Date: 19 Oct 2004 11:30:01 -0400 From: Lowell Gilbert <freebsd-questions-local@be-well.ilk.org> To: Richard Bradley <rtb27@cam.ac.uk> Cc: freebsd-questions@freebsd.org Subject: Re: How to run a stream based command in place on a file Message-ID: <44r7nuwwsm.fsf@be-well.ilk.org> In-Reply-To: <200410181334.37665.rtb27@cam.ac.uk> References: <200410181334.37665.rtb27@cam.ac.uk>
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Richard Bradley <rtb27@cam.ac.uk> writes: > Hi, > > 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? In this specific case, sed(1) has a '-i' option on recent FreeBSD systems, specifically to handle this case. For other commands, you typically write to a different file, and copy it back.
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