On Sun, 17 Aug 2008 18:33:28 -0700, David Wolfskill <[EMAIL PROTECTED]> wrote:
> 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=""
>       while read bar ... ; do
>        ...
>         foo=$bar
>        ...
>       done <$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=""
>       cat $filename | while read bar ... ; do
>        ...
>         foo=$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.

Hi David,

You are right that feeding data to a looping construct through a pipe
may run in a subshell.  The ``Single UNIX Specification'' says

    "... each command of a multi-command pipeline is in a subshell
    environment; as an extension, however, any or all commands in a
    pipeline may be executed in the current environment."

You can read the online text of the Single UNIX Specification at:

    http://www.unix.org/single_unix_specification/

A simple 'registration' is required, and you need a browser that
supports cookies.  But after that, you can follow the links to the shell
command language page at

    http://www.opengroup.org/onlinepubs/000095399/utilities/xcu_chap02.html

Near section `` 2.12 Shell Execution Environment'' you can find the text
I quoted.

What I usually do in similar shell scripts is something like:

    cat "${filename}" | sed -n -e '/foo/ s/bar/baz/' | \
        xargs -n1 blah

This isn't exactly the same as assigning $foo to the results of the
loop, but you can also use:

    foo=`cat $filename | while read bar ; do \
             stuff ...
             echo "$bar"
             more stuff...
         done`

> Now here's a copy of the in-development script:
>
> #! /bin/sh
>
> cmd="netstat -nibd -f inet"
> ctr=0
> clist=""
> hlist=`$cmd | head -1`
> for f in $hlist; do
>   ctr=$(( $ctr + 1 ))
>   eval c$ctr=\"$f\"
>   eval h_$f=c$ctr
> done
> cmax=$ctr
>
> t_file=`mktemp /tmp/XXXXXXXXX`
> $cmd | tail +2 >$t_file
> while read $hlist dummy; do
>   if [ "$Name" = "lo0" ]; then
>     continue
>   fi
>   for f in $hlist; do
>     eval val=\"\$$f\"
>     case $val in
>     -) eval ${f}_$Name=0;;
>     *) eval ${f}_$Name="$val";;
>     esac;
>   done
>   nics="$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
> 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
> 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)?

Ah, that's much better.  Now I see what you are trying to do.  Would you
be ok with an awk(1) script instead of /bin/sh?  It tends to be nicer
for this sort of thing, i.e.:

$ expand david.awk | cat -n
     1  #
     2  # Gather the field names if this is a header-line.
     3  #
     4  $0 ~ /^Name/ {
     5          for (k = 1; k <= NF; k++)
     6                  tag[k] = $k;
     7  }
     8
     9  #
    10  # For all other lines, just print the tagged field values.
    11  #
    12  $0 !~ /^Name/ {
    13          name = $1;
    14          for (k = 1; k <= NF; k++) {
    15                  if ($k == "-")
    16                          $k = "0";
    17                  printf "%s_%s: %s\n", tag[k], name, $k;
    18          }
    19  }

$ netstat -nibd -f inet | awk -f david.awk
Name_re0: re0
Mtu_re0: 1500
Network_re0: 192.168.1.0/2
Address_re0: 192.168.1.3
Ipkts_re0: 1672873
Ierrs_re0: 0
Ibytes_re0: 1411912899
Opkts_re0: 1418782
Oerrs_re0: 0
Obytes_re0: 948973554
Coll_re0: 0
Drop_re0: 0
Name_lo0: lo0
Mtu_lo0: 16384
Network_lo0: 127.0.0.0/8
Address_lo0: 127.0.0.1
Ipkts_lo0: 9019
Ierrs_lo0: 0
Ibytes_lo0: 2835806
Opkts_lo0: 9019
Oerrs_lo0: 0
Obytes_lo0: 2835806
Coll_lo0: 0
Drop_lo0: 0

With a bit of preprocessing, it may be possible to extract the network
names and print the "(end) NICs: XXX XXX" part too.

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