Re: What does `echo xxx 1>&2xxx` do?

[Koichi Murase]

2021年5月8日(土) 21:20 Peng Yu <pengyu.ut@gmail.com>:
> How the parsing is done?

If we literally interpret the manual, the distinction between
filenames and file descriptors is not processed at the parsing level.
The manual says

https://www.gnu.org/software/bash/manual/bash.html#Redirecting-Standard-Output-and-Standard-Error
> When using the second form, word may not expand to a number or
> ‘-’. If it does, other redirection operators apply (see
> Duplicating File Descriptors below) for compatibility reasons.

https://www.gnu.org/software/bash/manual/bash.html#Duplicating-File-Descriptors
> The operator
>
>       [n]>&word
>
> is used similarly to duplicate output file descriptors. If n is not
> specified, the standard output (file descriptor 1) is used. If the
> digits in word do not specify a file descriptor open for output, a
> redirection error occurs. If word evaluates to ‘-’, file
> descriptor n is closed. As a special case, if n is omitted, and word
> does not expand to one or more digits or ‘-’, the standard output
> and standard error are redirected as described previously.

So, "1>&2" and "1>&2xxx" are both parsed as "n>& word" and then the
behavior is switched based on the expanded result of "word".

> The shell grammar seems to be extremely context-sensitive.

Yes.

> So in "1>&2xxx", 2xxx is recognized as a word?

Yes.

> In "1>&2", "2" is also recognized as a word?

Yes.

> "1" is recognized as a word?

No. It is a part of the redirection construct of "n>&"

> What about "1 >&2"? "1" is recognized as a word as well?

Yes. But in that case, "1" is not a part of the redirection but a
separate argument.

> How does bash knows "1 >&2" and "1>&2" are actually different?

If there is no space, it is treated as a part of the redirection.