Date: Sun, 17 Aug 2008 18:33:28 -0700 From: David Wolfskill <david@catwhisker.org> To: freebsd-questions@freebsd.org Subject: Shell scripts: variable assignment within read loops Message-ID: <20080818013328.GY44815@bunrab.catwhisker.org>
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I am writing a (Bourne) shell script that is intended (among other
things) to obtain information from a command, such as:
netstat -nibd -f inet
by reading and parsing the output.
However, the "obvious" (to me) approach of piping the output of the
command to the standard input of a "while read ..." statement turns out
to be not very useful; it appears that while
foo=3D""
while read bar ... ; do
...
foo=3D$bar
...
done <$filename
echo $foo
will assign to foo the value of the bar variable form the last record
read (in FreeBSD 6.3-STABLE, at least), the following fails to do so:
foo=3D""
cat $filename | while read bar ... ; do
...
foo=3D$bar
...
done
echo $foo
Well, that's not *quite* accurate:the assignment is done all right, but
in the latter case, it appears to be done in a subshell, so by the time
we get to the "echo" statement, any variable assignments from within the
read loop have vanished.
Here's a cut/pasted, somewhat contrived example:
#! /bin/sh
foo=3D0
echo "0 foo: $foo"
while read line; do
echo "1.0 foo: $foo"
foo=3D1$line
echo "1.1 foo: $foo"
done </etc/resolv.conf
echo "2 foo: $foo"
echo ""
foo=3D2
echo "3 foo: $foo"
eval "cat /etc/resolv.conf | grep '.'"
cat /etc/resolv.conf | while read line; do
echo "4.0 foo: $foo"
foo=3D3$line
echo "4.1 foo: $foo"
done
echo "5 foo: $foo"
exit 0
and its output on my laptop:
0 foo: 0
1.0 foo: 0
1.1 foo: 1search catwhisker.org
1.0 foo: 1search catwhisker.org
1.1 foo: 1nameserver 172.16.8.12
1.0 foo: 1nameserver 172.16.8.12
1.1 foo: 1nameserver 172.16.8.11
1.0 foo: 1nameserver 172.16.8.11
1.1 foo: 1nameserver 172.16.8.1
2 foo: 1nameserver 172.16.8.1
3 foo: 2
search catwhisker.org
nameserver 172.16.8.12
nameserver 172.16.8.11
nameserver 172.16.8.1
4.0 foo: 2
4.1 foo: 3search catwhisker.org
4.0 foo: 3search catwhisker.org
4.1 foo: 3nameserver 172.16.8.12
4.0 foo: 3nameserver 172.16.8.12
4.1 foo: 3nameserver 172.16.8.11
4.0 foo: 3nameserver 172.16.8.11
4.1 foo: 3nameserver 172.16.8.1
5 foo: 2
Note that the "2 foo" line shows a value read from the last line of
/etc/resolv.conf, while the "5 foo" line fails to do so. (In a Solaris
9 environment, the output from each stanza is the same as the output
from teh second stanza in FreeBSD.)
Now here's a copy of the in-development script:
#! /bin/sh
cmd=3D"netstat -nibd -f inet"
ctr=3D0
clist=3D""
hlist=3D`$cmd | head -1`
for f in $hlist; do
ctr=3D$(( $ctr + 1 ))
eval c$ctr=3D\"$f\"
eval h_$f=3Dc$ctr
done
cmax=3D$ctr
t_file=3D`mktemp /tmp/XXXXXXXXX`
$cmd | tail +2 >$t_file
while read $hlist dummy; do
if [ "$Name" =3D "lo0" ]; then
continue
fi
for f in $hlist; do
eval val=3D\"\$$f\"
case $val in
-) eval ${f}_$Name=3D0;;
*) eval ${f}_$Name=3D"$val";;
esac;
done
nics=3D"$Name $nics";
done</$t_file
rm $t_file
echo "(end) NICs: $nics"
for n in $nics; do
for f in $hlist; do
eval "echo ${f}_$n: \$${f}_$n"
done
done
exit 0
And its output on my laptop:
(end) NICs: ath0=20
Name_ath0: ath0
Mtu_ath0: 1500
Network_ath0: 172.17
Address_ath0: 172.17.1.30
Ipkts_ath0: 725191
Ierrs_ath0: 0
Ibytes_ath0: 185144197
Opkts_ath0: 821917
Oerrs_ath0: 0
Obytes_ath0: 74260936
Coll_ath0: 0
Drop_ath0: 0
and (somewhat more interestingly) on my "firewall" machine:
(end) NICs: dc0 de0 fxp0=20
Name_dc0: dc0
Mtu_dc0: 1500
Network_dc0: 172.16.8/24
Address_dc0: 172.16.8.1
Ipkts_dc0: 2501577
Ierrs_dc0: 0
Ibytes_dc0: 215386153
Opkts_dc0: 20269087
Oerrs_dc0: 0
Obytes_dc0: 2553930555
Coll_dc0: 0
Drop_dc0: 0
Name_de0: de0
Mtu_de0: 1500
Network_de0: 63.193.123/24
Address_de0: 63.193.123.122
Ipkts_de0: 5936847
Ierrs_de0: 0
Ibytes_de0: 734092787
Opkts_de0: 18557543
Oerrs_de0: 0
Obytes_de0: 2551089632
Coll_de0: 0
Drop_de0: 0
Name_fxp0: fxp0
Mtu_fxp0: 1500
Network_fxp0: 172.17
Address_fxp0: 172.17.0.1
Ipkts_fxp0: 10013
Ierrs_fxp0: 0
Ibytes_fxp0: 1366082
Opkts_fxp0: 1253115
Oerrs_fxp0: 0
Obytes_fxp0: 70429903
Coll_fxp0: 0
Drop_fxp0: 0
As you see, I am circumventing the issue by writing to a transient file.
In the intended application, the script is to be used to gather
resource-utilization information; thus, I want its "footprint" to be
smaller, rather than larger. Granted, in my case, I would be writing a
tiny text file to a swap-backed tmpfs, but in production, I won't have
the luxury of knowing that in advance: the intent is that the script
must run on a minimal FreeBSD system, with no "ports" or other 3rd-party
software installed.
Is there some other -- possibly better -- way to do this (using Bourne
shell scripting)?
Thanks!
(Please select recipients for your replies carefully: I'm not subscribed
to -questions. I've provided a hint in the form of a Reply-To header,
though I realize that not all mailers honor it. Please do include me in
replies.)
Peace,
david
--=20
David H. Wolfskill david@catwhisker.org
Depriving a girl or boy of an opportunity for education is evil.
See http://www.catwhisker.org/~david/publickey.gpg for my public key.
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