C Test For File Existence Before Calling execvp

Question

I'm writing a UNIX minishell on ubuntu, and am trying to add built-in commands at this point. When it's not a built-in command I fork and then the child executes it, however for built-in commands I'll just execute it in the current process.

So, I need a way to see if the files exist(if they do it's not a built-in command), however execvp uses the environment PATH variable to automatically look for them, so I have no idea how I would manually check beforehand.

So, do you guys know how I could test an argument to see if it's a built-in command simply by supplying the name?

Thanks guys.

Answer 1

I have tested the answer by Tom

It contained a number of problems. I have fixed them here and provided a test program.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/stat.h>

int is_file(const char* path) {
    struct stat buf;
    stat(path, &buf);
    return S_ISREG(buf.st_mode);
}

/*
 * returns non-zero if the file is a file in the system path, and executable
 */
int is_executable_in_path(char *name)
{
    char *path = getenv("PATH");
    char *item = NULL;
    int found  = 0;

    if (!path) 
        return 0;
    path = strdup(path);

    char real_path[4096]; // or PATH_MAX or something smarter
    for (item = strtok(path, ":"); (!found) && item; item = strtok(NULL, ":"))
    {
        sprintf(real_path, "%s/%s", item, name);
        // printf("Testing %s\n", real_path);
        if ( is_file(real_path) && !(
               access(real_path, F_OK) 
            || access(real_path, X_OK))) // check if the file exists and is executable
        {
            found = 1;
        }
    }

    free(path);
    return found;
}

int main()
{
    if (is_executable_in_path("."))
        puts(". is executable");
    if (is_executable_in_path("echo"))
        puts("echo is executable");
}

Notes

1. the test for access return value was reversed
2. the second strtok call had the wrong delimiter
3. strtok changed the path argument. My sample uses a copy
4. there was nothing to guarantee a proper path separator char in the concatenated real_path
5. there was no check whether the matched file was actually a file (directories can be 'executable' too). This leads to strange things like . being recognized as an external binary

Answer 2

You can iterate yourself through the PATH directories, and for each entry in PATH (You will have to split PATH with :, probably using strtok) concatenate at the end of each path the name of the command called. When you have create this path, check if the file exists and if it is executable using access.

int is_built_in(char *path, char *name)
{
  char *item = strtok(path, ":");

  do {
    char real_path[4096] = strcat(item, name); // you would normally alloc exactly the size needed but lets stick to it for the sake of the example
    if (!access(real_path, F_OK) && !access(real_path, X_OK)) // check if the file exists and is executable
      return 0;
  } while ((item = strtok(NULL, ":")) != NULL);
  return 1;
}

Answer 3

What you can do is you can change the path to the particular directory and then use #include<dirent.h> header file and its readdir and scandir functions to walk through the directory or stat structure to see if the file exists in the directory or not.

Answer 4

Why do you want to test before calling execvp? That's the wrong approach. Just call execvp and it will tell you if the program does not exist.