notes: bash has hashmaps, oh god

Signed-off-by: Xe Iaso <me@xeiaso.net>

1 files changed, 75 insertions, 0 deletions

diff --git a/lume/src/notes/2024/bash-hashmap.mdx b/lume/src/notes/2024/bash-hashmap.mdx
new file mode 100644
index 0000000..bafb478
--- /dev/null
+++ b/lume/src/notes/2024/bash-hashmap.mdx
@@ -0,0 +1,75 @@
+---
+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.