Why does popen() invoke a shell to execute a process? - c

Question

I'm currently reading up on and experimenting with the different possibilities of running programs from within C code on Linux. My use cases cover all possible scenarios, from simply running and forgetting about a process, reading from or writing to the process, to reading from and writing to it.
For the first two, popen() is very easy to use and works well. I understand that it uses some version of fork() and exec() internally, then invokes a shell to actually run the command.
For the third scenario, popen() is not an option, as it is unidirectional. Available options are:
Manually fork() and exec(), plus pipe() and dup2() for input/output
posix_spawn(), which internally uses the above as need be
What I noticed is that these can achieve the same that popen() does, but we can completely avoid the invoking of an additional sh. This sounds desirable, as it seems less complex.
However, I noticed that even examples on posix_spawn() that I found on the Internet do invoke a shell, so it would seem there must be a benefit to it. If it is about parsing command line arguments, wordexp() seems to do an equally good job.
What is the reason behind benefit of invoking a shell to run the desired process instead of running it directly?
Edit: I realized that my wording of the question didn't precisely reflect my actual interest - I was more curious about the benefits of going through sh rather than the (historical) reason, though both are obviously connected, so answers for both variations are equally relevant.

Answer 1

Invoking a shell allows you to do all the things that you can do in a shell.
For example,
FILE *fp = popen("ls *", "r");
is possible with popen() (expands all files in the current directory).
Compare it with:
execvp("/bin/ls", (char *[]){"/bin/ls", "*", NULL});
You can't exec ls with * as argument because exec(2) will interpret * literally.
Similarly, pipes (|), redirection (>, <, ...), etc., are possible with popen.
Otherwise, there's no reason to use popen if you don't need shell - it's unnecessary. You'll end up with an extra shell process and all the things that can go wrong in a shell go can wrong in your program (e.g., the command you pass could be incorrectly interpreted by the shell and a common security issue). popen() is designed that way. fork + exec solution is cleaner without the issues associated with a shell.

Answer 2

The glib answer is because the The POSIX standard ( http://pubs.opengroup.org/onlinepubs/9699919799/functions/popen.html ) says so. Or rather, it says that it should behave as if the command argument is passed to /bin/sh for interpretation.
So I suppose a conforming implementation could, in principle, also have some internal library function that would interpret shell commands without having to fork and exec a separate shell process. I'm not actually aware of any such implementation, and I suspect getting all the corner cases correct would be pretty tricky.

Answer 3

The 2004 version of the POSIX system() documentation has a rationale that is likely applicable to popen() as well. Note the stated restrictions on system(), especially the one stating "that the process ID is different":
RATIONALE
...
There are three levels of specification for the system() function. The
ISO C standard gives the most basic. It requires that the function
exists, and defines a way for an application to query whether a
command language interpreter exists. It says nothing about the command
language or the environment in which the command is interpreted.
IEEE Std 1003.1-2001 places additional restrictions on system(). It
requires that if there is a command language interpreter, the
environment must be as specified by fork() and exec. This ensures, for
example, that close-on- exec works, that file locks are not inherited,
and that the process ID is different. It also specifies the return
value from system() when the command line can be run, thus giving the
application some information about the command's completion status.
Finally, IEEE Std 1003.1-2001 requires the command to be interpreted
as in the shell command language defined in the Shell and Utilities
volume of IEEE Std 1003.1-2001.
Note the multiple references to the "ISO C Standard". The latest version of the C standard requires that the command string be processed by the system's "command processor":
7.22.4.8 The system function
Synopsis
#include <stdlib.h>
int system(const char *string);
Description
If string is a null pointer, the system function determines
whether the host environment has a command processor. If string
is not a null pointer, the system function passes the string
pointed to by string to that command processor to be executed
in a manner which the implementation shall document; this might then
cause the program calling system to behave in a non-conforming
manner or to terminate.
Returns
If the argument is a null pointer, the system function
returns nonzero only if a command processor is available. If
the argument is not a null pointer, and the system function
does return, it returns an implementation-defined value.
Since the C standard requires that the systems "command processor" be used for the system() call, I suspect that:
Somewhere there's a requirement in POSIX that ties popen() to the system() implementation.
It's much easier to just reuse the "command processor" entirely since there's also a requirement to run as a separate process.
So this is the glib answer twice-removed.