UNIX simple shell in C, execve and parameters - c

Question

[...] Preprocesser directives
void read_command()
{
int i; //index to the arrays stored in parameter[]
char *cp; //points to the command[]
const char *hash = " "; //figures out the strings seperated by spaces
memset(command, 0, 100); //Clear the memory for array
parameter[0] = "/bn/"; //Initialize the path
//Get the user input and check if an input did occur
if(fgets(command, sizeof(command), stdin) == NULL)
{
printf("Exit!\n");
exit(0);
}
//Split the command and look store each string in parameter[]
cp = strtok(command, " "); //Get the initial string (the command)
strcat(parameter[0], cp); //Append the command after the path
for(i = 1; i < MAX_ARG; i++)
{
cp = strtok(NULL, " "); //Check for each string in the array
parameter[i] = cp; //Store the result string in an indexed off array
if(parameter[i] == NULL)
{
break;
cp = NULL;
}
}
//Exit the shell when the input is "exit"
if(strcmp(parameter[0], "exit") == 0)
{
printf("Exit!\n");
exit(0);
}
}
int main()
{
[...]
read_command();
env = NULL; //There is no environment variable
proc = fork();
if(proc == -1) //Check if forked properly
{
perror("Error");
exit(1);
}
if (proc == 0) //Child process
{
execve(parameter[0], parameter, env); //Execute the process
}
else //Parent process
{
waitpid(-1, &status, 0); //Wait for the child to be done
}
[...]
}
The basic idea of the code is to read the input command by the user (done in the read_command() function) (ex: ls -l). Then I divide the input string in little strings and store them in an array. The point is to store the command in parameter[0] (ex: ls) and the parameters in parameter[1,2,3 etc.] (ex: -l). However, I think I executing the execve() function incorrectly.

Answer 1

There are all types of issues with your code including the following (some of them are correctly pointed out by Jonathan Leffler):
"/bin/" is misspelled as "/bn/"
Since parameter[0] points to a string literal ("/bn/") in strcat(parameter[0], cp); you are trying to append to this string literal which is incorrect. You should allocate a buffer to hold the concatenated string instead.
Your tokenizing code doesn't handle the trailing newline in command properly.
env should point to a NULL-terminated array of strings.
In general, I think you should focus on implementing and testing parts of your code properly before integrating them in a larger program. If you tested the read_command before trying to pass its results to execve, you would notice that it doesn't work.