UNIX Utilities IV: cat

This is probably one of the most useful commands on any UNIX system, it almost becomes a third arm for most users, essential for navigating a world where everything is a file.

/**
 * A copy of the "cat" command on UNIX, missing a few options, but it
 * does what is normally asked of it (reading files).
 * 
 * About 1/4 the size of the GNU one without stripping.
 * 
 * Copyright 2012-01-25 Joseph Lewis
 */
#include <iostream>
#include <fstream>
#include <string.h>
#include <stdio.h>
using namespace std;

int main(int nargs, char* vargs[])
{
 // General help behaviour.
 if(strcmp(vargs[0],"--help") == 0 || strcmp(vargs[0], "-h") == 0)
 {
  cout << "Usage: " << vargs[0] << " filename [filename ...]" << endl;
  cout << "\tPrints the contents of the given file(s)" << endl;
  return 1;
 }
 
 // If the only arg is the program name, read from stdin.
 if(nargs == 1)
 {
  while(cin.good())
  {
   string line;
   getline(cin,line);
   cout << line << endl;
  }
  
  return 0;
 }
 
 // If the program has args, try to read the files.
 for(int i = 1; i < nargs; i++)
 {
  string line;
  ifstream myfile;
  myfile.open (vargs[i]);
  
  if(myfile.bad() || myfile.fail())
  {
   cout << "cat: "<< vargs[i] << ": No such file or directory" << endl;
  }

  while ( myfile.good() )
  {
   getline (myfile,line);
   cout << line << endl;
  }
  myfile.close();
 }
 return 0;
}

UNIX Utilities III: Yes

This isn't a hugely popular program, but it does have its uses: the basic "yes" command, simply repeats whatever you put in to it, over and over and over again. This can be handy for things like testing buffering on a new shell you're developing.

Note that the usage is incredibly similar to echo, but simply provides it in a for loop:

/**
 * A utility that emulates the UNIX command "yes", repeating the input
 * until killed.
 * 
 * Used in places like Jurassic Park (the movie) when Nedry leaves a 
 * trap that outputs the string "You didn't say the magic word!" over
 * and over.
 * 
 * Copyright 2011-12-23 Joseph Lewis <joehms22@gmail.com>
 */

#include <iostream>

using namespace std;

int main(int nargs, char* vargs[])
{
 string s;
 
 for(int i = 0; i < nargs - 1; i++)
 {
  if(i != 0)
   s += " ";
  s += vargs[i + 1];
 }
 s += "\n";
 
 while(true)
 {
  cout << s;
 }
}

UNIX Utilities II: True and False

It is occasionally useful to check the outputs of commands in bash scripts; it is even more useful to test your scripts for all eventualities before you release them, to make sure strange errors don't begin occurring once they have been run in a variety of environments; this is where "true" and "false" come in.

These two simple programs simply return 0, or 1 as an exit status when run. This is probably the only case in a UNIX system where you will see 1 denoting a success.

Once compiled, these programs end up being about 8.3 kb on my system, which is almost a third the size of the GNU versions, how this happened, I have no idea; they are in essence the simplest programs in the world:

/**
 * true - Probably the simplest program in the world, does nothing, and
 * succeeds at it. Not very realistic, I know, but this is UNIX after 
 * all, not the real world where not doing anything makes you a failure.
 * 
 * Copyright 2011-12-23 Joseph Lewis <joehms22@gmail.com>
 */

int main()
{
 return 0;
}

/**
 * false
 * Probably the (second) simplest program in the world, does nothing, 
 * and fails at it. Note that this is the second because "true" is the 
 * first.
 * 
 * Copyright 2011-12-23 Joseph Lewis <joehms22@gmail.com>
 */

int main()
{
 return 1; // FAIL
}

UNIX Utilities: Echo

This is the first post of what I hope to be many describing UNIX command line utilities, first up is the "echo" command:

/**
 * A copy of the "echo" command on UNIX, missing a few options, but it
 * does what is normally asked of it.
 * 
 * Copyright 2011-12-23 Joseph Lewis <joehms22 gmail com>
 */
#include <iostream>

using namespace std;

int main(int nargs, char* vargs[])
{
 for(int i = 0; i < nargs - 1; i++)
 {
  if(i != 0)
   cout << " ";
  cout << vargs[i + 1];
 }
 cout << "\n";
}