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Re: [Help-bash] eval

From: Eric Blake
Subject: Re: [Help-bash] eval
Date: Sat, 21 May 2016 06:38:35 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 05/21/2016 06:31 AM, Wu Shuangrong wrote:
> let say:
> $> a=‘one two     three”

Umm, that's not what you typed.  Your use of UTF-8 quotes ‘ and ” are
NOT valid shell quotes.  Assuming you meant:

$> a='one two     three'

> $> eval ‘b=$a’

Again, that's not what you typed.  Assuming you meant:

$> eval 'b=$a'

Note the single quotes. That is the same as if you had typed:

$> b=$a

which is well-formed, and gives b the contents of $a, without word
splitting the contents of a.

> $> printf “%s\n” “$b”

Again, not what you typed.  Assuming:

$> printf "$s\n" "$b"

> you will get “one two    three”.

Yes, that's correct, because that's what you assigned to b.

> What’s confusing me is that why this is working this way. From my point of 
> view, after expansing,

You meant expansion, not expansing.

> 'b=$a’ will become ‘b=one two   three”,

No, that's only true if you had written:

$> eval "b=$a"

Note the difference between single and double quotes.  With single
quotes, $ is a literal character, so you are executing the literal string:

$> b=$a

But with double quotes, you first expand the string, and the string
includes $a, so you would be executing the literal string:

$> b=one two     three

which, at the point of the eval, would have run the command 'two' with
argument 'three' and with 'b=one' in its environment.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library

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