How does execve run commands?-CodePudding

I attempted to use execve to run the command: ls -l /tmp, I'm familiar with the system call usage but I do not know how it runs commands.

I've tried this and i keep getting errors

char *argv[] = {"ls", "-l", NULL};

int exec;

exec = execve("/tmp", argv, NULL);
printf("%d\n", exec);
return (0);

CodePudding user response：

You don't use execve correctly. The first argument has to be the executable path.

So your code should be

int  main(void) {
    char *argv[] = {"ls", "-l", "/tmp", NULL};

    execve("/bin/ls", argv, NULL); 
    // Or  "/usr/bin/ls", depending where  
    //    is stored 'ls' on your system

    // On success theses lines are not reached
    perror("execve failed");

    return 0;
}

Note that (from man execve):

RETURN VALUE
On success, execve() does not return, on error -1 is returned, and errno is set to indicate the error.

If the execve call is a success, your program will be replaced by the launched one (ls here), so the two last lines won't be executed

CodePudding user response：

The first argument to execve is the path to the program to execute.^[1]

char *argv[] = { "ls", "-l", "/tmp", NULL };
execve( "/usr/bin/ls", argv, NULL );

Note that we had to provide the full path to the binary. To avoid this by searching the PATH when needed, we can use one of the "p" variants instead.

char *argv[] = { "ls", "-l", "/tmp", NULL };
execvpe( argv[0], argv, NULL );         # execvpe is a GNU extension.

char *argv[] = { "ls", "-l", "/tmp", NULL };
execvp( argv[0], argv );

Finally, the purpose of exec* is to change the program the current process executes. As such, it only returns on error, and it always returns -1 on error. If you wanted to print the error, you could used perror.

To set the path to use as the current work directory, use chdir before exec*.

CodePudding user response：

The first argument to execve must be the path to the program to execute:

exec = execve("/bin/ls", argv, NULL);

The second argument, argv, must be a list of all the arguments:

char *argv[] = {"ls", "-l", "/tmp", NULL};