Date: Thu, 08 May 2014 22:07:18 +0100 From: Matthew Seaman <matthew@FreeBSD.org> To: freebsd-questions@freebsd.org Subject: Re: Bourne variable unset outside while() Message-ID: <536BF206.4060005@FreeBSD.org> In-Reply-To: <CAHzLAVFhyPtkWCj1uj0aq0AusfrWL55UUn-mkxiyzUus7x4vdA@mail.gmail.com> References: <CAHzLAVFhyPtkWCj1uj0aq0AusfrWL55UUn-mkxiyzUus7x4vdA@mail.gmail.com>
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On 08/05/2014 19:48, Rick Miller wrote:
> I have a Bourne shell script that echoes the contents of a data structu=
re
> to a while() loop that iterates over the data and is expected to append=
a
> string from the current iteration to a different list. Inside the whil=
e(),
> it works fine, but echo'ing the list outside the loop produces empty
> output. I expect the variable to maintain it's final state after
> processing the while(), but it does not appear to be doing so.
>=20
> /** The script **/
>=20
> #! /bin/sh -x
>=20
> fs=3D"freebsd-ufs gprootfs 1G
> freebsd-swap gpswapfs 1G
> freebsd-ufs gpvarfs 1G";
>=20
> echo "${fs}" |
> while read -r fstype fslabel fssize; do
>=20
> labels=3D"${labels} ${fslabel}";
> echo "${labels}";
>=20
> done
>=20
> echo -e "\nlabels =3D ${labels}";
>=20
> /** End the script **/
>=20
> The output from this script is at http://pastebin.com/mxNLLWtm
>=20
> This almost appears to be an issue with scope, but I've not encountered=
> this before, especially in such simple context. I must be missing
> something obvious and just looking for feedback to send me off in the r=
ight
> direction.
Piping input to a 'while read' in shell implies that the content of the
while loop will be run in a sub-shell. Meaning it can't affect the
value of any variables in the parent process, ie. outside the while loop.=
This is a fairly obscure gotcha in shell programming -- finding out
about it is like one of those rite of passage on the way to shell-godhood=
=2E
Either put the tail of your script inside the while loop, using some
sort of if- condition or other wise arranging for it to only run the
last time around the loop, or print the values you need as a statement
you can eval, or use a temporary file or ... Well, there are many ways
to solve this problem. Good luck and have fun.
Cheers,
Matthew
--=20
Dr Matthew J Seaman MA, D.Phil.
PGP: http://www.infracaninophile.co.uk/pgpkey
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