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From: Ralf Fassel <ralfixx@gmx.de>
Newsgroups: comp.os.linux.misc
Subject: Re: correct quoting for variables in bash scripts?
Date: Tue, 18 Oct 2022 11:31:11 +0200
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* Eli the Bearded <*@eli.users.panix.com>
| In your example neither is needed. Firstly there's nothing adjoining to
| confuse the variable name. Secondly, it is [[ not [.
>
| The one [ ] square bracket form of the test command needs empty string
| protection because variable expansion happens before thecommand is run. 
>
| The two [[ ]] square bracket form of the test command is explicitly a
| shell built-in, so it can recognize empty strings without help.

Interesting, didn't know that difference:

    $ unset bar
    $ [[ $bar = x ]] && echo yes
    $ type [[
    [[ is a shell keyword
    
    $ [ $bar = x ] && echo yes
    bash: [: =: unary operator expected
    $ type [
    [ is a shell builtin

Most probably the error thrown with "[" is to ensure compatibility with
/usr/bin/test?

R'