This gist contains the cat Unix command—written in Go.

Cat in Go.md

Cat in Go

This gist contains the cat Unix command—written in Go.

The following Go program concatenates files passed as command-line arguments. If you don't pass any command-line arguments, then cat reads from stdin (it's the cat Unix utility).

Getting started

The following procedure shows how to build and run cat.

1. Navigate to a temporary directory.

   cd $(mktemp -d)
   

2. Download cat.go Go code.

   wget https://gist.github.com/mbigras/a1c96ec65deacb5fa037215849b5c2ba/raw/90bd1bc0d2db776604395b17baf6d846c1e1433e/cat.go
   

3. Build cat Go program.

   go build cat.go
   

4. Pass some input to cat interactively from stdin.

   ./cat
   

   Type something. Press return. Press control-D. Your output should look like the following.

   $ ./cat
   hello world # Manually type "hello world".
   hello world
   

5. Pass some input through a Unix pipe—also stdin.

   echo hello world | ./cat
   

   Your output should look like the following.

   $ echo hello world | ./cat
   hello world
   

6. Pass your input through a couple files.

   echo hello > foo.txt
   echo world > bar.txt
   ./cat foo.txt bar.txt
   

   Your output should look like the following.

   $ echo hello >foo.txt
   $ echo world >bar.txt
   $ ./cat foo.txt bar.txt
   hello
   world
   

7. Pass your input through a couple temporary files.

   ./cat <(echo hello) <(echo world)
   

   Your output should look like the following.

   $ ./cat <(echo hello) <(echo world)
   hello
   world
   

8. Pass some bad input.

   ./cat idontexist
   

   Your output should look like the following.

   $ ./cat idontexist
   2023/01/02 14:33:45 open idontexist: no such file or directory
   

9. Clean up.

   Navigate away and the temporary directory will get automatically cleaned up.

cat.go

// Program cat concatenates files passed as command-line arguments. If you don't pass any command-line arguments, then cat reads from stdin. Note: This implementation comes from dup2 in Go Programming Language by Kernighan and Donovan.
package main

import (
	"bufio"
	"fmt"
	"log"
	"os"
)

func main() {
	files := os.Args[1:]

	if len(files) == 0 {
		printLines(os.Stdin)
	} else {
		for _, arg := range files {
			f, err := os.Open(arg)
			if err != nil {
				log.Fatal(err)
			}
			printLines(f)
			f.Close()
		}
	}
}

func printLines(f *os.File) {
	input := bufio.NewScanner(f)
	for input.Scan() {
		fmt.Println(input.Text())
	}
	// Note: Ignore potential errors from input.Err() because that's what dup2 does. Todo: Figure out if I should account for EOF or other errors.
}