Re: CONDITIONAL EXPRESSIONS, string == pattern

[Eric Cook <llua@gmx.com>]

On 7/24/21 10:14 PM, scowles@ckhb.org wrote:
> I would like to execute test on patterns that are dynamically generated and that change between invocations depending on other data.  When I generate a scalar parameter containing the pattern, construct the test statement, then invoke it, the test fails.
> However, if I force an eval on the test statement, the test succeeds.  I do not understand zsh parsing in this case.  Could someone please let me know the mechanics I am missing?
>
> thanks very much.
>
> zsh version:  5.8, patch 460
> platform:  x64, ubu 21.04, current patches
>
> # test code begin:
>
> unset vs vp r
> local -a r
>
> vs=' str1  a2'
> vp=
> vp+=[[:blank:]]##
> vp+=str1
> vp+=[[:blank:]]##
> vp+=[[:alnum:]]
> vp+=[[:alnum:]]
>
> r=( ${(f)"$( eval echo 'test ${vs} == ${vp} && echo hi || echo lo' )"} )
> echo ${r[-1]}
>
> test ${vs} == ${vp} && echo hi || echo lo
>
> # test code end.
>
> first test output:   hi
> second test output:  lo
>


Your code assumes EQUALS is disabled which allowed test == to not error with a command not found error.

so in:

eval echo 'test ${vs} == ${vp} && echo hi || echo lo'

zsh does the normal expansions of the line and passes the arguments to eval, since the single quotes prevented
any possible expansions the result is:

echo test ${vs} == ${vp} && echo hi || echo lo

so the above line goes through the steps of expansions and ends up like so:

echo test ' str1  a2' '==' '[[:blank:]]##str1[[:blank:]]##[[:alnum:]][[:alnum:]]' && echo hi || echo lo

so /echo/ prints out the strings given to it, which succeeds, causing echo hi to run.

Nothing related to pattern matching happened since test doesn't perform pattern matching.

CONDITIONAL EXPRESSIONS
        A conditional expression is used with the [[ compound command to test attributes of files and to  com‐
        pare  strings.   Each  expression can be constructed from one or more of the following unary or binary
        expressions

[[ does perform pattern matching on its right hand operand but when said pattern is inside a parameter you have
to perform an expansion that allows for it to be treated as a pattern since the results of a parameter expansion
are treated literal unlike other shells.

ultimately, the script boils down to:

setopt extendedglob
unset vs vp r

vs=' str1  a2'
vp=[[:blank:]]##str1[[:blank:]]##[[:alnum:]][[:alnum:]]

if [[ ${vs} == ${~vp} ]]; then
   echo hi
else
   echo lo
fi