How to execute Python script from Java (via command line)?

Question

I can execute Linux commands like ls or pwd from Java without problems but couldn't get a Python script executed.

This is my code:

Process p;
try{
 System.out.println("SEND");
 String cmd = "/bash/bin -c echo password| python script.py '" + packet.toString() + "'";
 //System.out.println(cmd);
 p = Runtime.getRuntime().exec(cmd); 
 BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()));
 String s = br.readLine(); 
 System.out.println(s);
 System.out.println("Sent");
 p.waitFor();
 p.destroy();
} catch (Exception e) {}

Nothing happened. It reached SEND but it just stopped after it...

I am trying to execute a script which needs root permissions because it uses serial port. Also, I have to pass a string with some parameters (packet).

• VishalDevgire: is your python script writing somethong to it's standard out?
• rkosegi: What about use Apache commons exec ?

Answer 1

You cannot use the PIPE inside the Runtime.getRuntime().exec() as you do in your example. PIPE is part of the shell.

You could do either

• Put your command to a shell script and execute that shell script with .exec() or

• You can do something similar to the following

  String[] cmd = {
   "/bin/bash",
   "-c",
   "echo password | python script.py '" + packet.toString() + "'"
   };
  Runtime.getRuntime().exec(cmd);
  

• Biribu: How do I do a shell script for that? I have never created a shell script
• pts: This answer works even without creating a shell script file. Just copy-paste it to your code.
• Alper: @pts Yes, it works but I couldn't explain it clearly before. I updated the post, I hope it better now.
• Nitesh kumar: I have a two python versions running in my system. one the default one for which I have set the env path variable, the other one is from anaconda package which is in different location for instance (c:/Users/Nitesh/appdata/anaconda/python.exe) I want this python which is given by anaconda to run the python script through java. What should I give in the array of string cmd. I tried changing the 2 element (-c) to the fully qualified path of my anaconda python, It didn't work yet.
• kautuksahni: Is it good practice to call scripts from JAVA? Any performance issues?

Answer 2

@Alper's answer should work. Better yet, though, don't use a shell script and redirection at all. You can write the password directly to the process' stdin using the (confusingly named) Process.getOutputStream().

Process p = Runtime.exec(
 new String[]{"python", "script.py", packet.toString()});
BufferedWriter writer = new BufferedWriter(
 new OutputStreamWriter(p.getOutputStream()));
writer.write("password");
writer.newLine();
writer.close();

• Chinmaya B: Is this going to work with Windows as well as Linux?
• jtahlborn: @ChinmayaB - yes, this is OS agnostic.

Answer 3

You would do worse than to try embedding jython and executing your script. A simple example should help:

ScriptEngine engine = new ScriptEngineManager().getEngineByName("python");
// Using the eval() method on the engine causes a direct
// interpretataion and execution of the code string passed into it
engine.eval("import sys");
engine.eval("print sys");

If you need further help, leave a comment. This does not create an additional process.

Answer 4

First, open terminal and type "which python3". You will get the complete path of python3. For example "/usr/local/bin/python3"

String[] cmd = {"/usr/local/bin/python3", "arg1", "arg2"};
Process p = Runtime.getRuntime().exec(cmd);
p.waitFor();
String line = "", output = "";
StringBuilder sb = new StringBuilder();
BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()));
while ((line = br.readLine())!= null) {sb = sb.append(line).append("\n"); }
output = sb.toString();
System.out.println(output);