Date: Thu, 4 Apr 2013 15:10:57 +0000 From: "Teske, Devin" <Devin.Teske@fisglobal.com> To: Mark Felder <feld@feld.me> Cc: Devin Teske <dteske@freebsd.org>, FreeBSD Questions <freebsd-questions@freebsd.org> Subject: Re: OT: posix sh problem Message-ID: <13CA24D6AB415D428143D44749F57D7201EF2689@ltcfiswmsgmb21> In-Reply-To: <13CA24D6AB415D428143D44749F57D7201EF2627@ltcfiswmsgmb21> References: <op.wu0nsgsf34t2sn@tech304.office.supranet.net> <13CA24D6AB415D428143D44749F57D7201EF2411@ltcfiswmsgmb21> <op.wu0tttba34t2sn@tech304.office.supranet.net> <13CA24D6AB415D428143D44749F57D7201EF2627@ltcfiswmsgmb21>
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Oh, and just to cover all bases=85 If you suspect you have sub-shells in the loop, use "export" to export the = vars so that the sub-shells get the vars in the loop. --=20 Devin On Apr 4, 2013, at 8:04 AM, Teske, Devin wrote: >=20 > On Apr 4, 2013, at 7:42 AM, Mark Felder wrote: >=20 >> On Thu, 04 Apr 2013 08:54:30 -0500, Teske, Devin <Devin.Teske@fisglobal.= com> wrote: >>=20 >>> Wait, you can't? Then I've been doing something wrong all these years=85 >>> #!/bin/sh >>> printf "line1\nline2\n" | while read line >>> do >>> echo "line=3D[$line]" >>> done >>=20 >> You sort-of can, but it's not portable at all. As detailed here: http://= www.etalabs.net/sh_tricks.html >>=20 >>>> One common pitfall is trying to read output piped from commands, such = as: >>>> foo | IFS=3D read var >>>> POSIX allows any or all commands in a pipeline to be run in subshells, >=20 > And for most purposes that's fine. Read-on=85 >=20 >=20 >>>> and which command (if any) runs in the main shell varies greatly betwe= enimplementations >=20 > =85 that is only if you truly need the variables to be read into the main= shell. This is most always not what you want. >=20 > The page you linked about doesn't talk about the special case of "while",= in example: >=20 > foo | IFS=3D while read var >=20 > On the back-end nothing changes (the same caveat applies -- variables set= on the right side of the pipe may not be available to the main shell; as-p= er the quoted text). However, the high-level task changes from: >=20 > I want to read some text from a pipe into some variables >=20 > to instead: >=20 > I want to read some text from a pipe and process it word-by-word (in you= r case) and act on the words in a loop >=20 > So in other words=85 the only reason for wanting the variables in the mai= n shell is if you want to act on the last set of variables for the last lin= e after the loop has run (and presumably already processed the last line). = This is what I am saying anyone will rarely ever want. In other words, once= the loop (potentially running in a sub-shell) has completed, you likely do= n't care about the variable contents and are willing to throw them away any= how. >=20 >=20 >=20 >>>> =97 in particular Bash and ksh differ here. The standardidiom for over= coming this problem is to use a here document: >>>>=20 >>>> IFS=3D read var << EOF >>>> $(foo) >>>> EOF >>=20 >=20 > But you're not processing a single line; you're processing the entire inp= ut at-once and performing an action (writing to the screen) that also doesn= 't care whether it's in a sub-shell or not. >=20 > SO=85 >=20 > I say rock-on with the original syntax. It's portable. You don't need tho= se vars when the loop ends. >=20 >>=20 >> I was having problems with the variables magically becoming empty, remem= bered I had Rich's site bookmarked, checked to see if it mentioned and it w= as. I'll admit there's a high chance that due to lack of sleep user error w= as the culprit. >=20 > I'm interested in why you need the variables after the loop has completed= . Put your code in the loop where the variables are defined and have values. > --=20 > Devin >=20 > _____________ > The information contained in this message is proprietary and/or confident= ial. If you are not the intended recipient, please: (i) delete the message = and all copies; (ii) do not disclose, distribute or use the message in any = manner; and (iii) notify the sender immediately. In addition, please be awa= re that any message addressed to our domain is subject to archiving and rev= iew by persons other than the intended recipient. Thank you. _____________ The information contained in this message is proprietary and/or confidentia= l. If you are not the intended recipient, please: (i) delete the message an= d all copies; (ii) do not disclose, distribute or use the message in any ma= nner; and (iii) notify the sender immediately. In addition, please be aware= that any message addressed to our domain is subject to archiving and revie= w by persons other than the intended recipient. Thank you.
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