Re: [Help-bash] Readig multiple null-separated values

[František Kučera]

Dne 29. 07. 19 v 15:07 Greg Wooledge napsal(a):
> > In Bash 4.4, the builtin `mapfile' has an opton `-d delim' which
> > enables to change the delimiter from newlines. It accepts an empty
> > string which actually behaves as NUL separator. For example, the
> > provided example can be written with `mapfile' as follows:
> >
> >    $ printf 'a\0aaa\0b\0bbb\0c\0ccc' | { mapfile -d '' arr; declare -p arr; }
> >    declare -a arr=([0]="a" [1]="aaa" [2]="b" [3]="bbb" [4]="c" [5]="ccc")
> And prior to bash 4.4, the only choice was IFS= read -r -d '' which works
> basically the same way, just with string variables instead of array
> variables.
>
> $ for var in a b c; do IFS= read -r -d '' "$var"; done < <(printf 
> 'a\0aaa\0b\0bbb\0c\0ccc\0')

Thanks for responses. This is similar to my function, just in-lined.

The mapfile can read null-separated values, but it reads them into a 
single-dimensional array – but the logical structure in my case is not 
an array but a two-dimensional table (rows, colums) and it is also 
useful to reference the values by their names instead of numeric indexes.

My first attempt was to convince the read command to read e.g. three 
null-separated into three variables (the first record/row) and then 
stop, so the "while" body will be executed, and in the next cycle the 
read would read the next three values (second record) into the 
variables. But this does not work, because if the delimiter is set to 
\0, the read stops reading even if there are more variables waiting to 
be filled. It is good default behavior but there might be an option to 
read all variables even if the delimiter occurred (i.e. option to 
continue over delimiters until all variables are filled and then return).

> Since the OP seems to be asking for a Swiss Army Knife that handles either
> arrays or strings (and opens wine bottles too), the only answer we can
> give is: there is no single tool.  What you use depends on the situation.
>
>   * If you need a fixed number of string values, you can simply call
>     read however many times you require.
>
>   * If you need an array, and you're in bash 4.4 or newer, you can use
>     mapfile.
>
>   * If you need an array, and you're targeting older bash releases, you
>     can call read inside a while loop, and build up the array that way.

To illustrate a use-case – I sometimes pass a stream of null-separated 
values to this function from Relational pipes 
<https://relational-pipes.globalcode.info/> if I need to process such 
data in a script, e.g. call some shell commands for each record. For 
example:

find -type f -print0 | relpipe-in-filesystem --file path --xattr 
xdg.origin.url | relpipe-out-nullbyte | while read_nullbyte file url; do 
echo "File $file was downloaded from $url and has SHA-256 hash: 
$(sha256sum "$file")"; done

Echoing the string is too simple task – just for illustration – inside 
the for cycle there might be any other Bash code which works with 
variables of current record loaded from the stream. The data can come 
from various sources like XML, Recfiles, databases etc. and can be 
filtered or JOINed in particular pipeline steps. It is similar to 
declarative SQL, but without database/server and it is stream-oriented.

Franta