Check if Swift program is outputting to terminal

Question

How can I check from my Swift script whether or not I am outputting to a terminal?

In a bash or zsh script, for example, this can be accomplished with the -t 1 conditional expression to check if file descriptor 1 (stdout) is open and associated with a terminal device:

if [[ -t 1 ]]; then
  echo "\e[31m"HELLO"\e[0m"           # colorize output
else
  echo hello
fi

Notice the difference in formatting when the script is run in an interactive shell and not being piped to another program:

./script
↪︎ HELLO

./script | tee
↪︎ hello

What is the equivalent in Swift?

#!/usr/bin/env swift

if ❓ {
  print("\u{001B}[31m" + "HELLO" + "\u{001B}[0m")
} else {
  print("hello")
}

Answer 1

The isatty function returns 1 if the given file descriptor is associated with a terminal. Like many C library functions, isatty can be called from Swift via the Darwin module:

import Darwin

if isatty(STDOUT_FILENO) == 1 {
  print("\u{001B}[31m" + "HELLO" + "\u{001B}[0m")
} else {
  print("hello")
}

To make this run on both macOS and Linux, replace the import Darwin by

#if os(Linux)
  import Glibc
#else
  import Darwin
#endif