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Re: File locking within zsh?
- X-seq: zsh-users 10241
- From: Tim Writer <tim@xxxxxxxxxxx>
- To: "Brian K. White" <brian@xxxxxxxxx>
- Subject: Re: File locking within zsh?
- Date: 10 May 2006 17:53:33 -0400
- Cc: <zsh-users@xxxxxxxxxx>
- In-reply-to: <009101c673f7$06b3f090$6500000a@venti>
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- Sender: tim@xxxxxxxxxxx
"Brian K. White" <brian@xxxxxxxxx> writes:
> ----- Original Message -----
> From: "Tim Writer" <tim@xxxxxxxxxxx>
> To: "Lloyd Zusman" <ljz@xxxxxxxxxx>
> Cc: <zsh-users@xxxxxxxxxx>
> Sent: Tuesday, May 09, 2006 11:31 PM
> Subject: Re: File locking within zsh?
> > Lloyd Zusman <ljz@xxxxxxxxxx> writes:
> >> Do any of you know of any functions, primitives, tricks, hacks, or even
> >> outright abominations which will allow me to do cooperative file locking
> >> from within zsh?
> >> I know that I can do this with a number of compiled executables, but I'm
> >> looking for a zsh-only solution.
> >> Assuming some sort of zsh locking operator called "lock", consider
> >> this example (within a zsh script):
> >> lock -x -t 0 file # for this example of a hypothetical operator, '-x'
> >> # means to wait until I get an exclusive lock, and
> >> # '-t 0' means no time out
> >> print foo bar baz >>file
> >> # do a whole lot of other stuff to "file"
> >> unlock file # release the lock
> >> In this example, any other zsh script which asks for an exclusive lock
> >> on "file" using this hypothetical "lock" operator will block until the
> >> "unlock" operator has been invoked.
> >> Can this be done somehow in zsh, or do I have to rely on a compiled
> >> executable to accomplish this?
> > The usual way to lock within shell scripts is to use ln. This works on all
> > UNIX like systems because creating a hard link (with ln) is an atomic
> > operation which fails if the target already exists. Your example above canbe
> > written like this:
> > while ! ln file file.lock 2>/dev/null
> > do
> > sleep 1
> > done
> > # Lock obtained
> > print foo bar baz >>file
> > # do a whole lot of other stuff to "file"
> > rm -f file.lock
> > # Lock released
> > Wrapping this idiom into lock/unlock functions is left as an exercise for the
> > reader. :-)
> Is that better or worse, and why, than umask 222 ; >file ?
I think the ln solution is better because it's guaranteed to be atomic.
Granted, a sane implementation of ">file" will use open(2) with O_CREAT what
guarantees do you have that your shell (and I'm not talking about zsh here)
is sane? I have often seen code like this pseudocode:
if (file exists)
I don't know that any shells contain crap like this but I don't know they
tim writer <tim@xxxxxxxxxxx> starnix inc.
647.722.5301 toronto, ontario, canada
http://www.starnix.com professional linux services & products
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