Create an array with all network interfaces in bash

Question

I want to create an array in Bash which to contain all active network interfaces.

I have managed to create a for loop printing them but when I try to create the array, it just contains the last lo interface but not the other. This is my code:

#!/bin/bash

for iface in $(ifconfig | cut -d ' ' -f1| tr ':' '\n' | awk NF)
do
           printf "$iface%s\n"
           declare -a array_test=["$iface"]
done
for i in "${array_test[@]}"; do echo "$i"; done

And this is my output:

eno1
eno2
eno3
lo
[lo]

Also, how can I exclude the lo localhost interface from the array?

Answer 1

Here is a solution, assign the list and then add item to it:

#!/bin/bash

array_test=()
for iface in $(ifconfig | cut -d ' ' -f1| tr ':' '\n' | awk NF)
do
        printf "$iface\n"
        array_test+=("$iface")
done
echo ${array_test[@]}

If you want the output displayed one item per line:

for i in "${array_test[@]}"; do echo "$i"; done

To remove localhost from output:

if [ "$iface" != "lo" ] 
then
    array_test+=("$iface")
fi

Answer 2

My try:

readarray -t interfaces < <(ip l | awk -F ":" '/^[0-9]+:/{dev=$2 ; if ( dev !~ /^ lo$/) {print $2}}')
for i in "${interfaces[@]// /}" ; do echo "$i" ; done

Answer 3

bash will construct an array from any white-space delimited (spaces, tabs, newlines) list you give it. e.g. array=(a b c). We can use command substitution ($()) to generate such a white-space delimited list. For example:

$ ifaces=( $(ip addr list | awk -F': ' '/^[0-9]/ {print $2}') )

and now print out the array we just created:

$ declare -p ifaces
declare -a ifaces=([0]="lo" [1]="eth0" [2]="eth1" [3]="br1" [4]="br0" [5]="ppp0")

To exclude lo:

$ ifaces=( $(ip addr list | awk -F': ' '/^[0-9]/ && $2 != "lo" {print $2}') )
$ declare -p ifaces
declare -a ifaces=([0]="eth0" [1]="eth1" [2]="br1" [3]="br0" [4]="ppp0")

If you really want to use ifconfig rather than ip, try this:

ifaces=( $(ifconfig | awk -F':'  '/^[^ ]*: / && $1 != "lo" {print $1}') )

Answer 4

ifconfig only outputs the interfaces that are up.

On Linux, you can get a list of interfaces in the /sys/class/net directory. So, with zsh, you can do:

interfaces=(/sys/class/net/*(DN:t))

To exclude lo:

set -o extendedglob
interfaces=(/sys/class/net/^lo(DN:t))

For only the up ones, you could do:

interfaces=(/sys/class/net/*(DNe{'[[ $(<$REPLY/operstate) = up ]]'}:t)

or, assuming the default value of $IFS.

interfaces=($(ip -br link | LC_ALL=C awk '$2 == "UP" {print $1}'))

On my system at least the status for lo is UNKNOWN so would be excluded.

That would also work with bash provided interface names don't contain wildcard characters. In recent versions of Linux, IIRC, interface names are guaranteed not to contain space tab and newline characters (the default $IFS), so those would not cause problem. Other Unicode blanks are allowed though, hence the LC_ALL=C which should guarantee that awk won't split on those.

Answer 5

If you have '-j' support for ip (json output) then use the excellent jq utility;

# ip -j addr show | jq -r .[].ifname
lo
eth0
eth1
tunl0
gre0
gretap0
erspan0
ip_vti0
ip6_vti0
sit0
ip6tnl0
ip6gre0
dummy0
kube-ipvs0
cbr0
vethfbcab624

(example from a kubernetes node)