On Mon, Jun 2, 2014 at 4:44 PM, Chet Ramey <chet.ra...@case.edu> wrote:
> On 6/2/14, 8:21 AM, Greg Wooledge wrote:
> > On Fri, May 30, 2014 at 09:28:13PM -0500, Dan Douglas wrote:
> >> The problem is most people don't realize how "variables" are evaluated.
> >> Any time the shell needs to reference a variable, it takes a string
> >> like: "arr[$foo]" and, if there's an index, the string within the index
> >> gets processed for expansions. The arithmetic evaluator is no exception.
> >
> > I'm trying to understand this, but it's not clear to me yet.
> >
> > imadev:~$ x='$(date)'
> > imadev:~$ : $(($x))
> > bash: $(date): syntax error: operand expected (error token is "$(date)")
> >
> > That looks OK.
> >
> > imadev:~$ : $((a[$x]))
> > bash: Mon Jun 2 08:06:39 EDT 2014: syntax error in expression (error
> token is "Jun 2 08:06:39 EDT 2014")
> >
> > There's the code-injection problem that started the thread.
> >
> > imadev:~$ : ${a[$x]}
> > bash: $(date): syntax error: operand expected (error token is "$(date)")
> >
> > That also looks OK.
> >
> > Why is there no code injection in the last example? There is an index.
> > According to your paragraph, "... the string within the index gets
> > processed for expansions. The arithmetic evaluator is no exception."
>
> The arithmetic evaluator is, in fact, an exception. That, combined with
> the expansions that happen before the arithmetic evaluator gets hold of
> the expression -- and it is an expression -- leads to the difference.
>
> In the first case, the arithmetic evaluator sees `a[$(date)]' as the
> expression after parameter expansion is performed:
>
> "All tokens in the expression undergo parameter and variable expansion,
> command substitution, and quote removal. The result is treated as the
> arithmetic expression to be evaluated."
>
> Since that expression looks like a variable expansion, the following
> sentence in the description of arithmetic evaluation is applicable:
>
> "Within an expression, shell variables may also be referenced by name
> without using the parameter expansion syntax. A shell variable that
> is null or unset evaluates to 0 when referenced by name without using the
> parameter expansion syntax."
>
> The a[$(date)] is identified as an array index, so the $(date) is expanded
> like any other index, and evaluated as an expression.
>
> This is what lets you use things like 'x+1' and 'x[y+1]' in arithmetic
> expansions.
>
> The parameter expansion example (${a[$x]}) doesn't undergo that `extra'
> expansion. The index that ends up being passed to the evaluator is `$x',
> which is expanded to `$(date)'. That is treated as an expression and
> evaluation fails.
>
> Chet
> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
> ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, ITS, CWRU c...@case.edu
> http://cnswww.cns.cwru.edu/~chet/
>
Even if there is a perfectly good justification as to why this works, I
still think this is a terribly broken feature of the language.
The number of shell scripters, even experimented, that will realize that
$((a["$i"])) makes code injection possible is probably very close to 0
while the first thing an script kid would do to create trouble is to embed
$( ) in his strings.
