Check if program is being piped

Question

I'm writing a program that speeds up the git push operation. Here's what it needs to do:

printf("[github-username]\n[github-password]");

I'd then use it like so:

git-autologin | git push

But I don't want there to be any chance of someone simply typing git-autologin or git-autologin > file.txt and being able to see my username and password. I know it sounds silly 'cause anyone can still automate the git commands but it's unlikely that any untrusted user will ever get on my system.

My question: How could I tell if standard output is writing to a file/terminal or if it's being piped?
Edit: The git push pipe was simply an idea, not the only reason I'm asking.
Edit: Is there any way to determine the PID or more information about the process it's being piped to?

BTW if the 'git' tag needs to be removed please feel free to remove it, or comment. — MD XF, Oct 30 '16 at 02:04
For this particular solution you may want to read up on [git credentials helpers](https://git-scm.com/docs/gitcredentials). Actually, you would better off just using an ssh key for authentication, rather than a username/password. — larsks, Oct 30 '16 at 02:04
@larsks Thanks, I'll look into that but this isn't the only reason I'm asking, just the most explainable one — MD XF, Oct 30 '16 at 02:06
[This](http://stackoverflow.com/questions/911168/how-to-detect-if-my-shell-script-is-running-through-a-pipe) isn't exactly a duplicate, because it's shell rather than C, but I think it provides the general answer (which is, you can't really do what you want). — larsks, Oct 30 '16 at 02:08
You can't seek on a pipe or terminal; you can on files. You can tell if a device is a terminal by looking at the type information (`stat()` on the output file descriptor and `S_ISCHR` macro, or maybe check for `S_ISREG`); if it looks looks like a regular file, (`S_ISREG`), but you can't seek on it (`errno == ESPIPE` after an attempt to seek), it is probably a pipe. You can't tell if the command on the receiving end is `cat` very easily, though. — Jonathan Leffler, Oct 30 '16 at 02:21
This is silly. All someone has to do is run `strings git-autologin` and they'll see your credentials. — Barmar, Oct 30 '16 at 02:36
The literal answer is the `isatty` function (this is what libc uses internally to choose stdout buffering), but as already noted, `git-autologin | cat` makes this a bad idea. — torek, Oct 30 '16 at 02:46

Basile Starynkevitch · Accepted Answer · 2016-10-31T16:35:48.267

You can detect when stdout is a terminal by using (on Linux & POSIX!) the isatty(3) function, probably as isatty(STDOUT_FILENO)

So you could guess that if isatty(STDOUT_FILENO) is false, the standard output is would be redirected or piped.

Alternatively, use fstat(2) as fstat(STDOUT_FILENO, &stdoutstat);

But you should set up ssh correctly (with credentials, STFW for any SSH tutorial!) on your system, to avoid having git asking any password.

Check if program is being piped

1 Answers1