How to run a stream based command in place on a file

Richard Bradley rtb27 at
Mon Oct 18 05:35:42 PDT 2004


I want to run stream based commands like `sed` and `tr` on the contents of a 
file, and save the results to the same file.

Obviously I can do this with a temporary file:

$sed s/dog/cat/ myanimals.txt > tmp.txt
$mv tmp.txt myanimals.txt

But is there any way I can do this with a single command?

My first guess would be a "buffer" command that reads a file into memory (or 
into a temp file) then pipes it to stdout, e.g.

$cat myanimals.txt | buffer | sed s/dog/cat/ > myanimals.txt

But there isn't one which, in my experience of BSD, means it either wouldn't 
work or there is a better way to do it :-)

Having read through the Bash manual and run some experiments, it seems that 
the ">" operator truncates an output file to zero length before any commands 
are run.

So my missing command becomes:

$cat myanimals.txt | sed s/dog/cat | bufferedwrite myanimals.txt

I can't find anything like this anywhere -- any ideas what the "proper" way to 
do this is?

Thanks in advance,


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