Environment and CLI

ENV variables

Programs read their configuration from ENV, which maps process environment variables by name.

Fetch with a fallback keeps development simple while production passes the real value.

mode = ENV.fetch("APP_MODE", "development") puts mode

APP_MODE=production ruby app.rb

production

ARGV

Command line arguments arrive in ARGV as an array of strings.

Length counts the arguments and first reads the leading one.

puts ARGV.length puts ARGV.first

ruby args.rb one two

2
one

OptionParser

OptionParser turns flags such as verbose into an options hash with help text built in.

The on call registers each flag, and parse! consumes the flags from ARGV.

require "optparse" options = { verbose: false } OptionParser.new do |parser| parser.on("-v", "--verbose", "Run verbosely") do options[:verbose] = true end end.parse! p options

ruby flags.rb --verbose

{verbose: true}

Exit codes

A nonzero exit status signals failure to the shell and to calling scripts.

Warn writes the complaint to standard error, and exit 1 stops the program with status 1.

if ARGV.empty? warn("Need a file name") exit(1) end puts "Got #{ARGV.first}"

ruby check.rb

Need a file name

Next: Logging

Article author: Arthur Isaev