Question

I am starting a project of writing a simplified shell for linux in C. I am not at all proficient with C nor with Linux that’s exactly the reason I decided it would be a good idea.

Starting with the parser, I have already encountered some problems.

The code should be straightforward that’s why I didn’t include any comments.

I am getting a warning with gcc: "comparison with string literals results in unspecified behaviour" at the lines commented with "WARNING HERE" (see code below).

I have no idea why this causes an warning, but the real problem is that even though I am comparing an "<" to an "<" is doesn’t get inside the if…

I am looking for an answer for the problem explained, however if there’s something that you see in the code that should be improved please say so. Just take in mind I am not that proficient and that this is still a work in progress (or better yet, a work in start).

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

typedef enum {false, true} bool;

typedef struct {
    char **arg;
    char *infile;
    char *outfile;
    int background;
} Command_Info;

int parse_cmd(char *cmd_line, Command_Info *cmd_info)
{
    char *arg;
    char *args[100];    
    
    int i = 0;
    arg = strtok(cmd_line, " \n");
    while (arg != NULL) {
        args[i] = arg;
        arg = strtok(NULL, " \n");
        i++;
    }
    
    int num_elems = i;
    
    cmd_info->infile = NULL;
    cmd_info->outfile = NULL;
    cmd_info->background = 0;
    
    int iarg = 0;
    for (i = 0; i < num_elems; i++)
    {
        if (args[i] == "&") //WARNING HERE
            return -1;      
        else if (args[i] == "<") //WARNING HERE
            if (args[i+1] != NULL)
                cmd_info->infile = args[i+1];
            else
                return -1;
                        
        else if (args[i] == ">") //WARNING HERE
            if (args[i+1] != NULL)
                cmd_info->outfile = args[i+1];
            else
                return -1;          

        else 
            cmd_info->arg[iarg++] = args[i];
    }
    
    cmd_info->arg[iarg] = NULL;

    return 0;   
}

void print_cmd(Command_Info *cmd_info)
{
    int i;  
    for (i = 0; cmd_info->arg[i] != NULL; i++)
        printf("arg[%d]=\"%s\"\n", i, cmd_info->arg[i]);
    printf("arg[%d]=\"%s\"\n", i, cmd_info->arg[i]);    
    printf("infile=\"%s\"\n", cmd_info->infile);
    printf("outfile=\"%s\"\n", cmd_info->outfile);
    printf("background=\"%d\"\n", cmd_info->background);
}

int main(int argc, char* argv[])
{
    char cmd_line[100];
    Command_Info cmd_info;

    printf(">>> ");
    
    fgets(cmd_line, 100, stdin);

    parse_cmd(cmd_line, &cmd_info);
    
    print_cmd(&cmd_info);
    
    return 0;
}

Answer 1

You want to use strcmp() == 0 to compare strings instead of a simple ==, which will just compare if the pointers are the same (which they won’t be in this case).

args[i] is a pointer to a string (a pointer to an array of chars null terminated), as is "&" or "<".

The expression argc[i] == "&" checks if the two pointers are the same (point to the same memory location).

The expression strcmp( argc[i], "&") == 0 will check if the contents of the two strings are the same.