复制变量的值，而不是引用bash脚本

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

Answer 1

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 .

Answer 2

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 (ie 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 .