copy the value of the variable rather than reference bash script

Question

I am a bit new to the bash scripting. So please bear with me. I am trying to create a table and assign the values in for loop like this:

packages=("foo" "bar" "foobar")
packageMap=()
function test() {
    i=0;
    for package in "${packages[@]}"
    do
        echo $i
        packageMap[$package]=$i
        i=$(expr $i + 1)
    done
}

test
echo the first value is ${packageMap["foo"]}

The output for this is:

0
1
2
the first value is 2

While my expected output is:

0
1
2
the first value is 0

So basically the variable's reference is being assigned to this rather than the value. How to solve this?

My bash version:

GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin16)

TIA

BTW, i=$(( i + 1 )) is much more efficient than i=$(expr $i + 1), as $( ) needs to fork off a child process, and expr is implemented with an external command -- /bin/expr -- not part of the shell. The $(( )) form has been present in the POSIX sh standard since its initial publication in 1991, so there's also no concern about portability. — Charles Duffy
– Charles Duffy, Commented Jul 30, 2018 at 20:17
@CharlesDuffy : Apart of POSIX compatibility, is there an advantage of using i=$(( i + 1 )) over ((i++))? I find the latter easier to read and to type. — user1934428
– user1934428, Commented Jul 31, 2018 at 7:06
@user1934428, see BashFAQ #105 -- ((i++)) can (depending on the interpreter version) cause your script to exit when you're using set -e. ((++i)) is less prone to that issue (would only happen when i is initially negative). — Charles Duffy
– Charles Duffy, Commented Jul 31, 2018 at 11:11

chepner · Accepted Answer · 2018-07-30 20:26:47Z

6

bash 3.2 only has indexed arrays, so packageMap[$package] only works as intended if $package is an integer, not an arbitrary string.

(What you are observing is $package being evaluated in an arithmetic context, where foo, bar, and foobar are recursively expanded until you get an integer value. Undefined variables expand to 0, so packageMap[foo] is equivalent to packageMap[0].)

If you were using bash 4 or later, you could use an associative array:

packages=("foo" "bar" "foobar")
declare -A packageMap
test () {
    i=0
    for package in "${packages[@]}"
    do
        echo $i
        packageMap[$package]=$i
        i=$(($i + 1))
    done
}

Given that i is the same as the index of each element of packages, you could also write

test () {
    for i in "${!packages[@]}"; do
        package=${packages[i]}
        packageMap[$package]=$i
    done
}

instead of explicitly incrementing i.

edited Jul 30, 2018 at 20:26

answered Jul 30, 2018 at 20:11

chepner

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4 Comments

jeremysprofile Over a year ago

could you change i=$(($i+1)) to ((i++)) so that psykid learns the shortcut?

Charles Duffy Over a year ago

@jeremysprofile Better to suggest (( ++i )) so it doesn't trigger failures with set -e on first invocation (as the postincrement will evaluate to the initial value, and (( 0 )) is falsey). See the first exercise in BashFAQ #105 (granted, some versions of bash exempt arithmetic results from set -e behavior, but relying on that is creating a nonobvious version dependency).

psykid Over a year ago

@chepner, I tried to use this in bash 3.2 (before posting here), of course it didn't work. Unfortunately I cant install bash 4 since I don't have admin rights on that machine.

chepner Over a year ago

You don't need admin rights; you can install anything you want in your own home directory.

ghoti · Accepted Answer · 2018-07-31 01:19:24Z

0

As chep says, there are no associative arrays in bash 3. That said, if you don't mind wasting a bit of CPU, you can use functions to similar effect:

#!/bin/bash

packages=("foo" "bar" "foobar")

function packagemap () {
    local i
    for i in "${!packages[@]}"; do
        [[ ${packages[$i]} = $1 ]] && echo "$i" && return
    done
    echo "unknown"
}

echo "the first value is $(packagemap "foo")"

The ${!array[@]} construct expands to the set of indices for the array, which for a normally non-associative array consist of incrementing integers starting at 0. But array members can be removed without the indices being renumbered (i.e. unset packages[1]), so it's important to be able to refer to actual indices rather than assuming they're sequential with for loop that simply counts.

And I note that you're using Darwin. Remember that you really need it, you can install bash 4 using Homebrew or MacPorts.

edited Jul 31, 2018 at 1:19

answered Jul 30, 2018 at 20:39

ghoti

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1 Comment

psykid Over a year ago

Yes, I dont mind using up a bit of cpu since this script is for triggering the build command for my gradle packages. Since I cant install bash 4 on the machine I am using, I can only go in this way. Thanks for the solution.

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