Input and Command-Line Arguments

Reading from the keyboard

readline() reads one line of text typed by the user (without the newline):

# illustrative — waits for keyboard input
print("What's your name? ")
name = readline()
print("How old are you? ")
age = tryparse(Int, readline())
if age === nothing
    println("That's not a number.")
else
    println("Hi $name, next year you'll be $(age + 1).")
end

What's your name? Ada
How old are you? 36
Hi Ada, next year you'll be 37.

readline() always returns a String, so convert it with parse or tryparse. x === nothing is the idiomatic way to test for nothing (isnothing(x) also works).

Command-line arguments

Arguments after the script name arrive in the global vector ARGS, as strings:

# illustrative — run as: julia greet.jl Ada 3
name = length(ARGS) >= 1 ? ARGS[1] : "world"
times = length(ARGS) >= 2 ? parse(Int, ARGS[2]) : 1
for _ in 1:times
    println("Hello, $name!")
end
println("Script: ", PROGRAM_FILE)

Hello, Ada!
Hello, Ada!
Hello, Ada!
Script: greet.jl

PROGRAM_FILE holds the script's name. For real command-line tools, the ArgParse.jl or Comonicon.jl packages parse options like --verbose and generate help text.

Reading all of standard input

# illustrative — run as: echo "3 4 5" | julia sum.jl
nums = parse.(Int, split(read(stdin, String)))
println(sum(nums))

12

read(stdin, String) reads everything piped in. split breaks it into words and parse.(Int, ...) converts each one.