double quote of variable in test or [ ] or [[ ]]

Hello, I am trying to follow on some bash scripting tutorial and realized that we need to be careful on using a variable with or without double quote in test/[]/[[]] operator. But I am getting some confusion when and where do we use that. Could someone please help me to understand it with example. Thanks!
beer9Asked:
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TintinConnect With a Mentor Commented:
Hopefully the following example makes it clearer

#!/bin/bash
foo="some value"
[ $foo = "some value" ] && echo test1
[ "$foo" = "some value" ] && echo test2
[[ $foo = "some value" ]] && echo test3

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Output is:


test.sh: line 3: [: too many arguments
test2
test3

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Short answer is that you should use quotes around variables when you use [, but is not needed when you use [[
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farzanjConnect With a Mentor Commented:
The newer standard is to use [[ ]] in both Bash and Korn.  [ ] is for backward compatibility with Borne shell.

With [[ ]], you can use all the C++ like operator like ==, <=, etc.
With [[ ]], you can use the latest Bash regular expression matching as well.
All the new power of of Bash obtained by using [[ ]].  But some people still want to keep using [ ].
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