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Re: mkfifo and tee within a function


From: Chet Ramey
Subject: Re: mkfifo and tee within a function
Date: Tue, 05 Dec 2006 23:23:02 -0500
User-agent: Thunderbird 1.5.0.8 (Macintosh/20061025)

Nathan Coulter wrote:
> Hi,
> 
> Within a function, I'd like to print some input to the terminal and at
> the same time store it in a fifo fostore some input in a fifo,  but am
> getting mixed results.  In this example, I thought "hello" would be
> output twice:
> 
> $>cmd_print () { mkfifo zout ; (tee zout &); cat zout ; rm zout; }
> $>printf 'hello\n' | cmd_print

When job control is not active, Posix requires that a command run in the
background with `&' behave as if its standard input were /dev/null in the
absence of any explicit redirection.  Bash counts a pipe directly into
such a command as one such redirection, but not the pipe into the shell
function.  This function works:

cmd_print1 () { mkfifo zout1 ; (tee zout1 </dev/stdin &) ;  cat zout1 ; rm
zout1; }

It adds an explicit redirection from stdin.

> 
> This outputs "hello" twice:
> 
> $>cmd_print () { mkfifo zout ; (cat - | tee zout &); cat zout ; rm zout; }
> $>printf 'hello\n' | cmd_print
> hello
> hello

This behaves as you expect because of the pipe into the asynchronous `tee'
command.

> 
> but this doesn't output anything:
> 
> $>cmd_print () { mkfifo zout ; (cat - | tee zout &) ; rm zout; }
> $>cmd_print <<EOF
> $> date
> $> EOF

I believe this is caused by a race condition:  the rm runs before the
pipeline, and the `tee' is either orphaned or writes to a regular file
named `zout'.  I'm not sure how to make this form work reliably.

> 
> My real goal is to feed data to a function and then source it: 
> $> cmd_print () { mkfifo zout ; (cat - > zout ) & ; source zout ; rm
> zout; }
> $> cmd_print <<EOF
> $> date
> $> EOF
> $>

There is a limitation in current releases of bash, which will be fixed
in the next release, that restricts `source' to work only on regular
files.  Once that is fixed, the same sort of explicit redirection of
input from /dev/stdin as in the first example will work:

cmd_print4 () { mkfifo zout4 ; (cat - > zout4) </dev/stdin & source zout4 ;
rm zout4; }


Chet
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                       Live Strong.  No day but today.
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/




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