0

In 

#include <unistd.h>
int execve(const char * pathname , char *const  argv [], char *const  envp []);

argv [0] of execve corresponds to argv[0] in the main function of the new program, and is thus typically the same as the basename component of the pathname argument. Is it a requirement that we should always follow?

When a program file can be invoked by different values of argv[0], does that rule/convention mean that when I invoke a program file and want argv[0] of its main() to have a specific value, I have to create a symlink to the program file named after the value of argv[0], and invoke the program by the symlink? Or not necessarily? For example, this answer to “Why does argv include the program name?” says

Bash runs in POSIX mode when argv[0] is sh. It runs as a login shell when argv[0] begins with -.

Does that mean that

  • to invoke bash in POSIX mode, I have to create a symlink /path/to/sh to /bin/bash and invoke bash by execve("/path/to/sh", argv, ...) with argv[0] pointing to string "sh"? Is this way preferrable over execve("/bin/bash", argv, ...) where argv[1] points to string "--posix" and argv[0] doesn't matter?

  • to invoke bash as a login shell, I have to create a symlink /path/to/- to /bin/bash and invoke bash by execve("/path/to/-", argv, ...) with argv[0] pointing to string "-"? Is this way preferable over execve("/bin/bash", argv, ...) where argv[1] points to string "--login" and argv[0] doesn't matter?

When is it useful that argv [0] of execve is not the same as the basename component of the pathname argument?

Stephen Kitt
  • 434,908
Tim
  • 101,790
  • What question are you getting at here? Besides the title, you have eight questions in the Question. You don't invoke bash from the command-line via execve(), so I don't know which way you're going with this question. – Jeff Schaller Nov 28 '18 at 18:59
  • Number of question marks doesn't matter. To entertain you, I can end every sentence with a question mark. If I know my answer clearly, I wouldn't post my confusion. If my confusion confuses you, you don't understand that I am asking questions. – Tim Nov 28 '18 at 19:13
  • Just my opinion, but your Question diverges between using a system call execve() and using symlinks to invoke bash. Maybe I'm not smart enough to understand where you're going. – Jeff Schaller Nov 28 '18 at 19:22
  • It is more about acceptance than smartness. – Tim Nov 28 '18 at 19:23
  • Questions should be answerable, because you want Answers. Are you actually concerned about the system call, or is that just an inherent part of your actual question about invoking programs in different ways? – Jeff Schaller Nov 28 '18 at 19:33
  • I voted as "too broad" because "your question ... has many valid answers (but no way to determine which - if any - are correct)," (from https://unix.stackexchange.com/help/closed-questions in the help center) – Jeff Schaller Nov 28 '18 at 19:42
  • 1
    Jeff’s not being rude. After all the time you’ve been using the site, it seems reasonable to expect that you would have understood by now that it works best when a question is a single question, and not a number of more-or-less related questions. Especially in this case since at least some of your questions can be answered from the linked question. – Stephen Kitt Nov 28 '18 at 21:21

1 Answers1

3

The accepted answer to the question you linked to says

To begin with, note that argv[0] is not necessarily the program name.

which addresses your main question.

Your various questions about starting a program seem to stem from a misunderstanding. You don’t need to use links (symbolic or hard) to start a program with argv[0] set to a value other than its name; you need to be able to control the array given to execve. This is what login does when it starts the shell: it prefixes it with - in argv[0].

Bash starts in POSIX mode when its argv[0] is sh, so that it can be used as /bin/sh. If that required using --posix, you’d need to change a lot of shell scripts.

The same reasoning applies to using - for a login shell: this convention works with any shell, whether it supports a --login option or not.

The login use-case is an example of when it’s useful for argv[0] to not be equal to the program’s name.

Stephen Kitt
  • 434,908