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Re: Incorrect alias expansion within command substitution

From: Alex fxmbsw7 Ratchev
Subject: Re: Incorrect alias expansion within command substitution
Date: Sat, 5 Feb 2022 21:39:30 +0100

On Sat, Feb 5, 2022 at 7:55 PM Chet Ramey <chet.ramey@case.edu> wrote:
> On 2/4/22 6:17 PM, Alex fxmbsw7 Ratchev wrote:
> > what about this viewing point
> > aliases can start, $(('s, but not end... this is unlogic
> Well, I don't know about `unlogic' but there's an understanding deficit,
> for sure.
> >
> > alias -- \
> > p='printf %s\\n ' \
> > assign='assign=$(( ' begin='$(( ' \
> >
> > for data in "1 + 2"
> > do
> > alias -- data="$d "
> > p begin data ))
> > done
> >
> > this works and results 3
> OK, let's go through it. I'll gloss over some non-essential details and
> simplify others. I'll skip over the bulk of the for loop parsing and just
> cover the simple command where alias expansion takes place.
> Since `p' is in a command position, it gets alias expanded to
> `printf %s\\n '. The lexical analysis restarts, the two words get scanned,
> and we start a simple command. The expansion ends with a space, so the next
> token (`begin') undergoes alias expansion, gets expanded to `$(( ', and the
> lex restarts. We read `$((' and know we're reading the start of a word that
> begins with an arithmetic expansion. We keep reading as if we are reading a
> double-quoted string, looking for `))', since that's what you do while
> reading an arithmetic expansion.

so the alias expansion fails at the current command is no alias
anymore, so alias parsing rules dont apply anymore, .. no ... hmm .. ?
would make much sense then and i have nothing else to say..

btw the printf leet were examples about the other, not to work, not to
say i tried to code so

> There is no alias expansion because we are
> not reading a new token and the contents of the arithmetic expansion are
> treated like a double-quoted string, both disqualifying conditions. We find
> those two characters, closing the arithmetic expansion, and go on looking
> for the end of the word. We read a newline, which delimits the token, which
> is the (shell grammar) WORD `$((  data ))'.
> After we finish parsing the for loop, we go back to execute it. We run
> a useless `alias' command, then come to the simple command
> printf %s\\n $((data))
> (I removed the spaces from the arithmetic expansion to make it clear that
> it's one word.)
> We begin to expand the words in this simple command. When we come to the
> arithmetic expansion, we take the contents (`data') and run them through
> the arithmetic evaluator. We get an identifier (`data'), assume it's a
> shell variable, look up its value (`1 + 2', since that's how the for loop
> set it) and evaluate that as an expression. It evaluates to 3, and the
> string "3" becomes the result of the arithmetic expansion.
> We run
> printf %s\n 3
> and get `3' as output.
> Understand?

yes, now, fail at no more alias so no alias expanded

cheers, thank you much for your kind good effords

> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>                  ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/

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