[TUHS] redirection wildness in v7

Will Senn will.senn at gmail.com
Fri Nov 10 01:42:25 AEST 2017

On 11/09/2017 09:30 AM, Dan Cross wrote:
> So...what's that unnamed process? To figure out what's going on, we'll
> have to look at the source for the shell itself. In this case, the
> answer to the mystery lies in /usr/src/cmd/sh/xec.c, in the (extremely
> long) function `execute`. This is the heart of the shell's
> interpreter. Basically, the string entered by the user has been parsed
> into an in-memory data structure (essentially an abstract syntax
> tree), and this function is responsible for interpreting that data
> structure and invoking the entered commands. It is called recursively;
> you'll note that the bulk of it is a switch statement on the token
> type for the current node in the syntax tree.
> The magic here is in the case for TFIL, which sets up a pipe between
> the left-hand side of tree and the right-hand side.
> In this case, the left-hand-side is an empty command; basically,
> equivalent to typing return at the shell prompt. Note that that just
> happens to be perfectly syntactically valid, so the parser doesn't
> generate an error. We may reasonably assume that this results in a
> left-hand child of the pipe node in the AST that is set with TFORK and
> an empty command vector: indeed, we see this in the `term` function in
> cmd.c.
> In the context of a pipeline, a new copy of the shell *is* forked for
> this, and if we follow the TFORK logic, we can see that, in the
> handling of the child, we do things like setting up pipes and I/O
> redirection and then either execute a builtin command, or *if the
> command is not empty* we execute the command via `execa`. This is not
> a builtin command, but since this command is empty, we don't do
> anything in the child other than exit.
> Thus, the empty command produces no output, despite having its input
> redirected from a file and therefore nothing is put into the write-end
> of the pipe and sed sees nothing on the read-end (other than EOF).

This is a great answer. I appreciate the detail and the source pointers 
(although that code is pretty complicated). My mental model is 
significantly enhanced, if not totally accurate, by your description. 
I've seen some other constructs along these same lines and they make 
more sense now - such as:

 > myfile

to create a file.

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