Re: read and env variables + POSIX => SEGFAULT

[Chet Ramey]

On 10/12/15 7:39 PM, Linda Walsh wrote:

>>>> a= read a <<< x;echo $?
>>> 0
>>>> declare -p a
>>> declare -- a="x"
>>> #  the manpage claims "one line is read from [the input], and the result
>>> #  is split by words and assigns 1st word to 1st var and so forth, but
>>> #  apparently the reading of 1 line is optional -- though this is
>>> consistent
>>> #  with the fact that read can be told to read some number of characters
>>> and #  return when the limit is reached.  So technically, read doesn't
>>> "read one line",
>>> #  but read whatever is on 'input' up to 1 line.  (DOC clarification?)
>>
>> This is terribly wrong.
>>
>> The command in question is `a= read a <<< x'.
>>
>> The here-string construct takes the following word and, like a here
>> document, makes it the standard input to the command.  The standard
>> input is then a file consisting of a single line: x\n.
>>
>> It's basically shorthand for
>>
>> read a <<EOF
>> x
>> EOF
>>
>> So, `read' reads the single line from its standard input and assigns it
>> to the variable `a'.
> ----
> I wasn't sure if it put the "\n" at the end in a 1-line example.

Yes.  All the lines in a here-document or here-string are newline-
terminated.

> Does it also use a tmp file and use process-substitution, or is
> that only when parens are present?

Here-documents and here-strings use temporary files and open them as
the standard input (or specified file descriptor) for the command.

> 
> read a < <( echo x)
> 
> I'm under the impression, uses a tmp file.

Why would you think that?  The documentation clearly says it uses a named
pipe or a file descriptor associated with a /dev/fd filename (which happens
to be a pipe in this case).

> 
> does the read a <<< x
> also use a tmp file?

Yes.

> 
> I.e. is
>> readarray -t a < <( echo -e 'x\ny\n')
>> declare -p a
> declare -a a='([0]="x" [1]="y")'
> 
> implemented the same way as
>> a=(x y)
>> b=$(printf "%s\n" ${a[@]})
>> readarray -t ar <<< "${b[@]}"
>> declare -p a
> declare -a a='([0]="x" [1]="y")'

No, if you mean the type of object the `readarray' command has associated
with its standard input.  The first case is a pipe or FIFO; the second is
a regular file.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    chet@case.edu    http://cnswww.cns.cwru.edu/~chet/