Question

I’m currently programming a shell in C and I’m running into a few issues. When I try to compare my command to an “exit” for example, it just runs write over it and acts like they don’t equal according to gdb. I end with a segfault. If anyone could help me figure out what’s wrong, I’d greatly appreciate it. This is my first shell ever btw!

#include <stdio.h>
#include <string.h>
#include <strings.h>
#include <limits.h>
#include <unistd.h>
#include <stdlib.h>
#include <pwd.h>
#include <dirent.h>e
#include <sys/types.h>
#include <sys/wait.h>    
#include <signal.h>
#include "sh.h"

int sh( int argc, char **argv, char **envp ){

    char *prompt = calloc(PROMPTMAX, sizeof(char));
    char *commandline = calloc(MAX_CANON, sizeof(char));
    char *command, *arg, *commandpath, *p, *pwd, *owd;
    char **args = calloc(MAXARGS, sizeof(char*));
    int uid, i, status, argsct, go = 1;
    struct passwd *password_entry;
    char *homedir;
    struct pathelement *pathlist;

    uid = getuid();
    password_entry = getpwuid(uid);
    homedir = password_entry->pw_dir; 

    if ( (pwd = getcwd(NULL, PATH_MAX+1)) == NULL ){
    perror("getcwd");
    exit(2);
    }

    owd = calloc(strlen(pwd) + 1, sizeof(char));
    memcpy(owd, pwd, strlen(pwd));
    prompt[0] = ' '; prompt[1] = '\0';

    pathlist = get_path();

    prompt = "[cwd]>";

    while ( go ){
    printf(prompt);

    commandline = fgets(commandline, 100, stdin);
    command = strtok(commandline, " ");

    printf(command);

    if (strcmp(command, "exit")==0){
        exit(0);
    }

    else if (strcmp(command, "which")==0){
    //  which();
    }

    else if (strcmp(command, "where")==0){
    //  where();
    }

    else if (strcmp(command, "cd")==0){
        chdir(argv[0]);
    }

    else if (strcmp(command, "pwd")==0){
        getcwd(pwd, PATH_MAX);
    }

    else if (strcmp(command, "list")==0){
        if (argc == 1){

        }

        else if (argc > 1){

        }
    }

    else if (strcmp(command, "pid")==0){
        getpid();
    }

    else if (strcmp(command, "kill")==0){

    }

    else if (strcmp(command, "prompt")==0){
        prompt = "argv[0] + prompt";
    }

    else if (strcmp(command, "printenv")==0){

    }

    else if (strcmp(command, "alias")==0){

    }

    else if (strcmp(command, "history")==0){

    }   

    else if (strcmp(command, "setenv")==0){

    }

    else{
        fprintf(stderr, "%s: Command not found.\n", args[0]);
    }



}
return 0;

} 

Most of it is still bare bones so bear with me.

Answer 1

If you change:

printf(command);

into:

printf("<<%s>>\n",command);

you’ll see why it will never match any of those strings. That’s because fgets does not strip off the trailing newline ^(a):

[cwd]>ls
<<ls
>>

Which means it will execute this line of code:

fprintf(stderr, "%s: Command not found.\n", args[0]);

And, since you’ve initialised all those args[] values to NULL with your calloc, BANG! goes your code (trying to dereference a null pointer is undefined behaviour).

Go and have a look at this answer for a robust user input solution with prompting, buffer overflow protection, ignoring the remainder of too-long-lines and, most importantly here, stripping off the newline.

---

^(a) As an aside, you shouldn’t be passing user input to printf as the format string anyway. Guess what happens if I enter %s%s%s%s%s%s at your prompt 🙂

And, unrelated to your problem, ISO C mandates that sizeof(char) is always 1, so you don’t need to use it in your allocation statements – I find it just clogs up the code unnecessarily.