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Re: Probably very stupid script/bash question

Mark Clarkson wrote:
On Wed, 05 Mar 2008 12:16:02 -0800
Bob McGowan <bob_mcgowan@symantec.com> wrote:

Brian wrote:
So can you explain exactly what the first < <( echo "$teststring" )
does exactly please?
In any case, I'd be interested in knowing where you found this

The bash man page seems to be one of the few that can be read a
thousand times and /still/ find something new in it each time!

The following extract is from 'man bash' ;-) ,although what it doesn't
make clear is that <(list) is preceeded by '< ', which I guess is why
it's often missed.


   Process Substitution
       Process  substitution  is supported on systems that support
named pipes (FIFOs) or the /dev/fd method of naming open files.  It
takes the  form of  <(list) or >(list).  The process list is run with
its input or out- put connected to a FIFO or some file in /dev/fd.  The
name of this file is  passed  as  an argument to the current command as
the result of the expansion.  If the >(list) form is used, writing to
the file will  pro- vide  input  for list.  If the <(list) form is
used, the file passed as an argument should be read to obtain the
output of list.

       When available, process substitution is performed
simultaneously  with parameter  and variable expansion, command
substitution, and arithmetic expansion.



Interesting.  But...

If I do the "process substitution" using a stand alone programs, it works as described:

    $ wc <(echo this is a test)
          1       4      15 /dev/fd/63

And, note the '/dev/fd' "device", just as the man page describes.

If I put the above, verbatim, in a script file, it also works exactly as described.

It "breaks" when the command used is a builtin command:

    $ read list <(echo this is a test)
    bash: read: `/dev/fd/63': not a valid identifier

In this case, I typed a <CR> after the first line and it "hung", in fact it was waiting for input to the 'read' builtin. So I typed a '^D', with the error message then following.

The string '< <(' does not appear anywhere in the bash man page, hence my failure to make the association with 'process substitution'.

But, my question is still not completely answered. What prompted Brian (or the original poster, if different), to add a second '<' to the '<('?

I suppose the next question should be what type of bug is this? Is it a documentation issue, where the '< <(...)' syntax simply needs explaining or is it a bash bug, in that a 'read list <(...)' should work?

Bob McGowan

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