Indirection on a Bash array

Question

Is there a possibility to write the following script without the loop?

IPv4_first=1.1.1.1
IPv4_second=2.2.2.2
IPv4_third=3.3.3.3

IPv4_all=() 

for var in ${!IPv4_@}
do
   IPv4_all+=(${!var})
done

printf "'%s'\n" "${IPv4_all[@]}"

Something like:

IPv4_all=${!${!IPv4_@}}

Answer 1

This might be the ugliest Bash code I've ever written, but...

IPv4_first=1.1.1.1
IPv4_second=2.2.2.2
IPv4_third=3.3.3.3

names=(${!IPv4_@})
eval "IPv4_all=(${names[@]/#/$})"
printf "'%s'\n" "${IPv4_all[@]}"

Look Ma, no loop!

${names[@]/#/$} prepends $ to the start of every element of the array, by matching an empty string anchored to the start of each element. That gives an array of variable dereferences, which we can expand inside eval to get the variable references inside the array initialiser. These need to be two separate lines because you can't apply multiple parameter expansions at the same time.

The output is:

'1.1.1.1'
'2.2.2.2'
'3.3.3.3'

as expected.

It's possible to replace the line with:

IPv4_all=($(eval "echo ${names[@]/#/$}"))

rather than evalling the array assignment. I'm not sure whether that's any better or not.

If your variable values might contain spaces or other IFS characters, you can change the eval:

eval "IPv4_all=($(printf '"$%s" ' "${names[@]}"))"

That properly double-quotes all the variable dereferences.

Answer 2

You don't need IPv4_all variable:

eval printf "\'%s\'\\\n" $(printf "$%s\n" ${!IPv4_@})

Output:

'1.1.1.1'
'2.2.2.2'
'3.3.3.3'

Answer 3

My competition entry on the ugliest/most convoluted bash code ;-):

eval 'declare(){ v=${2%%=*};[[ $v = IPv4_* ]]&&IPv4_all+=("${!v}");};'"$(declare -p)"
unset -f declare

Answer 4

This works for me, I think, though I'll accept that I could be missing some fundamental point:

IPv4_first=1.1.1.1
IPv4_second=2.2.2.2
IPv4_third=3.3.3.3

IPv4_all=( $(set | sed '/IPv4_.*[=)]/!d;s///') )

printf "'%s'\n" "${IPv4_all[@]}"

OUTPUT

'1.1.1.1'
'2.2.2.2'
'3.3.3.3'

This is better:

eval IPv4_all=( "$(set |
    grep -E '^IPv4_[_[:alnum:]]*=([^(]|$)' |
    sed 's/\([^=]*\).*/${\1+"$\1"} /')"
)

grep only gets safe lines that match your target var. sed surrounds them in parameter expansion tokens so they evaluate away to nothing if they're not actually current shell variable names.