The configuration file for arduino-cli is typically located at "~/.arduino15/arduino-cli.yaml" in GNU/Linux systems. You may need to create the file first or run "$ arduino-cli config init".

To check if your arduino-cli installation is actually using your configuration, you can run the following command, which outputs the configuration used:

$ arduino-cli config dump

Here are available options:

Section Option Description
default_board string Default FQBN board name used if none specified
board_manager.additional_urls list of strings URLs to third-party board package indices
directories data, downloads, user Paths for CLI data, downloads, and user sketch directory
logging level, file, format CLI logging level, log file path, and output format
network.proxy string Proxy URL for network access
locale string CLI output language
sketch.always_export_binaries bool Export compiled binaries alongside sketch
compile warnings, optimize_for, build_cache Compiler warning level, optimization, build cache
upload verify, programmer Upload verification and programmer type
library.enable_unsafe_install bool Allow installing unsigned libraries
core.enable_unsafe_install bool Allow installing unsigned cores
telemetry.enabled bool Enable or disable telemetry data sending
updater.enable_notification bool Show update notifications
errors.show_stack_trace bool Show stack traces on errors for debugging
cli.network_timeout_seconds int Network timeout duration in seconds
custom any Custom user-defined options

Here is a big example of configuration with nearly all available options: 

# Arduino CLI Configuration File (~/.arduino15/arduino-cli.yaml)
# Complete example including nearly all available options with detailed explanations

# --------------------------------------------------
# Default Board: Fully Qualified Board Name (FQBN) used when no --fqbn is specified
# Example: "arduino:avr:uno"
default_board: arduino:avr:uno

# --------------------------------------------------
# Board Manager: Additional URLs for 3rd-party board package indices
# These URLs will be used when updating cores
board_manager:
  additional_urls:
    - https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_index.json
    - http://arduino.esp8266.com/stable/package_esp8266com_index.json

# --------------------------------------------------
# Directory paths used by Arduino CLI
# Defaults:
#   data: ~/.arduino15
#   downloads: ~/.arduino15/downloads
#   user: ~/Arduino
directories:
  # Directory where Arduino CLI stores data (cores, libraries, tools, etc.)
  data: /home/user/.arduino15
  # Directory for downloads (cores, tools, libs)
  downloads: /home/user/.arduino15/downloads
  # User sketch directory where your .ino files are stored
  user: /home/user/Arduino

# --------------------------------------------------
# Logging configuration: controls CLI logging behavior
logging:
  # Log level: trace, debug, info, warn, error, fatal, panic
  level: info
  # Log file path (optional)
  file: /home/user/arduino-cli.log
  # Log output format: "text" or "json"
  format: text

# --------------------------------------------------
# Network proxy settings (optional)
network:
  # Proxy URL, e.g. "http://user:[email protected]:8080"
  proxy: ""

# --------------------------------------------------
# CLI output language / locale (e.g., en, de, fr, es)
locale: en

# --------------------------------------------------
# Serial Monitor options
monitor:
  # Default baud rate if not specified by the board or overridden on CLI
  baudrate: 9600
  # Echo input typed in monitor back to the screen
  echo: false
  # Quiet mode reduces output noise
  quiet: false
  # Raw mode disables formatting, outputs raw bytes
  raw: false
  # Adds timestamp to each line of output
  timestamp: false

# --------------------------------------------------
# Sketch build options
sketch:
  # If true, exports binaries after compilation
  always_export_binaries: false

# --------------------------------------------------
# Compile options
compile:
  # Warning level: none, default, more, all
  warnings: default
  # Optimization target: "size" or "speed"
  optimize_for: speed
  # Enables build caching for faster subsequent builds
  build_cache: true

# --------------------------------------------------
# Upload options
upload:
  # Verify the upload after flashing
  verify: true
  # Programmer type (e.g., arduino, avrisp, usbtiny)
  programmer: arduino

# --------------------------------------------------
# Library manager options
library:
  # Allow installing unsigned (unsafe) libraries
  enable_unsafe_install: false

# --------------------------------------------------
# Core manager options (board packages)
core:
  # Allow installing unsigned (unsafe) cores
  enable_unsafe_install: false

# --------------------------------------------------
# Telemetry settings (send usage data to improve Arduino CLI)
telemetry:
  enabled: false

# --------------------------------------------------
# Updater settings
updater:
  # Enable notification about new Arduino CLI releases
  enable_notification: true

# --------------------------------------------------
# Error handling options
errors:
  # Show full stack traces on errors (useful for debugging)
  show_stack_trace: false

# --------------------------------------------------
# Advanced CLI options (may vary with versions)
cli:
  # Timeout for network operations in seconds
  network_timeout_seconds: 60

# --------------------------------------------------
# Custom user options (placeholders for future or user-specific config)
custom:
  example_option: example_value