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---
title: Today I learned that bash has hashmaps
date: 2024-01-15
tags:
- bash
- til
- crimes
---
Hashmaps (associative arrays) are a great way to store a bag of key-value data. At work I was writing something that needed me to spawn a bunch of GPU instances, GPU availability is spread out by region and GPU type. I wanted to store a mapping of GPU kind to region name and for some reason I thought it would be a good idea to do it in bash. I was horrified to find out that bash has hashmaps, and decided to write this note to tell you how to use them.
Here's how you do it:
First, declare the hashmap with the `declare` builtin:
```shell
# -A means associative array (hashmap)
declare -A FLY_REGIONS
```
Then add some values into it [from the fly.io documentation](https://fly.io/docs/gpus/gpu-quickstart/):
```shell
FLY_REGIONS["a100-40gb"]="ord"
FLY_REGIONS["a100-80gb"]="mia"
FLY_REGIONS["l40s"]="ord"
```
You can also predeclare the array with values in a read-only state:
```sh
# -A means associative array (hashmap)
# -r means read-only
declare -A -r FLY_REGIONS=(
["a100-40gb"]="ord"
["a100-80gb"]="mia"
["l40s"]="ord"
)
```
You can look up values with the same syntax you used to set them:
```shell
echo "${FLY_REGIONS["a100-40gb"]}" # ord
```
You can iterate over the keys with a `for` loop on the `@` value:
```shell
for size in "${!FLY_REGIONS[@]}"; do
echo "size: $size, region: ${FLY_REGIONS[$size]}"
done
```
You can iterate over the values with a `for` loop on the `*` value:
```shell
for region in "${FLY_REGIONS[*]}"; do
echo "region: $region"
done
```
You can delete individual keys with `unset`:
```shell
unset FLY_REGIONS["a100-40gb"]
```
If you need to delete the entire hashmap, you can also do it with `unset`:
```shell
unset FLY_REGIONS
```
Please don't use this for evil.
|
