Ooooh that shell, Can't you shell that shell

Staying up late to work on a fun project. Building a custom PXE boot image that will be used to pre-load our servers and get them ready for Puppet management. It has been about eight years since I had built a custom boot image. At that time busybox was my goto for small, powerful environments that needed to run from very small RAM mounted file systems.

BusyBox combines tiny versions of many common UNIX utilities into a single small executable. It provides replacements for most of the utilities you usually find in GNU fileutils, shellutils, etc. The utilities in BusyBox generally have fewer options than their full-featured GNU cousins; however, the options that are included provide the expected functionality and behave very much like their GNU counterparts. BusyBox provides a fairly complete environment for any small or embedded system.

When most distros offer boot images that do little more than boot, or fail to boot; for my systems I used busybox to create a very compact environment that would allow you to perform diagnostics and fix any problems.

As great as busybox was at the time, I found it lacked a lot of compatibilities with even old POSIX systems and scripts. Arguments that you rely on for find, grep and even ls, just were not there. Things have changed; somewhat. Busybox has grown up and is full of new abilities.

Screenshot of "make menuconf" providing the ability
to choose the features to build into the binary.

Recursion

My son is learning C++ in school and was given an assignment to create a program that would convert Roman Numerals to decimal integers. Most people new to C++ would do this using a while loop. Up on completion of his version I will show him an alternate method that does not use a while loop but rather is a good example of recursion.

This example makes it much harder to debug as the calculation happens in the recursion call, and I have placed that in the return of the same function, but this is just for fun.

#include <iostream>
#include <string>

using namespace std;

int Decode(const char* CodeString, int Position, int PreviousValue) {
    // When we have reached the end of the string
    if(CodeString[Position] == '\0') return(0);

    int NextValue = 0;
    switch (CodeString[Position]) {
        case 'M': NextValue = 1000;  break;
        case 'D': NextValue = 500;  break;
        case 'C': NextValue = 100;  break;
        case 'L': NextValue = 50;   break;
        case 'X': NextValue = 10;   break;
        case 'V': NextValue = 5;    break;
        case 'I': NextValue = 1;    break;
    }
    if(PreviousValue < NextValue) NextValue -= PreviousValue * 2;
    return(NextValue + Decode(CodeString, Position+1, NextValue));
}

int main()
{
    cout << "Please enter a value as valid formated Roman numerals ";
    string UserInput = "";
    cin >> UserInput;
    int Answer = Decode(UserInput.c_str(), 0, 0);
    cout << "The answer is " << Answer << endl;
    return 0;
}

Super Simple Linked List In C

When writing code in a high level scripting language like Perl, Ruby or Python it is easy to take for granted what a nice feature it is to have automatic types like arrays or hashs. Variable that you can just dump data in to and pull it back out any way you like. These features were not always there, someone had to write the code to make it to this.

A linked list in C is a way to store values, as many as you like for as much memory as you have. Memory for a variable is created on the fly as needed and stored in a location that knows information about it's neighbour.  A good linked list will have features to let you search, insert, delete from any point, move backwards and forwards in the list and most importantly a good list would protect your code from accidental memory bugs. My example today has non of those features.

This is the bare minimum of a simple linked list. It is a single linked list so you can only move forward. A double linked list lets you move backwards also.

#include<stdio.h>
#include<stdlib.h>

struct DataNode {
 void *Load;
 struct DataNode *Next;
};

struct DataNode *TheFirst = NULL;
struct DataNode *Current = NULL;
struct DataNode *Last = NULL;

void* NewList(void *Load) {
 struct DataNode *Pointer = (struct DataNode*)malloc(sizeof(struct DataNode));
 if(NULL == Pointer) {
  return NULL;
 }
 Pointer->Load = Load;
 Pointer->Next = NULL;

 TheFirst = Last = Current = Pointer;
 return(Pointer);
}

void* Add(void *Load) {
 if(TheFirst == NULL) {
  return (NewList(Load));
 }

 struct DataNode *Pointer = (struct DataNode*)malloc(sizeof(struct DataNode));
 Pointer->Load = Load;
 Pointer->Next = NULL;

 Last->Next = Pointer;
 Current = Last = Pointer;
 return(Pointer);
}

void* First() {
 Current = TheFirst;
 if(Current == NULL) return(NULL);
 return(Current->Load);
}

void* GetNext() {
 struct DataNode *Pointer = Current;

 // Advance to next
 if(Pointer->Next) {
  Current = Pointer->Next;
  return(Current->Load); 
 }
 return(NULL);
}

int DeleteFirst() {
 int Return = 0;
 if(TheFirst) {
  if(TheFirst->Next) {
   Current = TheFirst->Next;
   Return = 1;
  } else {
   Current = NULL;
  }
 }
 free(TheFirst);
 TheFirst = Current;
 return(Return);
}

int main(void) {
 char* ToDoList;
 Add("Pet the dog");
 Add("Feed the fish");
 Add("Let the cat out");
 Add("Do the dishes");
 Add("Gas up the car");
 Add("Watch some TV");
 ToDoList = First();
 while(ToDoList != NULL) {
  printf("%s\n", ToDoList);
  ToDoList = GetNext();
 }
 while(DeleteFirst()) {}
 return 0;
}

Daemonize your code

If you write a shell script or program and need to fork it to the background as a daemon there could be some issues. The normal standard daemon library may not be able to totally disconnect from any open file descriptors. This simple C++ program can do it all for you and it is so easy to use, just put this program in front of your normal command and it will turn any normal program in to a system level daemon.

#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <syslog.h>
#include <string.h>

int main(int argc, char *argv[]) {
 int i;
 // int reterr;
 pid_t pid, sid;
 
 //Fork the Parent Process
 pid = fork();
 
 if (pid < 0) { exit(EXIT_FAILURE); }
 
 //We got a good pid, Close the Parent Process
 if (pid > 0) { exit(EXIT_SUCCESS); }
 
 //Change File Mask
 umask(0);
 
 //Create a new Signature Id for our child
 sid = setsid();
 if (sid < 0) { exit(EXIT_FAILURE); }
 
 //Change Directory
 //If we cant find the directory we exit with failure.
 if ((chdir("/")) < 0) { exit(EXIT_FAILURE); }
 
 //Close Standard File Descriptors
 close(STDIN_FILENO);
 close(STDOUT_FILENO);
 close(STDERR_FILENO);
 
 //----------------
 //Main Process
 //----------------
 for(i=0; i < argc - 1; i++) {
  argv[i]=argv[i+1];
 }
 argv[argc-1] = '\0';
 execv(argv[0], argv);
 //reterr = execv(argv[0], argv);
 //printf("execv failed with '%s'\n", strerror(errno));

 //Close the log
 closelog ();
}