shell2.c

//Venkata Sai deepak Aitha, Chandan dev, shanmukha priya, Madhu anchuri, saransh singh
//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);
           		 }
        	}
   	 }
     }
}