Bash - Pointer to Value in Associate Array?

Question

Is there a way in Bash to make a pointer to the value of a key in an associate array? Like this:

declare -A mapp
mapp=( ["key"]="${value}" )

for k in "${!mapp[@]}"; do 
    pointer="${mapp["${k}"]}"   # How do I do this?
done

Usually, you do not need to use a pointer, but I'm curious to see if there's a way to make one.

In a simpler situation (i.e., for normal/string variables), I would make a pointer like this:

pointer=b
read -p "Enter something: " b
eval pointer=\$${pointer}

How would I do this for an associate array? This doesn't work (skip the strikethroughed code):

~~declare -A mapp mapp=( ["first"]="${a}" ["second"]="${b}" ) for k in "${!mapp[@]}"; do v=mapp["${k}"] read -p "Enter ${k}: " new eval v=\$${v} # Doesn't work done~~

declare -A mapp
mapp=( ["first"]="${a}"  ["second"]="${b}" )

for k in "${!mapp[@]}"; do 
    v=mapp["${k}"]

    read -p "Enter ${k}: " k

    eval v=\$${v}    # Doesn't work

done

This doesn't work either (skip the strikethroughed code):

~~declare -A mapp mapp=( ["first"]="${a}" ["second"]="${b}" ) for k in "${!mapp[@]}"; do v=mapp read -p "Enter ${k}: " new eval v=\$${v["${k}"]} # Doesn't work (and has terrible readability) done~~

declare -A mapp
mapp=( ["first"]="${a}"  ["second"]="${b}" )

for k in "${!mapp[@]}"; do 
    v=mapp

    read -p "Enter ${k}: " k

    eval v=\$${v["${k}"]}    # Doesn't work (and has terrible readability)

done

So you don't mean "pointer" here at all really? You mean how do you indirectly/etc. get the value from array? — Etan Reisner
– Etan Reisner, Commented Apr 16, 2015 at 18:54
Sorry, I just realized I messed up something in my code. I'm trying to use the pointer to change the value in the array. — GreenRaccoon23
– GreenRaccoon23, Commented Apr 16, 2015 at 18:57
What about pointer="${mapp["${k}"]}" didn't work to get pointer to have the value you wanted? (Other than the inner quotes being unnecessary and wrong.) Or do you really mean a "pointer" (of sorts) so you can then use $pointer to get the current value from the associative array? — Etan Reisner
– Etan Reisner, Commented Apr 16, 2015 at 18:58
None of your examples are about assigning through the "pointer". Perhaps you want to fix that? — Etan Reisner
– Etan Reisner, Commented Apr 16, 2015 at 19:00

glenn jackman · Accepted Answer · 2015-04-16 19:05:01Z

8

In bash 4.3, you can use a nameref:

$ mapp=([key]=value)
$ declare -n x=mapp[key]  # NO dollar sign!
$ x=7
$ echo ${mapp[key]}
7

Before 4.3, you need to use the declare command differently to do the indirection.

$ mapp=([key]=value)
$ x=mapp[key]  # NO dollar sign!
$ declare "$x=7"
$ echo ${mapp[key]}
7

edited Apr 16, 2015 at 19:05

glenn jackman

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answered Apr 16, 2015 at 19:02

chepner

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1 Comment

GreenRaccoon23 Over a year ago

Perfect! Thanks! I never thought to check the options for declare.

glenn jackman · Accepted Answer · 2015-04-16 19:01:30Z

1

No problem:

$ declare -A ary=([foo]=bar [baz]=qux)
$ key=foo
$ pointer="ary[$key]"
$ echo "$pointer"
ary[foo]
$ echo "${!pointer}"
bar

A "pointer" in this sense is an indirect variable

answered Apr 16, 2015 at 19:01

glenn jackman

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3 Comments

glenn jackman Over a year ago

Ah, I missed the point about using the "pointer" to set a value.

Etan Reisner Over a year ago

That's ok. So did the OP for a while. =)

GreenRaccoon23 Over a year ago

Touché Etan Reisner :P

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