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John, You and I have extensive experience with most of the Unix shells. The cat command is a standard Unix/Linux command. /dev/null is universal in all Unix and variants. And, all Unix file systems work on the same basic concept from a user and API standpoint. (Under the covers there are significant differences). The process overhead in Unix and Linux is relatively small, and with virtual memory, there is a good chance that the cat command may already be memory resident. Unix and Linux have a large number of small commands. Because of that and because of virtual memory there has been a lot of work on reducing the overhead. I maintain that the additional overhead of loading an external command like cat is insignificant when compared to the savings of using a shell builtin. The shell may have to page in the echo command. On Fri, 07 Feb 2003 17:55:07 UTC John Chambers <jc at trillian.mit.edu> wrote: > Dave Gavin writes: > | the "rm filename; touch filename" variation doesn't necessarily > preserve| ownership or permissions depending on who does it and what > their umask is.| My personal favorite is "cat /dev/null > filename". > > This may be the most universal, and it doesn't have the problem with > "rm;touch" of a window when the file doesn't exist. Is there any case > where this wouldn't work in any unix shell? > > If you're worried about the overhead of the unnecessary process, > there doesn't seem to be any universal command that works in all > known shells. Using echo doesn't quite work, because it produces a > single \n by default, and there are at least two different ways of > suppressing this character. > > Isn't it amazing how much discussion it can take to figure out how to > produce nothing, reliably? > > I've always sorta liked the observation that the Arabs introduced the > concept of zero to Western Civilization around 1500 years ago, but > most of the computer industry still hasn't caught on to the idea. The > zero case is almost always special, and takes special code to handle. > Unix software is usually better at this than others, but even on unix > systems you run into lots of cases where making the zero case work > takes half the time. > > I haven't worked on IBM mainframes for over 20 years now, but I well > remember the grief caused by the fact that closing an output file > without writing a record was a fatal error, as was attempting to > write a zero-length record. This recently bit me again, though. I'm > working on a project where some of the files get sent to mainframes. > One of the constraints is that, if my code writes \n\n, the mainframe > (Cobol) software gets a fatal error when it attempts to deal with the > zero-length record. So far, I haven't sent them any null files, but I > can well imagine what that would do to their software. -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 189 bytes Desc: not available URL: <http://lists.blu.org/pipermail/discuss/attachments/20030207/f3a5ee8d/attachment.sig>
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