cor

Colorize JSON-structured log lines from stdin.

cor reads newline-delimited JSON log entries from stdin and prints colorized, human-readable output to stdout. Non-JSON lines pass through unchanged.

$ echo '{"level":"info","ts":"2026-01-15T10:30:01.456Z","msg":"server started","host":"0.0.0.0","port":8080}' | cor

2026-01-15T10:30:01.456   INFO: server started
                                host: 0.0.0.0
                                port: 8080

Demo

Default output

cat logs.jsonl | cor

Level filtering

cat logs.jsonl | cor --level warn

Include specific fields

cat logs.jsonl | cor -i method,path,status

Exclude fields

cat logs.jsonl | cor -e func,query

JSON passthrough

cat logs.jsonl | cor --json --level error

Field truncation

cat logs.jsonl | cor --max-field-length 20

Features

• Auto-detects fields from logrus, zap, slog, pino, bunyan, structlog, and more
• Embedded JSON — handles lines like 2026-01-15 10:30:00 {"level":"info",...}
• Level filtering — --level warn suppresses debug and info
• Numeric levels — bunyan/pino 30→info, 40→warn, etc.
• Custom keys — --message-key, --level-key, --timestamp-key
• Field filtering — --include-fields or --exclude-fields
• JSON passthrough — --json outputs filtered JSON for piping
• Truncation — long values truncated at 120 chars (configurable)
• Config file — ~/.config/cor/config.toml for persistent settings
• NO_COLOR — respects no-color.org convention
• Fast — ~400K lines/sec, O(line-length) memory, streaming I/O

Install

From crates.io

cargo install cor

From source

git clone https://github.com/alexsavio/jgb.git
cd jgb
cargo install --path .

Usage

# Pipe any JSON log stream
my-app | cor

# Filter by level
kubectl logs my-pod | cor --level warn

# Custom keys
my-app | cor --message-key event --level-key severity

# Only show specific fields
my-app | cor --include-fields=host,port,status

# Hide noisy fields
my-app | cor --exclude-fields=pid,hostname

# Output filtered JSON (for piping)
my-app | cor --level error --json | jq .

# Disable truncation
my-app | cor --max-field-length=0

# Force colors in pipes
my-app | cor --color=always | less -R

Output format

YYYY-MM-DDTHH:MM:SS.mmm  LEVEL: message
                                      key: value
                                other_key: other_value

• Timestamp — bold YYYY-MM-DDTHH:MM:SS.mmm in UTC
• Level — colored and bold, right-justified in a 5-char field
  • TRACE · DEBUG · INFO · WARN · ERROR · FATAL
• Message — plain text
• Extra fields — one per line, key right-justified to 25 chars, bold gray

Log levels

These levels are recognized (case-insensitive, with aliases):

Custom level aliases can be defined in the config file.

Auto-detected fields

cor scans for well-known field names used by popular logging frameworks:

CLI flags (--message-key, --level-key, --timestamp-key) override auto-detection.

Embedded JSON

Lines with a text prefix before JSON are detected automatically:

2026-01-15 10:30:00.123 {"level":"info","msg":"server started","port":8080}
[INFO] 2026-01-15T10:30:01Z {"level":"debug","msg":"config loaded"}
myapp | {"level":"warn","msg":"disk space low"}

The prefix is preserved in the output after the level badge.

Nested objects

Nested JSON objects are flattened using dot notation:

{"level":"info","msg":"req","http":{"method":"GET","status":200}}

INFO: req
              http.method: GET
              http.status: 200

Config file

cor loads ~/.config/cor/config.toml (or $XDG_CONFIG_HOME/cor/config.toml) if present. CLI flags always take precedence.

# Default minimum level
level = "info"

# Color mode: auto, always, never
color = "auto"

# Timestamp display format (strftime)
timestamp_format = "%Y-%m-%dT%H:%M:%S%.3f"

# Max field value length (0 = unlimited)
max_field_length = 120

# Examples of custom timestamp formats:
# timestamp_format = "%H:%M:%S%.3f"    # time only with milliseconds
# timestamp_format = "%H:%M:%S"        # time only, no milliseconds

# Override field key names
[keys]
message = "msg"
level = "level"
timestamp = "ts"

# Map custom level names → standard levels
[levels]
"verbose" = "debug"
"critical" = "fatal"
"success" = "info"

Environment variables

--color=always and --color=never override all environment variables.

CLI reference

cor [OPTIONS]

Options:
  -c, --color <COLOR>              Color mode [default: auto] [values: auto, always, never]
  -l, --level <LEVEL>              Minimum severity level [values: trace, debug, info, warn, error, fatal]
  -m, --message-key <KEY>          Override message field key
      --level-key <KEY>            Override level field key
  -t, --timestamp-key <KEY>        Override timestamp field key
  -i, --include-fields <FIELDS>    Only show these fields (comma-separated)
  -e, --exclude-fields <FIELDS>    Hide these fields (comma-separated)
  -j, --json                       Output raw JSON instead of colorized text
  -M, --max-field-length <N>       Max field value length [default: 120]
      --config <PATH>              Path to config file
  -h, --help                       Print help
  -V, --version                    Print version

License

MIT — Alexandre Savio