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using fork/execvp to catenate a file

Discussion in 'C' started by ohaqqi, Jul 2, 2007.

  1. ohaqqi

    ohaqqi New Member

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    Hi guys,
    I'm still working on my shell. I'm trying to implement a function typefile that will take a command line input as follows:

    > type <file1>

    This command will implement a catenation of file1, equal to the command cat <file1>

    I need to use execvp() and fork() system calls to create a new process that will type/cat any text file. In my code below, the execvp() call in the function at the bottom gets me the following:

    o-shell.c:128: warning: passing arg 2 of `execvp' from incompatible pointer type

    I know for sure i'm calling execvp incorrectly, any tips/pointers would be great. I want to use /bin/cat followed by filename as the argument.

    Code:
    #include <stdio.h>
    #include <unistd.h>
    #include <sys/types.h>
    #include <string.h>
    
    int main(int argc, char *argv[])
    {
    	//VARIABLE DECLARATIONS
    
    	char command[50];				//Command line input
    
    	//tokens for strtok() funtion
    	char * com;						//Primary command
    	char * arg1;					//First argument of command (usually a file name)
    	char * arg2;					//2nd argument of command (usually a file name)
    
    	int args;						//Number of arguments entered
    
    	//FUNCTION DECLARATIONS
    
    	void typefile(char * filename);
    
    	printf("***************************************************\n");
    	printf("Welcome to O-Shell! A UNIX shell by O Haqqi\n\n");
    	printf("Type 'help' for list of commands, 'exit' to exit\n");
    	printf("***************************************************\n");
    
    	for(;;)
    	{
    
    	printf("o-shell>");						//user prompts for o-shell
    
    	fgets(command, 1024, stdin);		//command line input from user
    
    	com = strtok(command, " \r\n\t");
    
    	args = 0;
    	arg1 = strtok(NULL, " \r\n\t");
    	if(arg1) args++;						//to see if arguments being tracked
    	arg2 = strtok(NULL, " \r\n\t");
    	if(arg2) args++;						//to see if arguments being tracked
    
    	if(com != NULL)			//Can only execute commands if a command is entered
    							//Otherwise it keeps looping and printing prompt
    	{
    	printf("command entered: %s\n", com);
    	printf("There are %d arguments\n", args);
    
    	if ((strcmp(com,"help")) == 0 || (strcmp(com,"Help")) == 0)
    	{
    		printf("***************************************************\n");
    		printf("\nO-Shell Help Menu:\n");
    		printf("\nhelp - Print help menu\n");
    		printf("\nexit - Exit O-Shell\n");
    		printf("\ntype <file> - Print contents of <file> to terminal\n");
    		printf("\ncopy <file1> <file2> - Copy contents of <file1> to <file2>.");
    		printf("\n     <file2> must be a non-existent file and will be created\n");
    		printf("\ndelete <file> - Delete <file> from system\n");
    		printf("***************************************************\n");
    	}
    
    	else if ((strcmp(com,"exit")) == 0 || (strcmp(com,"Exit")) == 0)
    	{
    		printf("Exiting O-Shell...Goodbye!\n");
    
    		//if exit command is entered, exit() system call is performed
    		exit(1);
    	}
    
    	else if ((strcmp(com,"type")) == 0 || (strcmp(com,"Type")) == 0)
    	{
    		if((arg1 == NULL) || (arg2 != NULL))		//arg1 must exist, arg2 must be empty
    		{
    			printf("Incorrect number of arguments! Type 'help' for correct format.\n");
    		}
    
    		else
    		printf("Printing contents of file: %s\n", arg1);
    		printf("***************************************************\n");
    
    		typefile(arg1);			//call typefile function (see below)
    	}
    
    	else
    	printf("Invalid command!!! Type 'help' for list of valid commands.\n");
    
    	}
    
    	}
    
    	return 0;
    }
    
    /************************************************************************************************
    Function typefile() prints contents of a file to the terminal
    Format: type <file1>
    ************************************************************************************************/
    
    void typefile(char * filename)
    {
    	int pid, status;
    
    	const char cat[] = "/bin/cat";
    
    	pid = fork();
    
    	if(pid < 0)
    	printf("ERROR: Child pid = %i FORK FAILED! Please try again.\n", pid);
    
    	else if(pid > 0)
    	printf("ERROR: Child pid = %i Parent process executing, FORK FAILED!\n", pid);
    
    	else if(pid == 0)				//If the process id is zero, fork() was successful
    	{
    		printf("Child pid = %i FORK SUCCESSFUL...\n");
    
    		execvp(cat, filename);
    	}
    
    }
     
  2. DaWei

    DaWei New Member

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