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[Help-bash] Assigning builtins behavior

From: Quentin L'Hours
Subject: [Help-bash] Assigning builtins behavior
Date: Sun, 7 Jan 2018 13:10:57 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0


I've always thought builtins could not have a custom parsing, however it seems that export/readonly/declare/local (all the builtins creating an assignment) are able to disable field splitting when an assignment takes place, why is that? I know a simple assignment disables field splitting, but after all that's because the grammar states it should, but doesn't POSIX requires all simple commands to have the same splitting behavior?

bash$ foo=$(echo 1 bar=2)
bash$ echo "$foo"
1 b=2

This is logic, simple assignment doesn't triggers field splitting, that's the rule.

bash$ set a=$foo
bash$ echo "$1"

This is logic too, field splitting happens as expected

bash$ readonly a=$foo
bash$ echo "$a"
1 b=2

Why? I know simple assignment doesn't split but I would have thought this should still follow the simple commands arguments expansion and thus should do 2 assignments. I mean builtins shouldn't be able to have their custom parsing, if they do then they should be classified as keywords (like [[ ]]). What should be the POSIX way of doing things? Is this a bash extension?

As a side note ksh has the same behavior, but dash follows what I thought would be the "normal" behavior:

dash$ foo=$(echo 1 b=2)
dash$ readonly a=$foo
dash$ echo "$a"
dash$ echo "$b"



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