The code and data behind xeiaso.net
notes: bash has hashmaps, oh god
Signed-off-by: Xe Iaso <me@xeiaso.net>
+75
+75
lume/src/notes/2024/bash-hashmap.mdx
+75
lume/src/notes/2024/bash-hashmap.mdx
···
1
+
---
2
+
title: Today I learned that bash has hashmaps
3
+
date: 2024-01-15
4
+
tags:
5
+
- bash
6
+
- til
7
+
- crimes
8
+
---
9
+
10
+
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.
11
+
12
+
Here's how you do it:
13
+
14
+
First, declare the hashmap with the `declare` builtin:
15
+
16
+
```shell
17
+
# -A means associative array (hashmap)
18
+
declare -A FLY_REGIONS
19
+
```
20
+
21
+
Then add some values into it [from the fly.io documentation](https://fly.io/docs/gpus/gpu-quickstart/):
22
+
23
+
```shell
24
+
FLY_REGIONS["a100-40gb"]="ord"
25
+
FLY_REGIONS["a100-80gb"]="mia"
26
+
FLY_REGIONS["l40s"]="ord"
27
+
```
28
+
29
+
You can also predeclare the array with values in a read-only state:
30
+
31
+
```sh
32
+
# -A means associative array (hashmap)
33
+
# -r means read-only
34
+
declare -A -r FLY_REGIONS=(
35
+
["a100-40gb"]="ord"
36
+
["a100-80gb"]="mia"
37
+
["l40s"]="ord"
38
+
)
39
+
```
40
+
41
+
You can look up values with the same syntax you used to set them:
42
+
43
+
```shell
44
+
echo "${FLY_REGIONS["a100-40gb"]}" # ord
45
+
```
46
+
47
+
You can iterate over the keys with a `for` loop on the `@` value:
48
+
49
+
```shell
50
+
for size in "${!FLY_REGIONS[@]}"; do
51
+
echo "size: $size, region: ${FLY_REGIONS[$size]}"
52
+
done
53
+
```
54
+
55
+
You can iterate over the values with a `for` loop on the `*` value:
56
+
57
+
```shell
58
+
for region in "${FLY_REGIONS[@]}"; do
59
+
echo "region: $region"
60
+
done
61
+
```
62
+
63
+
You can delete individual keys with `unset`:
64
+
65
+
```shell
66
+
unset FLY_REGIONS["a100-40gb"]
67
+
```
68
+
69
+
If you need to delete the entire hashmap, you can also do it with `unset`:
70
+
71
+
```shell
72
+
unset FLY_REGIONS
73
+
```
74
+
75
+
Please don't use this for evil.