit code 0 indicates a success.)\n" +
+            "With the -c option, a build will only run if there has been\n" +
+            "an SCM change"
         );
     }
 

core/src/main/java/hudson/cli/CLICommand.java

@@ -51,8 +51,11 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.io.PrintStream;
 import java.lang.reflect.Type;
+import java.nio.charset.Charset;
+import java.nio.charset.UnsupportedCharsetException;
 import java.util.List;
 import java.util.Locale;
+import java.util.logging.Level;
 import java.util.logging.Logger;
 
 /**
@@ -303,6 +306,24 @@ public abstract class CLICommand implements ExtensionPoint, Cloneable {
         private static final long serialVersionUID = 1L;
     }
 
+    protected Charset getClientCharset() throws IOException, InterruptedException {
+        String charsetName = channel.call(new GetCharset());
+        try {
+            return Charset.forName(charsetName);
+        } catch (UnsupportedCharsetException e) {
+            LOGGER.log(Level.FINE,"Server doesn't have charset "+charsetName);
+            return Charset.defaultCharset();
+        }
+    }
+
+    private static final class GetCharset implements Callable<String, IOException> {
+        public String call() throws IOException {
+            return Charset.defaultCharset().name();
+        }
+
+        private static final long serialVersionUID = 1L;
+    }
+
     /**
      * Convenience method for subtypes to obtain environment variables of the client.
      */