Multi-dimensional arrays in Bash

Question

I am planning a script to manage some pieces of my Linux systems and am at the point of deciding if I want to use bash or python.

I would prefer to do this as a Bash script simply because the commands are easier, but the real deciding factor is configuration. I need to be able to store a multi-dimensional array in the configuration file to tell the script what to do with itself. Storing simple key=value pairs in config files is easy enough with bash, but the only way I can think of to do a multi-dimensional array is a two layer parsing engine, something like

array=&d1|v1;v2;v3&d2|v1;v2;v3

but the marshall/unmarshall code could get to be a bear and its far from user friendly for the next poor sap that has to administer this. If i can't do this easily in bash i will simply write the configs to an xml file and write the script in python.

Is there an easy way to do this in bash?

thanks everyone.

Answer 1

Bash doesn't have multi-dimensional array. But you can simulate a somewhat similar effect with associative arrays. The following is an example of associative array pretending to be used as multi-dimensional array:

declare -A arr
arr[0,0]=0
arr[0,1]=1
arr[1,0]=2
arr[1,1]=3
echo "${arr[0,0]} ${arr[0,1]}" # will print 0 1

If you don't declare the array as associative (with -A), the above won't work. For example, if you omit the declare -A arr line, the echo will print 2 3 instead of 0 1, because 0,0, 1,0 and such will be taken as arithmetic expression and evaluated to 0 (the value to the right of the comma operator).

Answer 2

Bash does not support multidimensional arrays, nor hashes, and it seems that you want a hash that values are arrays. This solution is not very beautiful, a solution with an xml file should be better :

array=('d1=(v1 v2 v3)' 'd2=(v1 v2 v3)')
for elt in "${array[@]}";do eval $elt;done
echo "d1 ${#d1[@]} ${d1[@]}"
echo "d2 ${#d2[@]} ${d2[@]}"

EDIT: this answer is quite old, since since bash 4 supports hash tables, see also this answer for a solution without eval.

Answer 3

This works thanks to 1. "indirect expansion" with ! which adds one layer of indirection, and 2. "substring expansion" which behaves differently with arrays and can be used to "slice" them as described https://stackoverflow.com/a/1336245/317623

# Define each array and then add it to the main one
SUB_0=("name0" "value 0")
SUB_1=("name1" "value;1")
MAIN_ARRAY=(
  SUB_0[@]
  SUB_1[@]
)

# Loop and print it.  Using offset and length to extract values
COUNT=${#MAIN_ARRAY[@]}
for ((i=0; i<$COUNT; i++))
do
  NAME=${!MAIN_ARRAY[i]:0:1}
  VALUE=${!MAIN_ARRAY[i]:1:1}
  echo "NAME ${NAME}"
  echo "VALUE ${VALUE}"
done

It's based off of this answer here

Answer 4

If you want to use a bash script and keep it easy to read recommend putting the data in structured JSON, and then use lightweight tool jq in your bash command to iterate through the array. For example with the following dataset:

[

    {"specialId":"123",
    "specialName":"First"},

    {"specialId":"456",
    "specialName":"Second"},

    {"specialId":"789",
    "specialName":"Third"}
]

You can iterate through this data with a bash script and jq like this:

function loopOverArray(){

    jq -c '.[]' testing.json | while read i; do
        # Do stuff here
        echo "$i"
    done
}

loopOverArray

Outputs:

{"specialId":"123","specialName":"First"}
{"specialId":"456","specialName":"Second"}
{"specialId":"789","specialName":"Third"}