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From: Eric Cook <llua@gmx.com>
To: zsh-users@zsh.org
Subject: Re: CONDITIONAL EXPRESSIONS, string == pattern
Date: Sat, 24 Jul 2021 23:30:26 -0400	[thread overview]
Message-ID: <45ef93bd-148b-54fc-9815-5b5dbe71d1f0@gmx.com> (raw)
In-Reply-To: <ae9325b1-1a8a-1384-3bd0-2286348843e@ckhb.org>

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.

        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

[[ 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'

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

  reply	other threads:[~2021-07-25  3:31 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-25  2:14 scowles
2021-07-25  3:30 ` Eric Cook [this message]
2021-07-25  3:48   ` Lawrence Velázquez

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