Get output of terminal multi-arg command using Java

Question

I'm trying to parse output in java from shell command: (sox can be installed from apt)

sox inputfile.flac -n stat

but I can't, few hours of searching and nothing.

code:

Process p = Runtime.getRuntime().exec(command);  
BufferedReader in = new BufferedReader(new InputStreamReader(p.getInputStream()));  

String line = null;
while ((line = in.readLine()) != null) {  
    System.out.println(line);  
} 

I tried with commands:

String command1 = "sox inputfile.flac -n stat";   //no output
String command2 = "bash -c sox inputfile.flac -n stat";   //output of 'sox' without arguments
String command3 = "sh -c sox inputfile.flac -n stat";   //output of 'sox' without arguments
String command4 = "sh -c \"sox inputfile.flac -n stat\"";   //error
String command5 = "bash -c \"sox inputfile.flac -n stat\"";   //error

According to documentation od 'sh':

The -c option causes the commands to be read from the string operand in- stead of from the standard input. Keep in mind that this option only ac- cepts a single string as its argument, hence multi-word strings must be quoted.

So I tried "command4"/"command5" -> Error code 2. However, I checked in normal linux Terminal and its working fine:

sh -c "sox inputfile.flac -n stat"
bash -c "sox inputfile.flac -n stat"
sox inputfile.flac -n stat

What am I missing?

Answer 1

What you're missing is that Process/Runtime isn't bash. Bash does things to a command line before actually running it. You can't do those things in ProcessBuilder and friends (Runtime.exec, Process, and ProcessBuilder).

Turning *.txt into a list of all files? That's bash.

Turning hello foo bar "baz boz" into the concept of: the app is called hello, and it gets 3 args, foo, bar, and baz boz? That is bash. Bash does the 'oh, quotes, those need to be bundled up'.

As a general rule, don't use the single-string version. Use java.lang.ProcessBuilder.

Note also that translating bash to /bin/bash is also a bashism.

Finally, note that depending on platform and phase of the moon, Runtime.exec and friends will do some bashisms. Maybe. The problem is, it's not reliable, so don't rely on it (for example, on windows, *.txt usually DOES work whereas on all non-windows OSes it almost never will, and Runtime.exec does do some application of $PATH, but not necessarily in the same way bash would).

// Always absolute-path your executables.
// Always pass args separately.
ProcessBuilder pb = new ProcessBuilder("/bin/bash", "-c", "sox inputfile.flac -n stat");
Process p = pb.start();

Note that the second string (the one with sox...) is passed to bash. So, if you want to get fancy with string escapes and stars in there, feel free. Bash will process it, not ProcessBuilder:

// This won't work:
ProcessBuilder pb = new ProcessBuilder("ls", "*.txt");

// but this will:
ProcessBuilder pb = new ProcessBuilder("/bin/bash", "-c", "ls *.txt");

*) It's rather common parlance, but where I say 'bashism', feel free to insert your shell of choice, or 'shellism'. The point is, it's a shell taking in what you type on the command line and processing it before actually passing it on to the OS. Runtime.exec and co also need to do this processing; its processing is FAR more limited, and you should avoid it as much as you can: Absolute paths, and list args separately.