Go pagination for CLI app output - detect if terminal and if it is use the available pager (e.g. less) #GoLang #CLI

go-cli-pager-less.go

package main

import (
	"fmt"
	"io"
	"os"
	"os/exec"
	"syscall"

	"golang.org/x/term"
)

// usePager attempts to pipe output to $PAGER if available and stdout is a TTY
// Returns a writer and a cleanup function
func usePager() (io.Writer, func() error) {
	// Don't use pager if output is redirected/piped
	if !term.IsTerminal(int(syscall.Stdout)) {
		return os.Stdout, func() error { return nil }
	}

	pager := os.Getenv("PAGER")
	if pager == "" {
		// Fallback to common pagers
		pager = "less"
		if _, err := exec.LookPath(pager); err != nil {
			pager = "more"
			if _, err := exec.LookPath(pager); err != nil {
				// No pager available, use stdout
				return os.Stdout, func() error { return nil }
			}
		}
	}

	cmd := exec.Command(pager)
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	// Set better defaults for less if it's being used
	if pager == "less" || (pager != "more" && os.Getenv("LESS") == "") {
		// -F: exit if content fits on one screen
		// -R: allow ANSI color codes
		// -X: don't clear screen on exit
		// -K: exit on Ctrl-C
		cmd.Env = append(os.Environ(), "LESS=-FRX")
	}

	stdin, err := cmd.StdinPipe()
	if err != nil {
		return os.Stdout, func() error { return nil }
	}

	if err := cmd.Start(); err != nil {
		return os.Stdout, func() error { return nil }
	}

	cleanup := func() error {
		stdin.Close()
		return cmd.Wait()
	}

	return stdin, cleanup
}

func main() {
	w, cleanupPager := usePager()
	defer cleanupPager()

	// Write your output to w instead of os.Stdout
	// ANSI colors will work properly with the -R flag
	for i := 1; i <= 100; i++ {
		if i%10 == 0 {
			fmt.Fprintf(w, "\033[1;32mLine %d: Highlighted output\033[0m\n", i)
		} else {
			fmt.Fprintf(w, "Line %d: Some output here\n", i)
		}
	}
}