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On Friday, May 25, 2012 08:52:00 PM Mike Frysinger wrote: |
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> On Friday 25 May 2012 18:33:43 Ciaran McCreesh wrote: |
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> > On Fri, 25 May 2012 15:02:32 -0500 Dan Douglas wrote: |
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> > > If it were made a policy now that ebuilds and eclasses cannot depend |
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> > > upon the subshell (for example, to set temporary positional |
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> > > parameters or isolate temporary variables), then maybe someday in the |
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> > > distant future this could be made the default, and in the meantime, |
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> > > an option for those with new enough shells. Since dependence on the |
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> > > subshell isn't very common, I think this should be feasible, and of |
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> > > course as a workaround all that's required is to wrap any such |
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> > > commands in parentheses. |
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> > |
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> > We'll be able to turn that on in a controlled way in EAPI 6. Having |
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> > said that, if we're reaching the point where speed of bash code is |
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> > at all relevant, then ebuilds are doing something wrong... |
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> |
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> i don't think speed is the main motivator, but rather avoiding behavior that |
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> bites new people all the time: |
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> count=0 |
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> printf '%s\n' a b c | \ |
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> while read line ; do |
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> : $(( count++ )) |
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> done |
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> echo $count |
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> |
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> w/out lastpipe, that shows 0. w/lastpipe, that shows 3. |
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> -mike |
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Right, performance is just a nice side-effect. It makes a number of things |
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cleaner (especially if the printf in that case were replaced with something |
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more complicated), and is more intuitive for beginners . |
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However, all involved code needs to be able to expect lastpipe to always be |
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either one way or the other, not mix-and-match. This means either EAPI 6 |
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requires Bash 4.2, or if Bash version detection is involved, a lot of the |
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benefit to lastpipe is lost. Code which can't predict the behavior has to be |
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written not only as though lastpipe were disabled, but also to account for the |
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possility that it is enabled to avoid name conflicts. |
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|
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In that example it would mean adding an explicit subshell, or saving and |
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restoring the value of "line" before/after, or putting it into a function and |
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using locals, or always making sure code executed after the loop initializes |
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"line" to a known state. |
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-- |
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Dan Douglas |
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