RunBash: Execute Real Shell Commands from Groovy

James Durbin bio photo By James Durbin

RunBash

This post is about running shell commands from within Groovy, specifically bash but it is easy to adapt to other shells. Groovy has built-in support for running commands like this:

"ls -l".execute()

That is about as simple as it can get and works great for many situations. However there is a key gotcha: execute() simply executes the given command passing it whatever else is in the string as options to the command. The options are not passed through any shell (e.g. bash) for wildcard expansion or other transformations. As a result, you can not natively do something like:

"ls *.groovy".execute()

In this case, no shell sees the * to expand it, and so the * just gets passed to ls exactly as an argument which ls itself does not know how to interpret. To address this, we can create a shell process with ProcessBuilder and pass the command to the shell for execution. A common use case for me is to want to just pipe the shell command’s output to stdout. With some Groovy meta-object programming we can make this a method of GString and String so that you can execute any kind of string simply by calling, for example, a .bash() method on the string. I have written a class called RunBash which provides this function. With this class you can properly execute the ls *.groovy example above with code like "ls *.groovy".bash(). You can even execute more complicated shell scripts like:

"""
 for file in \$(ls); do
   echo \$file
 done
""".bash()

To turn on this functionality it is necessary to call RunBash.enable() first. So a full example using the durbinlib implementation of RunBash is:

#!/usr/bin/env groovy

import durbin.util.*
RunBash.enable()

"""
for file in \$(ls); do
   echo \$file
done
""".bash()

Installation

You can obtain RunBash with durbinlib, which is my kitchen-sink of everday classes and scripts. Simply clone, ant build, and add to CLASSPATH and PATH like:

git clone git://github.com/jdurbin/durbinlib.git
cd durbinlib
ant install
export PATH=$PATH:pathtodurbinlib/scripts/ 
export CLASSPATH=pathtodurbinlib/target/jar/*

Source

To get an idea how this is implemented in Groovy code, a stripped-down but functional version of the class is shown below:

import java.io.InputStream;

class RunBash{
  
  static boolean bEchoCommand = false;
  
  // Add a bash() method to GString and String with meta-object programming
  // This is why it's necessary to call an enable function in your script. 
  static def enable(){
    GString.metaClass.bash = {->
      RunBash.bash(delegate)
    }    
    String.metaClass.bash = {->
        RunBash.bash(delegate)
    }    
  }
  
  static def bash(cmd){
    cmd = cmd as String

    // create a process for the shell
    ProcessBuilder pb = new ProcessBuilder("bash", "-c", cmd);
    pb.redirectErrorStream(true); // use this to capture messages sent to stderr
    Process shell = pb.start();
    shell.getOutputStream().close();
    InputStream shellIn = shell.getInputStream(); // this captures the output from the command

    // at this point you can process the output issued by the command
    // for instance, this reads the output and writes it to System.out:
    int c;
    while ((c = shellIn.read()) != -1){
      System.out.write(c);
    }

    // wait for the shell to finish and get the return code
    int shellExitStatus = shell.waitFor(); 

    // close the stream
    try {
      shellIn.close();
      pb = null;
      shell = null;
    } catch (IOException ignoreMe) {}
  }
}