The program counts the number of words or lines sent as input.

word counter

The program reads from standard input (STDIN) and returns the number of words or lines. It leverages a function called count which takes in the io.Reader interface and returns an int. Any concrete type with a Read method implements the io.Reader interface and is allowed to be passed to count as an argument. os.Stdin implements the io.Reader interface so the input data can be read with bufio.NewScanner(r). By default the scanner reads line by line so we need to instruct the function to read each word. The function then processes the input by iterating over each token with bufio.Scanner.Scan(). A command line flag is used to scan each line instead of words.

❯ cd wc
echo "My first command line tool with Go" | ./wc
7

❯ cat main.go | wc -l
38

Cross compilation is a process of compiling programs for different platforms than the one you are currently on. By default, the current operating system and architecture is detected when using the go build tool. We can prefix the command with the GOOS environment variable (which stands for Go Operating System) to change the target operating system.

GOOS=darwin go build
GOOS=linux go build
GOOS=windows go build

Run go env to reveal all your environment variable settings.

The result of the go build command is a static binary. All the dependencies are packaged inside. So no other runtime dependencies are required. Platforms with a static program binary don't even need Go installed.

On Windows, the word counter static binary could be used like so:

echo "Testing wc command on Windows" | wc.exe

todo

❯ cd todo/cmd/todo

❯ go build .

❯ ./todo -add This item comes from Args
❯ echo "This item comes from STDIN" | ./todo -add

❯ ./todo -list
  1: This item comes from Args
  2: This item comes from STDIN

❯ ./todo -complete 2

❯ ./todo -list
  1: This item comes from Args
X 2: This item comes from STDIN

mdp

❯ cd mdp

❯ go build .

❯ ./mdp -file README.md

❯ ./mdp -file README.md -t template-fmt.html.tmpl