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Re: read Built-in Parameter Behavior -- Null Byte Delimiter

From: Martijn Dekker
Subject: Re: read Built-in Parameter Behavior -- Null Byte Delimiter
Date: Sun, 17 Jan 2016 01:02:11 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:31.0) Gecko/20100101 Thunderbird/31.2.0

Adam Danischewski schreef op 16-01-16 om 20:28:
> Yet if we look for the null byte:

bash, like most UNIX shells, cannot store or handle null bytes. (zsh is
the only exception I know of.)

> $> while IFS= read -r -d'' a; do echo "got $a"; done < <(find . -type f
> -print0)
>  *                     returns nothing *

This is because of how shell quoting works. The "read" command doesn't
even enter into it. It's all happening before "read" is even executed.

'' does not represent a null byte; it's an empty, as in zero bytes,
surrounded by single quotes.

Before passing the arguments to the command (such as 'read'), the shell
performs quote removal. This removes the quotes from the empty.

That leaves the 'read' command with no way to distinguish between

    read -r -d'' a


    read -r -d a

Meaning, you were actually executing "read" with "a" as the delimiter,
and no variable, so your "echo" command naturally returned nothing.
Meanwhile "read" would have stored whatever it got in the variable REPLY
by default.

Separating the quoted empty from the -d makes it possible to distinguish
it as an empty argument (which is still different from a null byte).

Hope this helps.

- M.

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