Executing commands with Gradle?

I believe you're looking for this:

task stopServer(dependsOn: war, type: Exec) {
     commandLine "pkill", " -9", "tomcat"
}

The main difference is very subtle - I just deleted two characters. The << is gone from the task definition. The other difference is that the commandLine expects the executable to be passed in separately from the arguments to it.

I removed the << because of an important idea in gradle: the build lifecycle. There's configuration and execution phases (that's not all, but it's enough to explain this).

The << is like saying doLast - it adds the closure you pass to the end of the actions (the execution phase) for this task. So that means here, it's going to try and execute the command like normal (it's an Exec object, after all), and only then, once it's executed, will it call your block - the block setting commandLine. So when it's executing, execCommand really is null, and would be until your block was run. This is the heart of your problem.

Without the << (also known as left-shift), that same block runs during the configuration phase. So the exec command gets set before it runs, and it works.

Here is another solution that works well for the same error if you have a standalone exec task that you want to run from the commandline but do not want to run in Android Studio as part of your build.

This will always run the configuration of "myExecTask" but will only execute doMyExecTask when it is explicitly run via "gradle myExecTask"

/**
 * Actually run exec task in doLast phase
 */
task doMyExecTask << {
    def hasProperties = project.hasProperty('SOME_PROPERTY');

    if (hasProperties) {
        myExecTask.commandLine "echo", "$SOME_PROPERTY"
    } else {
        println "ERROR: Unable to run task. Missing properties."
    }
}

/**
 * Configure exec task, this always runs
 */
task myExecTask(type: Exec) {
    dependsOn doMyExecTask
    workingDir 'path/to/executable'
}