CODEWITHSUNDEEP
javacommand-lineterminal
devDude
devDude

Reputation: 17

Run system commands in java

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.

Upvotes: 0

Views: 114

Answers (3)

xiaofeng.li
xiaofeng.li

Reputation: 8587

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.

Upvotes: 0

devDude
devDude

Reputation: 17

    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();

Upvotes: 0

eg04lt3r
eg04lt3r

Reputation: 2610

Change:

new ProcessBuilder("ls -l");

To:

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

Upvotes: 1

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