CODEWITHSUNDEEP
javalinuxbashruntime.exec
Volker Seibt
Volker Seibt

Reputation: 1536

Redirecs with /bin/bash from Java Runtime exec()

I'm trying to use a redirect in a command executed with the exec()-method of java.lang.Runtime on Ubuntu 14.04

public static void main(String[] args) throws IOException, Exception {
    Runtime runtime = Runtime.getRuntime();
    String command = "echo bla > bla.txt";
    System.out.println("Command : " + command);
    Process process = runtime.exec(command);
    printLines(" stdout", process.getInputStream());
    printLines("  error", process.getErrorStream());
    process.waitFor();
    System.out.println("ExitValue : " + process.exitValue());
}

private static void printLines(String name, InputStream ins) throws Exception {
    try(Stream<String> lines = new BufferedReader(new InputStreamReader(ins)).lines()) {
        lines.forEach(line -> System.out.println(name + " : " + line));
    }
}

The output is:

Command : echo bla > bla.txt
 stdout : bla > bla.txt
ExitValue : 0

So bla > bla.txt is written to stdout but of course there is no redirect to bla.txt.

Perhapes shell redirects are not possible in a simple exec().

So I tried to change the command = "/bin/bash -c 'echo bla > bla.txt'" to use the whole echo and redirect as a parameter to /bin/bash.

With this I get the result:

Command : /bin/bash -c 'echo bla > bla.txt'
  error : bla: -c: line 0: unexpected EOF while looking for matching `''
  error : bla: -c: line 1: syntax error: unexpected end of file
ExitValue : 1

Of course /bin/bash -c 'echo bla > bla.txt'works fine on Ubuntu and creates the desired file.

I found no place where I could set the single quotes to get a satisfying result, and I also tried with all kinds of escape characters to escape the spaces or the redirect (>).

It works if I use a command array like

String cmdArray[] = {"/bin/bash", "-c", "echo bla > bla.txt"};
Process process = runtime.exec(cmdArray);

, but it has to be a single string because the entire command has to be build somewhere else.

I surely know

I'm just curious why this does not work.

Upvotes: 1

Views: 3048

Answers (2)

Kenster
Kenster

Reputation: 25380

String command = "echo bla > bla.txt";
Process process = runtime.exec(command);

The > blah.txt text in your command is shell syntax to redirect standard output. It's implemented by shells like sh or bash.

Runtime.exec() doesn't use a shell to run commands. The way it launches commands is described here and here. Basically, it uses its own logic to split the command string into arguments at spaces, then directly executes the resulting command.

If you want to invoke a command which should be interpreted as a shell command, you need to explicitly invoke a shell:

String[] command = { "/bin/bash", "-c", "echo bla > bla.txt" };
Process process = runtime.exec(command);

Upvotes: 1

Zbynek Vyskovsky - kvr000
Zbynek Vyskovsky - kvr000

Reputation: 18825

The cause is that exec uses simple StringTokenizer with any white space as a delimiter to parse the actual command. Therefore it's portable as it does work nowhere when you pass something complex :-)

The workaround you chose is correct way, portable and most of all safest asyou ddon't need to escape if the command contained for example quotes etc.

Upvotes: 1

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