Re: [Help-bash] prepend to stdin

[Chet Ramey]

On 12/22/15 3:18 PM, Brett Kail wrote:
> I'm writing a bash script, and I would like to execute a command passing as
> stdin the contents of a file followed by the script's stdin.  The command
> might or might not read stdin, but regardless, I would like the entire
> pipeline to end when the final command exits.  I tried this pipeline (using
> false as an example command):
> 
> cat file - | true
> 
> ...but it seems that something waits for a line of input before the
> pipeline somehow ends without error.  What is causing this wait?  Is there
> some way to disable the wait and have the entire pipeline end immediately
> when the final command exits?

You've created a race condition.  It depends on the behavior of the kernel
and its scheduling algorithm.  Consider the following possible sequence:

1. cat writes file to the pipe, which happens atomically because file is
   smaller than the pipe buffer size;
2. true exits;
3. cat processes its next input `file', which causes it to read a line from
   stdin

On another system (Mac OS X, say), with a different scheduler, true might
exit before cat writes the file, and the entire pipeline would exit when
cat got the SIGPIPE.  But you can't tell.

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