Python and Java program that work as pipes

--D. Thiebaut (talk) 08:45, 18 February 2017 (EST)

---

Two programs, one in Python and one in Java show how to code an application to take its input either from a file name given on the command line, or from stdin, fed through a pipe. Both programs are illustrated by having them filter a text file containing poems. The contents of the file is given at the end of this page.

Pipes in Python

Source

#! /usr/bin/env python
'''
gremlin.py
D. Thiebaut

This python program illustrates how to write a program
that will work equally well with information taken from
a file specified on the command line, or information
fed through stdin.

Typical use:

   chmod +x gremlin.py
   ./gremlin.py  someFileName.txt

   cat someFileName.txt | ./gremlin.py

'''

from __future__ import print_function
import sys

def main():
    # open the first argument on command line, or stdin if none is specified
    with open(sys.argv[1], 'r') if len(sys.argv) > 1 else sys.stdin as f:
        # read the whole contents and put it in memory
        lines = f.readlines()
        f.close()


    # filter each line and replace 'g' characters by "gremlin",
    # upper or lowercase.
    for line in lines:
        print( line.strip()
               .replace('g', 'gremlin' )
               .replace( 'G', 'Gremlin' ) )

main()

Output

Pipes in Java

Source

/*
  Gremlin.java
  D. Thiebaut
  
  A program illustrating how an app can work with both a file and with stdin as
  input.

  This program takes either a file given as argument, or stdin, and filters it, 
  replacing all 'g' characters by "gremlin" and all 'G' characters by "Gremlin"

  Typical use:
  javac Gremlin.java
  java Gremlin someFileName.txt
  cat someFileName.txt | java Gremlin
  
  Some code taken from https://www.caveofprogramming.com/java/java-file-reading-and-writing-files-in-java.html

 */
import java.io.*;
import java.util.Scanner;

class Gremlin {
    
    public static void main(String[] args) {
	// read from pipe
	if ( args.length == 0 ) {
	    Scanner sc = new Scanner(System.in);
	    //System.out.println("Printing the file passed in:");
	    while(sc.hasNextLine()) System.out.println(sc.nextLine());
	}

	// read from file specified as argument (reads only first of 
	// several arguments).
	else {
	    // The name of the file to open.
	    String fileName = args[0];

	    // This will reference one line at a time
	    String line = null;
	    
	    try {
		// FileReader reads text files in the default encoding.
		FileReader fileReader = new FileReader(fileName);
		
		// Always wrap FileReader in BufferedReader.
		BufferedReader bufferedReader = 
		    new BufferedReader(fileReader);
		
		while((line = bufferedReader.readLine()) != null) {
		    // filter away!
		    line = line.replace( "g", "gremlin" ).replace( "G", "Gremlin" );
		    System.out.println(line);
		}   
		
		// Always close files.
		bufferedReader.close();         
	    }
	    catch(FileNotFoundException ex) {
		System.out.println(  "Unable to open file '" + 
				     fileName + "'");                
	    }
	    catch(IOException ex) {
		System.out.println("Error reading file '" 
				   + fileName + "'");                  
	    }
	}
    }
}

Output

Poem File

The file, named snowPoems.txt, and used in these examples is the following:

Neither Snow - Poem by Billy Collins

When all of a sudden the city air filled with snow,
the distinguishable flakes
blowing sideways,
looked like krill
fleeing the maw of an advancing whale.

At least they looked that way to me
from the taxi window,
and since I happened to be sitting
that fading Sunday afternoon
in the very center of the universe,
who was in a better position
to say what looked like what,
which thing resembled some other?

Yes, it was a run of white plankton
borne down the Avenue of the Americas
in the stream of the wind,
phosphorescent against the weighty buildings.

Which made the taxi itself,
yellow and slow-moving,
a kind of undersea creature,
I thought as I wiped the fog from the glass,

and me one of its protruding eyes,
an eye on a stem
swiveling this way and that
monitoring one side of its world,
observing tons of water
tons of people
colored signs and lights
and now a wildly blowing race of snow. 

Billy Collins

----------------------------------------------------------

Shoveling Snow With Buddha - Poem by Billy Collins

In the usual iconography of the temple or the local Wok
you would never see him doing such a thing,
tossing the dry snow over a mountain
of his bare, round shoulder,
his hair tied in a knot,
a model of concentration.

Sitting is more his speed, if that is the word
for what he does, or does not do.

Even the season is wrong for him.
In all his manifestations, is it not warm or slightly humid? 
Is this not implied by his serene expression,
that smile so wide it wraps itself around the waist of the universe? 

But here we are, working our way down the driveway,
one shovelful at a time.
We toss the light powder into the clear air.
We feel the cold mist on our faces.
And with every heave we disappear
and become lost to each other
in these sudden clouds of our own making,
these fountain-bursts of snow.

This is so much better than a sermon in church,
I say out loud, but Buddha keeps on shoveling.
This is the true religion, the religion of snow,
and sunlight and winter geese barking in the sky,
I say, but he is too busy to hear me.

He has thrown himself into shoveling snow
as if it were the purpose of existence,
as if the sign of a perfect life were a clear driveway
you could back the car down easily
and drive off into the vanities of the world
with a broken heater fan and a song on the radio.

All morning long we work side by side,
me with my commentary
and he inside his generous pocket of silence,
until the hour is nearly noon
and the snow is piled high all around us; 
then, I hear him speak.

After this, he asks,
can we go inside and play cards? 

Certainly, I reply, and I will heat some milk
and bring cups of hot chocolate to the table
while you shuffle the deck.
and our boots stand dripping by the door.

Aaah, says the Buddha, lifting his eyes
and leaning for a moment on his shovel
before he drives the thin blade again
deep into the glittering white snow. 


Billy Collins