Using strtok to run multiple commands - c

Question

I'm trying to create a shell using C that can take multiple commands separated by a semicolon(;). Currently I'm trying to use strtok to separate the commands but I don't think I'm using it correctly. I'll post all the info I can without posting the entire code. Is strtok being used correctly?
char *semi=";";
else
{
char *token=strtok(str,semi);
if(token != NULL)
{
token=strtok(NULL,semi);
if((childpid = fork()) == 0)
{
if ((execvp(args[0], args))<0)//prints error message when unknown command is used
{
printf("Error! Command not recognized.\n");
}
execvp(args[0],args);
free(args);//deallocate args
exit(0);
}
Edit: As per instructed I removed a large chunk of the code originally posted to focus solely on the use of strtok. When compiled the makeshift shell will accept one command at a time. I'm trying to use ";" to separate and run two commands simultaneously. Am I using strtok correctly? If not, is there an alternative?

Answer 1

You should always check, if strtok() returns NULL. I would change the structure as follows:
char* semi = ";"; // Your semikolon
char *token = NULL; // Your token string
// ...
// Split first occour of semicolon
token = strtok(str,semi);
if(token == NULL){
perror("No command given ...");
return NULL;
}
do {
// Execute your code here
// fork() etc.
// You should get each line (each semikolon seperated string)
// and it should be stored into token
} while((token = strtok(NULL, semi) != NULL);
I hope, I did understand your problem right ...
But as I can see, you need to split the token again by spaces to get them into a char-Array for the argv[] (second parameter) of execvp(). Here the problem is, that strtok() internally uses a static (?) variable to store the last position. So using another strtok() inside the loop would "destroy" your text.
You could do something like this:
char *str; // Your string ...
char semi[1] = ";"; // Your semikolon AND space; strtok() will split at both
char *token = NULL; // Your token string
int len = 0;
char *token2;
int argvpos = 0;
// ...
// Split first occour of semicolon
token = strtok(str,semi);
if(token == NULL){
perror("No command given ...");
return EXIT_FAILURE;
}
do {
// save length of token
len = strlen(token);
// Split for blanks to get the arguments
token2 = strtok(token," ");
// Build array of arguments
while(token2 != NULL){
args[argvpos++] = token2;
token2 = strtok(NULL," ");
}
// Do something with token (as command)
// and args (as arguments)
// ...
} while((token = strtok(token+len+1, semi) != NULL);
// In the while condition you add the length to the token; so you get the "old" last position
I think it is not a good solution, but it should work. And I hope, I did understand you problem ;-)
Kind regards.

Answer 2

In order to work correctly, strtok should be used along with a while loop. Also, you don't need to run execvp twice.
I created a small sample program using your code to demonstrate how you can correctly use your code:
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
int main()
{
char str[] = "ls -1; echo 'hello world'"; // Input commands separated by ';'
// Break the commands string into an array
char *commands[10]; // Array to hold a max of 10 commands
char *semi = ";";
char *token = strtok(str, semi);
int i = 0;
while (token != NULL)
{
commands[i] = token;
++i;
token = strtok(NULL, semi);
}
int numCommands = i; // numCommands is the max number of input commands
// Run each input command in a child process
i = 0;
while (i < numCommands)
{
printf("Command: %s\n", commands[i]);
// Tokenize the command so that it can be run using execvp
char *args[10] = {}; // Array to hold command args
args[0] = strtok(commands[i], " ");
int tokenCounter = 0;
while (args[tokenCounter] != NULL)
{
tokenCounter++;
args[tokenCounter] = strtok(NULL, " ");
}
// Create a child process
int childpid = fork();
// If this is child process, run the command
if (childpid == 0)
{
if ((execvp(args[0], args)) < 0)
{
printf("Error! Command not recognized.\n");
}
exit(0);
}
// If this is the parent, wait for the child to finish
else if (childpid > 0)
{
wait(&childpid);
}
// If the child process could not be created, print an error and exit
else
{
printf("Error: Could not create a child process.\n");
exit(1);
}
++i;
}
return 0;
}