LinuxQuestions.org - Linux command input done in Java

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Linux command input done in Java

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?

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;



  }

...

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