Bourn Shell Scripts that Produce Multiple Files

Martin McCormick martin at
Wed Apr 6 06:45:19 PDT 2005

Mario Hoerich writes:
>This sounds a bit like a truncation issue.
>If you do something like
>    command1 > bar
>    # some code
>    command2 > bar
>then the second redirect will truncate the file to 0 bytes
>before redirecting the output from command2 into it.


	My code was very similar to what you describe only more like:

command1 > bar
#some code
command2 > bar2
#should leave bar alone and open bar2
#Instead, bar and bar2 both end up empty of anything.

>Try using >> instead of >, as it appends to the file.

	What's sad is, I think I tried that but forgot to remove some
older versions of the same files created by a less elegant method so
it looked like the new files had too many lines in them and I figured
that appending didn't work either.  I should have cleaned house first
and things would have been fine.

	Appending did solve the problem.

>If this doesn't help, please post the script (or a simplified
>version thereof).  We're not clairvoyant, you know... :)

	Here is the simplified origin of the problem:

#! /bin/sh
searchfor ()  {
#line of awk that produces standard output
return 0
while read currentnumber; do
#This line does work according to your suggestion.
searchfor  >>$currentnumber.txt
#This line was what I had which opens new files but never fills them:
#searchfor  >$currentnumber.txt
done <~/numbers

	The value held in $currentnumber.txt changes with each loop
iteration so we should be writing to bar2, bar3, etc and leaving bar1
alone.  It apparently does not work that way.  It reminds me of what
happens in C if a program ends without closing an open file for
whatever reason.  The buffer never gets emptied so the file is left
either empty or partially filled depending upon luck and how much data
got written to the disk before the abnormal end.

	My short-term problem is solved so thanks again, but it
appears that even opening new files without appending them confuses
the shell on previously-opened files such that you do not receive any
data in any of the files.

Martin McCormick WB5AGZ  Stillwater, OK 
OSU Information Technology Division Network Operations Group

More information about the freebsd-questions mailing list