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edited May 21, 2016 at 4:20

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There are alot of documents and discussion around the file descriptors of shells and how those file descriptors can be manipulated for redirection etc.
My question is, when a shell command is an external program, like rsync, cat, curl etc, do those commands have file descriptors in the same way the shell does?
To give a concrete example, if I run this command at a terminal prompt

% cat << EOF | php <?php echo "hello". PHP_EOL; echo "another line" . PHP_EOL; EOF

I'm assuming the contents of this heredoc are being fed to STDIN, but of which process? the shell, or does cat have a file descriptor 0 and the contents of the heredoc are being fed to cat's fd0 directly?

Even the Wikipedia file descriptor article doesn't seem to explain it.

There are alot of documents and discussion around the file descriptors of shells and how those file descriptors can be manipulated for redirection etc.
My question is, when a shell command is an external program, like rsync, cat, curl etc, do those commands have file descriptors in the same way the shell does?
To give a concrete example, if I run this command at a terminal prompt

% cat << EOF | php <?php echo "hello". PHP_EOL; echo "another line" . PHP_EOL; EOF

I'm assuming the contents of this heredoc are being fed to STDIN, but of which process? the shell, or does cat have a file descriptor 0 and the contents of the heredoc are being fed to cat's fd0 directly?

Even the Wikipedia file descriptor article doesn't seem to explain it.

There are alot of documents and discussion around the file descriptors of shells and how those file descriptors can be manipulated for redirection etc.
My question is, when a shell command is an external program, like rsync, cat, curl etc, do those commands have file descriptors in the same way the shell does?
To give a concrete example, if I run this command at a terminal prompt

% cat << EOF | php <?php echo "hello". PHP_EOL; echo "another line" . PHP_EOL; EOF

I'm assuming the contents of this heredoc are being fed to STDIN, but of which process? the shell, or does cat have a file descriptor 0 and the contents of the heredoc are being fed to cat's fd0 directly?

asked May 21, 2016 at 2:51

12.8k
17
74
115

When a shell executes an external command/program - does that program have file descriptors?

There are alot of documents and discussion around the file descriptors of shells and how those file descriptors can be manipulated for redirection etc.
My question is, when a shell command is an external program, like rsync, cat, curl etc, do those commands have file descriptors in the same way the shell does?
To give a concrete example, if I run this command at a terminal prompt

% cat << EOF | php <?php echo "hello". PHP_EOL; echo "another line" . PHP_EOL; EOF

I'm assuming the contents of this heredoc are being fed to STDIN, but of which process? the shell, or does cat have a file descriptor 0 and the contents of the heredoc are being fed to cat's fd0 directly?

Even the Wikipedia file descriptor article doesn't seem to explain it.