Java – Running external program with redirected stdin and stdout from Java

execjavastdinstdout

I'm trying to running an external program from a Java program and I'm having trouble. Basically what I'd like to do would be this:

 Runtime.getRuntime().exec("./extprogram <fileIn >fileOut");

However I've found that that doesn't work – Java apparentls needs to use a Process with input and output streams and other things which I'm not experienced with.

I've looked at a number of examples across the internet (many of which are from SO), and there doesn't seem to be a simple standard way of doing this, which for someone who doesn't fully understand what's going on, can be quite frustrating.

I'm also having trouble trying to build my own code off the examples of other people's code because generally it seems most other people 1. aren't interested in redirecting stdin, and 2. aren't necessarily redirecting stdout to a file, but instead to System.out.

So, would anyone be able to point me in the direction of any good simple code templates for calling external programs and redirecting stdin and stdout? Thanks.

Best Answer

You could try something like this:

ProcessBuilder pb = new ProcessBuilder();
pb.redirectInput(new FileInputStream(new File(infile));
pb.redirectOutput(new FileOutputStream(new File(outfile));
pb.command(cmd);
pb.start().waitFor();

Related Solutions

Bash – How to redirect and append both standard output and standard error to a file with Bash

cmd >>file.txt 2>&1

Bash executes the redirects from left to right as follows:

>>file.txt: Open file.txt in append mode and redirect stdout there.
2>&1: Redirect stderr to "where stdout is currently going". In this case, that is a file opened in append mode. In other words, the &1 reuses the file descriptor which stdout currently uses.

Linux – Confused about stdin, stdout and stderr

Standard input - this is the file handle that your process reads to get information from you.

Standard output - your process writes conventional output to this file handle.

Standard error - your process writes diagnostic output to this file handle.

That's about as dumbed-down as I can make it :-)

Of course, that's mostly by convention. There's nothing stopping you from writing your diagnostic information to standard output if you wish. You can even close the three file handles totally and open your own files for I/O.

When your process starts, it should already have these handles open and it can just read from and/or write to them.

By default, they're probably connected to your terminal device (e.g., /dev/tty) but shells will allow you to set up connections between these handles and specific files and/or devices (or even pipelines to other processes) before your process starts (some of the manipulations possible are rather clever).

An example being:

my_prog <inputfile 2>errorfile | grep XYZ

which will:

create a process for my_prog.
open inputfile as your standard input (file handle 0).
open errorfile as your standard error (file handle 2).
create another process for grep.
attach the standard output of my_prog to the standard input of grep.

Re your comment:

When I open these files in /dev folder, how come I never get to see the output of a process running?

It's because they're not normal files. While UNIX presents everything as a file in a file system somewhere, that doesn't make it so at the lowest levels. Most files in the /dev hierarchy are either character or block devices, effectively a device driver. They don't have a size but they do have a major and minor device number.

When you open them, you're connected to the device driver rather than a physical file, and the device driver is smart enough to know that separate processes should be handled separately.

The same is true for the Linux /proc filesystem. Those aren't real files, just tightly controlled gateways to kernel information.

Best Answer

Related Solutions

Bash – How to redirect and append both standard output and standard error to a file with Bash

Linux – Confused about stdin, stdout and stderr

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