Shing Lyu's Random Tech Facts

Generating a notification sound in command-line

You want to play a notification sound in a Linux shell script, but don’t want to download any sound files (e.g. MP3, WAV, MIDI). Here is a script that generates the sound on-the-fly:

First install SoX, the Swiss Army knife of sound processing programs.

sudo apt-get install sox

Then:

play -n synth 0.3 pluck A3 vol -20dB repeat 2

• play: the player command from SoX.
• -n: Play a “null file”.
• synth 0.3 pluck A3: Play the A3 (220.00 Hz) sound for 0.3 seconds, with a waveform that simulates a guitar string pluck.
• vol -20dB: The default is too loud, reduce the volume by -20dB.
• repeat 2: Repeat twice (i.e., total 3 times).

How to import serde's custom derive macors properly?

TL;DR Follow the official documentation

A little bit of history:

• Before Rust 2018, you do:

extern crate serde;
#[macro_use] extern crate serde_derive; // Imports the procedural macros

#[derive(Serialize, Deserialize)]
struct Foo;

• But Rust 2018 introduced new way of doing this:

use serde_derive::{Serialize, Deserialize}; // Imports the procedural macros

#[derive(Serialize, Deserialize)]
struct Foo;

• However, the serde crate re-exports the serde_derive::{Serialize, Deserialize} macros, hidden behind the feature flag derive. So if you enable the derive feature you can get both the Serialize/Deserialize traits and procedural macros (i.e., the custom derive) from the serde crate by one single use:

// Cargo.toml
[dependencies]
serde = { version = "1.0", features = ["derive"] }

// src/main.rs or lib.rs
use serde::{Serialize, Deserialize}; // Imports both the traits and procedural macros

#[derive(Serialize, Deserialize)]
struct Foo;

• You can choose not to use the derive feature. But you’ll run into this issue if any of your dependencies enables the derive feature. You can either:
  • Just enable derive (recommended).
  • Use serde::ser::Serialize and serde::de::Deserialize to get the trait and keep using serde_derive::{Serialize, Deserialize} to get the procedural macros.

Caveats in Terraforming WAF V2 for CloudFront

If you want to add a WAF V2 (aws_wafv2_web_acl) to a CloudFront distribution (aws_cloudfront_distribution) using Terraform, there are a few caveats:

• On aws_wafv2_web_acl:
  • Use scope = "CLOUDFRONT".
  • Use the AWS provider in us-east-1 region. (Although in the AWS Console it will still be listed under “Global”.)
• On aws_cloudfront_distribution:
  • You can use web_acl_id - aws_wafv2_web_acl.<name>.arn. Be careful that even though the name is id, but you need to pass the ARN instead. This bug only happens in V2.
  • The IAM user/role you use to execute terraform apply must have the waf:GetWebACL permission.

Check MongoDB index usage statistics

Run this command to get the usage of each index:

db.<collection>.aggregate( [ { $indexStats: { } } ] )

This is useful for figuring out which index is underutilized. You can find an example output in the documentation.

RFC2616 is dead, read RFC723X

While looking for the definition of HTTP Status Codes, I realize that RFC2616 is deprecated. You should look for RFC7231 instead.

Reference: mnot’s blog: RFC2616 is Dead