Unix Shell and History Feature

//Enter command  'history' for history feature and CTRL - c to exit the 'osh>' shell
/*Header files */
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

#define MAX_LINE 80 /* The maximum length of a command */
#define BUFFER_SIZE 50
#define buffer "\n\Shell Command History:\n"

//declarations
char history[10][BUFFER_SIZE]; //history array to store history commands
int count = 0;


//function to display the history of commands
void displayHistory()
{
   
    printf("Shell command history:\n");
   
    int i;
    int j = 0;
    int histCount = count;
   
    //loop for iterating through commands
    for (i = 0; i<10;i++)
    {
        //command index
        printf("%d.  ", histCount);
        while (history[i][j] != '\n' && history[i][j] != '\0')
        {
//printing command
            printf("%c", history[i][j]);
            j++;
        }
        printf("\n");
        j = 0;
        histCount--;
        if (histCount ==  0)
            break;
    }
    printf("\n");
}



//Fuction to get the command from shell, tokenize it and set the args parameter

int formatCommand(char inputBuffer[], char *args[],int *flag)
{
    int length; // # of chars in command line
    int i;     // loop index for inputBuffer
    int start;  // index of beginning of next command
    int ct = 0; // index of where to place the next parameter into args[]
    int hist;
    //read user input on command line and checking whether the command is !! or !n

  length = read(STDIN_FILENO, inputBuffer, MAX_LINE);

 
    start = -1;
    if (length == 0)
        exit(0);   //end of command
    if (length < 0)
    {
        printf("Command not read\n");
        exit(-1);  //terminate
    }
   
   //examine each character
    for (i=0;i<length;i++)
    {
        switch (inputBuffer[i])
        {
            case ' ':
            case '\t' :               // to seperate arguments
                if(start != -1)
                {
                    args[ct] = &inputBuffer[start];  
                    ct++;
                }
                inputBuffer[i] = '\0'; // add a null char at the end
                start = -1;
                break;
               
            case '\n':                 //final char
                if (start != -1)
                {
                    args[ct] = &inputBuffer[start];
                    ct++;
                }
                inputBuffer[i] = '\0';
                args[ct] = NULL; // no more args
                break;
               
            default :          
                if (start == -1)
                    start = i;
                if (inputBuffer[i] == '&')
                {
                    *flag  = 1; //this flag is the differentiate whether the child process is invoked in background
                    inputBuffer[i] = '\0';
                }
        }
    }
   
    args[ct] = NULL; //if the input line was > 80

if(strcmp(args[0],"history")==0)
        {
               if(count>0)
{

                displayHistory();
}
else
{
printf("\nNo Commands in the history\n");
}
return -1;
        }

else if (args[0][0]-'!' ==0)
{ int x = args[0][1]- '0';
int z = args[0][2]- '0';

if(x>count) //second letter check
{
printf("\nNo Such Command in the history\n");
strcpy(inputBuffer,"Wrong command");
}
else if (z!=-48) //third letter check
{
printf("\nNo Such Command in the history. Enter <=!9 (buffer size is 10 along with current command)\n");
strcpy(inputBuffer,"Wrong command");
}
else
{

if(x==-15)//Checking for '!!',ascii value of '!' is 33.
{ strcpy(inputBuffer,history[0]);  // this will be your 10 th(last) command
}
else if(x==0) //Checking for '!0'
{ printf("Enter proper command");
strcpy(inputBuffer,"Wrong command");
}

else if(x>=1) //Checking for '!n', n >=1
{
strcpy(inputBuffer,history[count-x]);

}

}
}
 for (i = 9;i>0; i--) //Moving the history elements one step higher
        strcpy(history[i], history[i-1]);
   
    strcpy(history[0],inputBuffer); //Updating the history array with input buffer
    count++;
if(count>10)
{ count=10;
}
}

int main(void)
{
    char inputBuffer[MAX_LINE]; /* buffer to hold the input command */
    int flag; // equals 1 if a command is followed by "&"
    char *args[MAX_LINE/2 + 1];/* max arguments */
    int should_run =1;
   
    pid_t pid,tpid;
    int i;
 
   
    while (should_run) //infinite loop for shell prompt
    {          
        flag = 0; //flag =0 by default
        printf("osh>");
        fflush(stdout);
        if(-1!=formatCommand(inputBuffer,args,&flag)) // get next command
{
pid = fork();
       
        if (pid < 0)//if pid is less than 0, forking fails
        {
           
            printf("Fork failed.\n");
            exit (1);
        }
       
        else if (pid == 0)//if pid ==0
        {
           
            //command not executed
            if (execvp(args[0], args) == -1)
            {

                printf("Error executing command\n");
            }
        }
       
        // if flag == 0, the parent will wait,
        // otherwise returns to the formatCommand() function.
        else
        {
            i++;
            if (flag == 0)
            {
                i++;
                wait(NULL);
            }
        }
    }
     }
}