Run system commands in java

Question

I am a little confused about why I can't run the following command ls -l If I run ls or pwd it works fine.

Am I missing something?

 ProcessBuilder pb = new ProcessBuilder("ls -l");
 pb.redirectErrorStream(true);
 Process process = pb.start();
 InputStream is = process.getInputStream();
 InputStreamReader isr = new InputStreamReader(is);
 BufferedReader br = new BufferedReader(isr);
 String line;
 while ( (line = br.readLine()) != null) {
 System.out.println(line);
 }
 br.close();

One more question: How can I run multiple system commands concurrently? Using while loop or for loops will run the command one by one. Any advice?

Thanks in advance.

• xiaofeng.li: Try new ProcessBuilder("ls", "-l");
• Boris the Spider: Why would you want to?! Use the functionality built into the JDK and keep your code platform independent.
• devDude: to see if it can be done or if it's possible. curious mind wants to know :)

Answer 1

Change:

new ProcessBuilder("ls -l");

To:

new ProcessBuilder("ls", "-l");

• devDude: Thanks. One more question. Let's say I have an array of system commands. How should i go about it?
• devDude: I got it...if anyone's interested here it is
• devDude: String[] st = {"ls", "bin"}; ProcessBuilder pb = new ProcessBuilder(st)

Answer 2

 String[] st = {"ls", "bin"};
 ProcessBuilder pb = new ProcessBuilder(st);
 pb.redirectErrorStream(true);
 Process process = pb.start();
 InputStream is = process.getInputStream();
 InputStreamReader isr = new InputStreamReader(is);
 BufferedReader br = new BufferedReader(isr);
 String line;
 while ( (line = br.readLine()) != null) {
 System.out.println(line);
 }
 br.close(); 

Answer 3

Using while loop or for loops will run the command one by one.

Only when you are doing the whole start-then-read-stdout business for each one of them, one-by-one. The processes are indeed run in parallel, it's just the reading part that's stopping you from running them concurrently. All you need to do is breaking the start and read into two parts:

 Stream.of(Arrays.asList("ls", "-l"),
 Arrays.asList("python", "-h"),
 Arrays.asList("df"))
 .map(cmd->{
 // Create a process for each command, but don't read the output
 try {
 return new AbstractMap.SimpleImmutableEntry<>(cmd,
 new ProcessBuilder(cmd)
 .redirectErrorStream(true)
 .start().getInputStream());
 } catch (IOException e) {
 e.printStackTrace();
 return null;
 }
 })
 .filter(p->p!=null)
 .parallel()
 .forEach(in->{
 // Read and print STDOUT for each process.
 try (BufferedReader reader = new BufferedReader(new InputStreamReader(in.getValue()))){
 String line;
 while ((line = reader.readLine()) != null) {
 System.out.printf("%20s: %s\n", in.getKey(), line);
 }
 } catch (IOException e) {
 e.printStackTrace();
 }
 });

The parallel() call is making the output really hard to read, it's there only to demonstrate that the processes are really running concurrently.

• devDude: Thanks for the advice. Need to practice more.
• Boris the Spider: cmd.split("\\s+") takes no accounting for quoting or escaped quotes and is generally a very bad idea.
• xiaofeng.li: Thanks, I will modify the answer.