logfmt — log string formatting cheat sheet

log::info!("hello {}, the answer is {}", "world", 42);

[2026-05-09T14:15:15Z INFO  env_logger_default] hello world, the answer is 42

let users = vec!["alice", "bob", "charlie"];
log::info!("users: {:?}", users);

[2026-05-09T14:15:15Z INFO  env_logger_default] users: ["alice", "bob", "charlie"]

#[derive(Debug)]
struct User { name: &'static str, age: u32, roles: Vec<&'static str> }

let user = User { name: "alice", age: 30, roles: vec!["admin", "editor"] };
log::info!("user: {:#?}", user);

[2026-05-09T14:15:15Z INFO  env_logger_default] user: User {
        name: "alice",
        age: 30,
        roles: [
            "admin",
            "editor",
        ],
    }

let name = "alice";
let age = 30;
log::info!("user {name} is {age} years old");

[2026-05-09T14:15:15Z INFO  env_logger_default] user alice is 30 years old

let n = 255u32;
log::info!("hex={:x}  octal={:o}  binary={:b}", n, n, n);

[2026-05-09T14:15:15Z INFO  env_logger_default] hex=ff  octal=377  binary=11111111

let n = 255u32;
log::info!("padded={:>5}", n);

[2026-05-09T14:15:15Z INFO  env_logger_default] padded=  255

log::info!("precision={:.2}", 3.14159);

[2026-05-09T14:15:15Z INFO  env_logger_default] precision=3.14

log::info!(answer = 42; "plain value");
log::info!(items:? = vec![1, 2, 3]; "? sigil = Debug");
log::info!(name:% = "world"; "% sigil = Display");

[2026-05-09T14:15:15Z INFO  env_logger_default] plain value
[2026-05-09T14:15:15Z INFO  env_logger_default] ? sigil = Debug
[2026-05-09T14:15:15Z INFO  env_logger_default] % sigil = Display

tracing::info!("hello {}, the answer is {}", "world", 42);

2026-05-09T14:15:15.224467Z  INFO tracing_full: hello world, the answer is 42

let users = vec!["alice", "bob", "charlie"];
tracing::info!("users: {:?}", users);

2026-05-09T14:15:15.225942Z  INFO tracing_full: users: ["alice", "bob", "charlie"]

#[derive(Debug)]
struct User { name: &'static str, age: u32, roles: Vec<&'static str> }

let user = User { name: "alice", age: 30, roles: vec!["admin", "editor"] };
tracing::info!("user: {:#?}", user);

2026-05-09T14:15:15.227643Z  INFO tracing_full: user: User {
    name: "alice",
    age: 30,
    roles: [
        "admin",
        "editor",
    ],
}

let name = "alice";
let age = 30;
tracing::info!("user {name} is {age} years old");

2026-05-09T14:15:15.229968Z  INFO tracing_full: user alice is 30 years old

let n = 255u32;
tracing::info!("hex={:x}  octal={:o}  binary={:b}", n, n, n);

2026-05-09T14:15:15.231669Z  INFO tracing_full: hex=ff  octal=377  binary=11111111

let n = 255u32;
tracing::info!("padded={:>5}", n);

2026-05-09T14:15:15.233263Z  INFO tracing_full: padded=  255

tracing::info!("precision={:.2}", 3.14159);

2026-05-09T14:15:15.234764Z  INFO tracing_full: precision=3.14

tracing::info!(answer = 42, "plain value");
tracing::info!(items = ?vec![1, 2, 3], "? sigil = Debug");
tracing::info!(name = %"world", "% sigil = Display");

2026-05-09T14:15:15.236170Z  INFO tracing_full: plain value answer=42
2026-05-09T14:15:15.236193Z  INFO tracing_full: ? sigil = Debug items=[1, 2, 3]
2026-05-09T14:15:15.236202Z  INFO tracing_full: % sigil = Display name=world

tracing::info!("hello {}, the answer is {}", "world", 42);

2026-05-09T14:15:15.238463Z  INFO tracing_compact: hello world, the answer is 42

let users = vec!["alice", "bob", "charlie"];
tracing::info!("users: {:?}", users);

2026-05-09T14:15:15.239893Z  INFO tracing_compact: users: ["alice", "bob", "charlie"]

#[derive(Debug)]
struct User { name: &'static str, age: u32, roles: Vec<&'static str> }

let user = User { name: "alice", age: 30, roles: vec!["admin", "editor"] };
tracing::info!("user: {:#?}", user);

2026-05-09T14:15:15.241629Z  INFO tracing_compact: user: User {
    name: "alice",
    age: 30,
    roles: [
        "admin",
        "editor",
    ],
}

let name = "alice";
let age = 30;
tracing::info!("user {name} is {age} years old");

2026-05-09T14:15:15.243983Z  INFO tracing_compact: user alice is 30 years old

let n = 255u32;
tracing::info!("hex={:x}  octal={:o}  binary={:b}", n, n, n);

2026-05-09T14:15:15.245662Z  INFO tracing_compact: hex=ff  octal=377  binary=11111111

let n = 255u32;
tracing::info!("padded={:>5}", n);

2026-05-09T14:15:15.247257Z  INFO tracing_compact: padded=  255

tracing::info!("precision={:.2}", 3.14159);

2026-05-09T14:15:15.248751Z  INFO tracing_compact: precision=3.14

tracing::info!(answer = 42, "plain value");
tracing::info!(items = ?vec![1, 2, 3], "? sigil = Debug");
tracing::info!(name = %"world", "% sigil = Display");

2026-05-09T14:15:15.250141Z  INFO tracing_compact: plain value answer=42
2026-05-09T14:15:15.250173Z  INFO tracing_compact: ? sigil = Debug items=[1, 2, 3]
2026-05-09T14:15:15.250182Z  INFO tracing_compact: % sigil = Display name=world

tracing::info!("hello {}, the answer is {}", "world", 42);

  2026-05-09T14:15:15.252405Z  INFO tracing_pretty: hello world, the answer is 42
    at src/bin/tracing_pretty.rs:14

let users = vec!["alice", "bob", "charlie"];
tracing::info!("users: {:?}", users);

  2026-05-09T14:15:15.253861Z  INFO tracing_pretty: users: ["alice", "bob", "charlie"]
    at src/bin/tracing_pretty.rs:18

#[derive(Debug)]
struct User { name: &'static str, age: u32, roles: Vec<&'static str> }

let user = User { name: "alice", age: 30, roles: vec!["admin", "editor"] };
tracing::info!("user: {:#?}", user);

  2026-05-09T14:15:15.255561Z  INFO tracing_pretty: user: User {
    name: "alice",
    age: 30,
    roles: [
        "admin",
        "editor",
    ],
}
    at src/bin/tracing_pretty.rs:22

let name = "alice";
let age = 30;
tracing::info!("user {name} is {age} years old");

  2026-05-09T14:15:15.257892Z  INFO tracing_pretty: user alice is 30 years old
    at src/bin/tracing_pretty.rs:27

let n = 255u32;
tracing::info!("hex={:x}  octal={:o}  binary={:b}", n, n, n);

  2026-05-09T14:15:15.259581Z  INFO tracing_pretty: hex=ff  octal=377  binary=11111111
    at src/bin/tracing_pretty.rs:31

let n = 255u32;
tracing::info!("padded={:>5}", n);

  2026-05-09T14:15:15.261166Z  INFO tracing_pretty: padded=  255
    at src/bin/tracing_pretty.rs:35

tracing::info!("precision={:.2}", 3.14159);

  2026-05-09T14:15:15.262689Z  INFO tracing_pretty: precision=3.14
    at src/bin/tracing_pretty.rs:38

tracing::info!(answer = 42, "plain value");
tracing::info!(items = ?vec![1, 2, 3], "? sigil = Debug");
tracing::info!(name = %"world", "% sigil = Display");

  2026-05-09T14:15:15.264076Z  INFO tracing_pretty: plain value, answer: 42
    at src/bin/tracing_pretty.rs:41

  2026-05-09T14:15:15.264103Z  INFO tracing_pretty: ? sigil = Debug, items: [1, 2, 3]
    at src/bin/tracing_pretty.rs:42

  2026-05-09T14:15:15.264113Z  INFO tracing_pretty: % sigil = Display, name: world
    at src/bin/tracing_pretty.rs:43

tracing::info!("hello {}, the answer is {}", "world", 42);

{"timestamp":"2026-05-09T14:15:15.266392Z","level":"INFO","fields":{"message":"hello world, the answer is 42"},"target":"tracing_json"}

let users = vec!["alice", "bob", "charlie"];
tracing::info!("users: {:?}", users);

{"timestamp":"2026-05-09T14:15:15.267890Z","level":"INFO","fields":{"message":"users: [\"alice\", \"bob\", \"charlie\"]"},"target":"tracing_json"}

#[derive(Debug)]
struct User { name: &'static str, age: u32, roles: Vec<&'static str> }

let user = User { name: "alice", age: 30, roles: vec!["admin", "editor"] };
tracing::info!("user: {:#?}", user);

{"timestamp":"2026-05-09T14:15:15.269662Z","level":"INFO","fields":{"message":"user: User {\n    name: \"alice\",\n    age: 30,\n    roles: [\n        \"admin\",\n        \"editor\",\n    ],\n}"},"target":"tracing_json"}

let name = "alice";
let age = 30;
tracing::info!("user {name} is {age} years old");

{"timestamp":"2026-05-09T14:15:15.272005Z","level":"INFO","fields":{"message":"user alice is 30 years old"},"target":"tracing_json"}

let n = 255u32;
tracing::info!("hex={:x}  octal={:o}  binary={:b}", n, n, n);

{"timestamp":"2026-05-09T14:15:15.273870Z","level":"INFO","fields":{"message":"hex=ff  octal=377  binary=11111111"},"target":"tracing_json"}

let n = 255u32;
tracing::info!("padded={:>5}", n);

{"timestamp":"2026-05-09T14:15:15.275832Z","level":"INFO","fields":{"message":"padded=  255"},"target":"tracing_json"}

tracing::info!("precision={:.2}", 3.14159);

{"timestamp":"2026-05-09T14:15:15.277784Z","level":"INFO","fields":{"message":"precision=3.14"},"target":"tracing_json"}

tracing::info!(answer = 42, "plain value");
tracing::info!(items = ?vec![1, 2, 3], "? sigil = Debug");
tracing::info!(name = %"world", "% sigil = Display");

{"timestamp":"2026-05-09T14:15:15.279472Z","level":"INFO","fields":{"message":"plain value","answer":42},"target":"tracing_json"}
{"timestamp":"2026-05-09T14:15:15.279508Z","level":"INFO","fields":{"message":"? sigil = Debug","items":"[1, 2, 3]"},"target":"tracing_json"}
{"timestamp":"2026-05-09T14:15:15.279523Z","level":"INFO","fields":{"message":"% sigil = Display","name":"world"},"target":"tracing_json"}

log::info!("hello {}, the answer is {}", "world", 42);

2026-05-09T14:15:15.282422Z  INFO tracing_log_bridge: hello world, the answer is 42

let users = vec!["alice", "bob", "charlie"];
log::info!("users: {:?}", users);

2026-05-09T14:15:15.284105Z  INFO tracing_log_bridge: users: ["alice", "bob", "charlie"]

#[derive(Debug)]
struct User { name: &'static str, age: u32, roles: Vec<&'static str> }

let user = User { name: "alice", age: 30, roles: vec!["admin", "editor"] };
log::info!("user: {:#?}", user);

2026-05-09T14:15:15.286161Z  INFO tracing_log_bridge: user: User {
    name: "alice",
    age: 30,
    roles: [
        "admin",
        "editor",
    ],
}

let name = "alice";
let age = 30;
log::info!("user {name} is {age} years old");

2026-05-09T14:15:15.289246Z  INFO tracing_log_bridge: user alice is 30 years old

let n = 255u32;
log::info!("hex={:x}  octal={:o}  binary={:b}", n, n, n);

2026-05-09T14:15:15.291155Z  INFO tracing_log_bridge: hex=ff  octal=377  binary=11111111

let n = 255u32;
log::info!("padded={:>5}", n);

2026-05-09T14:15:15.293082Z  INFO tracing_log_bridge: padded=  255

log::info!("precision={:.2}", 3.14159);

2026-05-09T14:15:15.294817Z  INFO tracing_log_bridge: precision=3.14

log::info!(answer = 42; "plain value");
log::info!(items:? = vec![1, 2, 3]; "? sigil = Debug");
log::info!(name:% = "world"; "% sigil = Display");

2026-05-09T14:15:15.296240Z  INFO tracing_log_bridge: plain value
2026-05-09T14:15:15.296266Z  INFO tracing_log_bridge: ? sigil = Debug
2026-05-09T14:15:15.296274Z  INFO tracing_log_bridge: % sigil = Display