Add Detect If Stdin Comes From A Redirect as a Go TIL

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1541 TILs and counting..._
+_1542 TILs and counting..._
 
 ---
 
@@ -405,6 +405,7 @@ _1541 TILs and counting..._
 - [Build For A Specific OS And Architecture](go/build-for-a-specific-os-and-architecture.md)
 - [Check If Cobra Flag Was Set](go/check-if-cobra-flag-was-set.md)
 - [Combine Two Slices](go/combine-two-slices.md)
+- [Detect If Stdin Comes From A Redirect](go/detect-if-stdin-comes-from-a-redirect.md)
 - [Do Something N Times](go/do-something-n-times.md)
 - [Find Executables Installed By Go](go/find-executables-installed-by-go.md)
 - [Format Date And Time With Time Constants](go/format-date-and-time-with-time-constants.md)

go/detect-if-stdin-comes-from-a-redirect.md

@@ -0,0 +1,59 @@
+# Detect If Stdin Comes From A Redirect
+
+Reading lines of input from `stdin` is flexible. And we may need our program to
+behave differently depending on where that input is coming from. For instance,
+if data is redirected or piped to our program, we scan and process it directly.
+Otherwise, we need to prompt the user to enter in specific info and go from
+there.
+
+We can detect whether [`os.Stdin`](https://pkg.go.dev/os#pkg-variables) is
+being piped to, redirected to, or whether we should prompt the user by looking
+at the file mode descriptor of
+[`os.Stdin.Stat()`](https://pkg.go.dev/os#File.Stat).
+
+```go
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+func main() {
+	file, err := os.Stdin.Stat()
+	if err != nil {
+		fmt.Printf("Error checking stdin: %v\n", err)
+		os.Exit(1)
+	}
+
+	fromTerminal := (file.Mode() & os.ModeCharDevice) != 0
+	fromAPipe := (file.Mode() & os.ModeNamedPipe) != 0
+
+	if fromTerminal {
+		fmt.Println("This is Char Device mode, let's prompt user for input")
+		termScanner := bufio.NewScanner(os.Stdin)
+		for termScanner.Scan() {
+			fmt.Printf("- %s\n", termScanner.Text())
+			break;
+		}
+	} else if fromAPipe {
+		fmt.Println("This is Named Pipe mode, contents piped in")
+		pipeScanner := bufio.NewScanner(os.Stdin)
+		for pipeScanner.Scan() {
+			fmt.Printf("- %s\n", pipeScanner.Text())
+		}
+	} else {
+		fmt.Println("This means the input was redirected")
+		redirectScanner := bufio.NewScanner(os.Stdin)
+		for redirectScanner.Scan() {
+			fmt.Printf("- %s\n", redirectScanner.Text())
+		}
+	}
+}
+```
+
+If `os.ModeCharDevice` then we are connected to a character device, like the
+terminal. We can see if input is being piped in by checking against
+`os.ModeNamedPipe`. Otherwise, there are a variety of file modes and I'm
+willing to assume we're dealing with a regular file redirect at that point.