Array disappear in bash script

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.

kojiro · Accepted Answer · 2012-12-07 12:47:50Z

6

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.

answered Dec 7, 2012 at 12:47

kojiro

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3 Comments

Dog eat cat world Over a year ago

Thanks, the first code works perfectly. But the array is lost outside the subshell. I suspect this is the intention, but does not work for me.

Arthur Over a year ago

Very nice. I prefer the POSIX compliant approach because it provides the input data at the start, supporting chronological reading.

kojiro Over a year ago

@Arthur you can provide the input data at the start either way. < <(netstat …) while read LINE; do …

gniourf_gniourf · Accepted Answer · 2012-12-07 14:33:09Z

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.

answered Dec 7, 2012 at 14:33

gniourf_gniourf

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Comments

F. Hauri - Give Up GitHub · Accepted Answer · 2012-12-07 18:11:22Z

1

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

edited Dec 7, 2012 at 18:11

answered Dec 7, 2012 at 13:55

F. Hauri - Give Up GitHub

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7 Comments

gniourf_gniourf Over a year ago

Too much eval for me :-(. Bash was never designed for that kind of stuff. If you really need these constructs, just use another language.

F. Hauri - Give Up GitHub Over a year ago

@gniourf_gniourf Are you comming from php? You could use eval in such case where you know which kind of data could be evalued. In this situation, command netstat -i won't return backtick or such cross-scripting echapments.

gniourf_gniourf Over a year ago

Nope, not coming from php at all. Just that I know what language is suited for what task (or at least, I believe I do).

gniourf_gniourf Over a year ago

Btw, you can achieve the same without any eval's (using indirect expansion), but what's the point anyways?

gniourf_gniourf Over a year ago

It's not really the purpose of this post to explain that but something around this (that will be safe regarding everything): array=(field0 field1); name="array[@]"; echo "${!name}"; keyfield0=0; namefield0="array[0]"; echo "${!namefield0}", then build up on this (with helper functions).

|

yes · Accepted Answer · 2022-12-02 10:51:15Z

0

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[@]}

answered Dec 2, 2022 at 10:51

yes

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