C中的Shell程序具有奇怪的fork行为

Question

I am writing a C program to emulate a simple shell. This shell will basically evaluate commands like any other shell (ls, cat, etc.), as well as handle pipelining and redirection.

Currently, I am trying to start out by getting user input, tokenizing it, and executing the command provided (eg executing only "ls" and not "ls -l"). However, I am having a lot of difficulty with the forking. It seems that every time I fork, something goes wrong and hundreds of identical processes are created, leading to my computer freezing and me having to restart. The code appears to be correct, but I have no idea what is causing this behaviour. Below is the relevant portion of my code (main method and input tokenizer method).

int main() {
    char inputLine[512];    //user input
    char *args[10];         //arguments
    char* pathVar = "/bin/";//path for argument
    char programPath[512];  //pathVar + args[0]
    int n;                  //count variable

    //loop
    while (1) {
        //print prompt, get input
        printf("input> ");
        fgets(inputLine, 512, stdin);
        n = tokenizer(inputLine, args);

        //fork process
        pid_t pid = fork();

        if (pid != 0) {     //if parent
            wait(NULL);
        } else {            //if child
            //format input for execution
            strcpy(programPath, pathVar);
            strcat(programPath, args[0]);

            //execute user command
            int returnVal = execv(programPath, args);
        }
    }

    return 0;
}

int tokenizer(char *input, char *args[]) {
    char *line;             //current line
    int i = 0;              //count variable

    line = input;
    args[i] = strtok(line, " ");
    do {
        i++;
        line = NULL;
        args[i] = strtok(line, " ");
    } while (args[i] != NULL);

    return i;
}

Answer 1

Putting it all together: You need to check fork and execv for failure. You should exit after an execv failure (and perhaps after a fork failure). And you need to add \\n to the strtok delimiters (or remove the newline from the input line in some other way).

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define MAXARGS 10
#define PATH "/bin/"

int main() {
    char inputLine[BUFSIZ];
    char *args[MAXARGS];
    char programPath[BUFSIZ + sizeof(PATH) + 10];

    while (1) {
        printf(":-> ");
        if (fgets(inputLine, BUFSIZ, stdin) == NULL) /* ctrl-D entered */
            break;

        tokenize(inputLine, args);

        pid_t pid = fork();
        if (pid == -1) {
            perror("fork");
            exit(EXIT_FAILURE);
        }

        if (pid != 0) {  /* parent */
            wait(NULL);

        } else {         /* child */
            strcpy(programPath, PATH);
            strcat(programPath, args[0]);

            execv(programPath, args); /* will not return unless it fails */

            perror("execv");
            exit(EXIT_FAILURE);
        }
    }

    return 0;
}

int tokenize(char *input, char *args[]) {
    int i = 0;

    args[0] = strtok(input, " \n");
    for (i = 0; args[i] && i < MAXARGS-1; ++i)
        args[++i] = strtok(NULL, " \n");

    return i;
}

Answer 2

You should check that execv doesn't fail and also be sure to exit() at the end of the child block.

            //execute user command
            int returnVal = execv(programPath, args);

            // check return from execv
            if (returnVal < 0) {
                perror("execv");
                exit(1);
            }

Also, beware using functions like strcpy in this context since they may lead to buffer overflows. If an untrusted attacker type is talking to your shell this type of security issue could let them break out of the "sandbox".