Array disappear in bash script - arrays

Question

I'm writing a script to collect some various network statistics.
What I'm trying to do is to produce some delta data from the netstat -i command.
I'm collecting the needed data with the following bash code:
declare -a array
n=0
netstat -i | tail -n +3 | while read LINE; do
echo "Setting array[$n] to $LINE"
array[$n]=$LINE
echo "array now have ${#array[#]} entries"
let n=$n+1
done
echo "array now have ${#array[#]} entries"
output from this command is:
Setting array[0] to eth0 1500 0 4946794 0 0 0 2522971 0 0 0 BMRU
array now have 1 entries
Setting array[1] to lo 16436 0 25059 0 0 0 25059 0 0 0 LRU
array now have 2 entries
Setting array[2] to vmnet1 1500 0 6 0 0 0 1126 0 0 0 BMRU
array now have 3 entries
Setting array[3] to vmnet8 1500 0 955 0 0 0 1054 0 0 0 BMRU
array now have 4 entries
Setting array[4] to wlan0 1500 0 613879 0 0 0 351194 0 0 0 BMU
array now have 5 entries
array now have 0 entries
As you can see, the array actually disappear after the while loop, and I do not understand why.

Answer 1

Any time you use a pipe you create an implicit subshell. When that subshell terminates, so do its variables. A quick fix for this is to not pipe stuff to read. You can accomplish the above using process substitution:
while read LINE; do
echo "Setting array[$n] to $LINE"
array[$n]=$LINE
echo "array now have ${#array[#]} entries"
let n=$n+1
done < <(netstat -i | tail -n +3)
A more POSIX compliant approach (read: more portable, less bashist) is to make everything happen in the subshell:
netstat -i | tail -n +3 | {
declare -a array
n=0
while read LINE; do
echo "Setting array[$n] to $LINE"
array[$n]=$LINE
echo "array now have ${#array[#]} entries"
let n=$n+1
done
echo "array now have ${#array[#]} entries"
}
You can read the fine points of this (and more) at Greg Wooledge's wiki.

Answer 2

If your only goal is to put the output of a command into an array (linewise), you'd better use the (sadly not very well-known) mapfile bash builtin, it's by far the most efficient (and the best suited for code golf, count how many character strokes I have compared to the other possibilities):
mapfile -t array < <(netstat -i | tail -n +3)
The other answers explain why your construct didn't work (pipe is in a subshell and all that).
help mapfile for all the details and possibilities of that command.

Answer 3

Ok, are you ready?
There is how to transform netstat -i | tail -n +3 in a bash Associative Array of array:
declare -A AANET
while read -a line ;do
declare -a AI$line
eval "AI$line=(${line[#]})"
AANET[$line]=AI$line
done < <(
netstat -i |
tail -n +3)
Than now:
echo ${!AANET[#]}
venet0 eth1 eth0 lo br0
echo ${AANET[eth0]}
AIeth0
And for sub-associative, we have to use eval:
eval echo \${${AANET[eth0]}[#]}
eth0 1500 0 17647 0 0 0 35426 0 0 0 BMPU
eval echo \${${AANET[eth0]}[1]}
1500
eval echo \${${AANET[eth0]}[3]}
17647
eval echo \${${AANET[eth0]}[7]}
35426
eval echo \${${AANET[eth0]}[#]:3:5}
17647 0 0 0 35426
An for assing a temporary variable:
eval currentBin=\${${AANET[eth0]}[3]} currentBout=\${${AANET[eth0]}[7]}
echo $currentBout
35426
echo $currentBin
17647
or even too:
eval "declare -a currentVals=(\${${AANET[eth0]}[#]:3:8})"
echo ${currentVals[0]}
17647
echo ${currentVals[4]}
35426
echo ${currentVals[#]}
17647 0 0 0 35426 0 0 0
Edit:
Ok, if it is possible without eval!
for aKey in ${!AANET[#]};do
fields=(${AANET[$aKey]}{[1],[3],[7]});
echo $aKey ${!fields} ${!fields[1]} ${!fields[2]}
done |
xargs printf "%-9s %12s %12s %12s\n" IFace MTU RX TX
IFace MTU RX TX
venet0 1500 0 0
eth1 1500 6400292 6942577
eth0 1500 17647 35426
lo 16436 83 83

Answer 4

Don't pipe to a loop, bash substitutes variables first and then starts a subshell without access to your array.
Do it like this. Nice and simple.
array=()
for alias in `netstat -i | tail -n +3`; do
array+=($alias)
done
echo ${array[#]}