Linux command input done in Java

fakie_flip · 08-26-2013, 10:31 PM

Many commands in Linux take input one of two ways, stdin, or as an argument to a file. Examples:

Code:

echo "text from stdin" | lpr
lpr filename.txt
  

echo "text from stdin" | nl
nl filename.txt

The same is true with awk, sed, grep, and many others. How can the same behavior happen from a command line app written in Java? I believe System.in represents stdin. Reading stdin is not difficult. Reading from a file is not difficult, but how can the app act accordingly to how it was called on the command line?

kbp · 08-27-2013, 12:31 AM

I haven't needed to often but in the past I've just tested whether stdin was a pipe and if not then check for data passed via argument .. here's a perl fragment that may clarify:

Code:

if ( -p STDIN ) {

  while (<STDIN>) {
    push(@pids, (split(/ /, $_)));
  }

}
else {

  $numArgs = @ARGV;
  if ( $numArgs > 0 ) {

    @pids = @ARGV;
    chomp @pids;

  }
...

NevemTeve · 08-27-2013, 02:36 AM

Check the number of arguments (argv.length). If positive, there are parameters (filenames), otherwise read strin.