Re: Possible bug in bash

[Chet Ramey]

On 1/15/23 7:42 AM, anonymous4feedback@outlook.com wrote:
> For the follow script
> alias al=' '
> alias foo=bar
> al for foo in v
> do echo foo=$foo bar=$bar
> done
> bash (version 5.1.16) prints foo=v bar=, while all other shells I tested (dash, 
> ksh, zsh, and yash) all prints foo= bar=v.

That's strange. I get the same results you do for bash, but I tried the
same shells you did and got the same results as bash (for different
reasons, I suspect).

$ cat x4
shopt -s expand_aliases 2>/dev/null
alias al=' '
alias foo=bar
al for foo in v
do echo foo=$foo bar=$bar
done
$ ./bash ./x4
foo=v bar=
$ dash ./x4
foo=v bar=
$ ksh93 ./x4
foo=v bar=
$ yash ./x4
foo=v bar=
$ mksh ./x4
foo=v bar=
$ zsh ./x4
foo=v bar=
$ ./bash --version
GNU bash, version 5.2.15(54)-maint (x86_64-apple-darwin21.6.0)
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.


> Apparently bash substitutes foo for bar in line 3 because the previous alias al 
> ends with a space. But it is unintuitive that the word after for is checked for 
> alias.

This is the opposite of what happens. In default mode, which you're
probably using, bash checks `for' for aliases, and, finding none, stops
checking. The other shells probably *don't* check whether `for' has an
alias, because POSIX says reserved words can't be alias expanded ("However,
reserved words in correct grammatical context shall not be candidates for
alias substitution.") Either way, the `check the next word for alias
expansion' flag gets turned off.

> According to the posix standard,
> If the value of the alias replacing the word ends in a <blank>, the shell shall check 
> the next command word for alias substitution; this process shall continue until a word is 
> found that is not a valid alias or an alias value does not end in a <blank>.


> But “command word” is not defined. It is ambiguous whether “for” in this 
> context is a command word, or whether tokens other than command word is allowed 
> between the first alias and the next command word.

`Command word' refers to the previous paragraph: since alias expansion is
attempted on "a resulting word that is identified to be the command name
word of a simple command," the "next command word" is presumably the word
following that one.

> The same is true for case
> alias al=' '
> alias foo=bar
> al case foo in foo) echo foo;; bar) echo bar;; esac

Everybody prints `foo'.

--
``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/