Re: Output redirection to block-defined filehandle

[Oleg Verych <olecom@xxxxxxxxx>]

2010/1/29 Martin Langhoff <martin.langhoff@xxxxxxxxx>:

Hallo, Martin.

> The flock binary in modern linuxes (from util-linux-ng) supports
> flocking a filehandle that is handled by the shell. Combining this
> with per-code-block filehandles / io redirection, it is an incredibly
> useful construct to protect a code block with a lock.
>
> The resulting code looks like this (from man flock on Fedora 11):
>
>      (
>         flock -s 200
>         # ... commands executed under lock ...
>       ) 200>/var/lock/mylockfile
>
> I am now discovering that this works in bash, but not in dash.

What about implementation of the race-condition protection or locking
using this scheme:
* `umask 777` -- lock situation using file to be created
* `{ echo “creating file using ‘>’ in a working block” ; do_the_task ;
} > lock_file`
* paralel / concurrent task: `echo “checking file for access using
‘>’” > lock_file && go || echo 'error: access denied' >&2`
* `{ mv lock_file || rm lock_file || …. ; } && chmod 666 lock_file` --
releasing the lock (order matters)

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