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Parsing JSON Logs in the Terminal ​

Process structured JSON logs with fx. Filter by log level, search for errors, and extract relevant information from log streams.

View Streaming Logs ​

Use fx with streaming mode for line-delimited JSON logs:

bash
tail -f app.log | fx

Each line is parsed as a separate JSON object and displayed interactively.

Filter by Log Level ​

Show only error logs:

bash
... | fx 'filter(l => l.level === "error")'

Search Log Messages ​

Find logs containing specific text:

bash
... | fx 'filter(l => l.message.includes("database"))'

Count Logs by Level ​

Aggregate log counts:

bash
cat app.log | fx --slurp 'x => {
  const counts = {};
  x.forEach(l => counts[l.level] = (counts[l.level] || 0) + 1);
  return counts;
}'

Output:

json
{
  "info": 1523,
  "warn": 45,
  "error": 12,
  "debug": 890
}

Extract Unique Error Types ​

List distinct error messages:

bash
cat app.log | fx --slurp 'filter(l => l.level === "error")' 'map(l => l.message)' uniq

Output:

json
[
  "Connection refused",
  "Timeout exceeded",
  "Invalid token"
]

Format Logs for Reading ​

Create a human-readable output:

bash
cat app.log | fx 'map(l => `[${l.level.toUpperCase()}] ${l.timestamp}: ${l.message}`)'

Output:

[ERROR] 2024-01-15T10:23:45Z: Connection refused
[WARN] 2024-01-15T10:23:46Z: High memory usage
[INFO] 2024-01-15T10:23:47Z: Request completed