Re: Probably very stupid script/bash question
On Wed, 05 Mar 2008 13:10:37 -0800
Bob McGowan <firstname.lastname@example.org> wrote:
> Mark Clarkson wrote:
> > On Wed, 05 Mar 2008 12:16:02 -0800
> > Bob McGowan <email@example.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
> >> construct.
> > 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.
> > <blockquote>
> > 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.
> > </blockquote>
> > Cheers
> > Mark.
> 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
This should explain it:
$ exec 4<>4
$ echo "this is a test" >4
$ wc /dev/fd/4
1 4 15 /dev/fd/4
which of course is equivalent to
$ wc <(echo this is a test)
and is not a redirection!
The man page tells me that <(list) substitutes the list and it's up to
the user to choose what to do with it. I chose to redirect it, hence
the additional '< '.