line filter

xxx-line-filter.go

@@ -0,0 +1,49 @@
+package main                                // A line-filter program reads input on stdin,
+                                            // processes it, and prints results to stdout.
+                                            // Here is an example line filter that writes a
+                                            // a captialized version of all text it reads.
+
+import "bufio"                              // Package `bufio` will help us read line-by-line.
+import "bytes"
+import "os"
+import "io"
+
+func main() {
+    newline := []byte("\n")                 // We'll need to add our own newlines
+                                            // between processed lines.
+
+    in  := bufio.NewReader(os.Stdin)        // The buffered reader gives us `ReadLine`.
+    out := os.Stdout
+
+    for {                                   // If succesful, each `ReadLine` returns bytes and a
+        inBytes, pfx, err := in.ReadLine()  // boolean indicating if don't have the whole line.
+        
+        if err == io.EOF {                  // The `EOF` error is expected when we reach the end
+            return                          // of the input, so exit gracefully in that case.
+        }                                   // Otherwise there is a problem.
+        if err != nil {
+            panic (err)
+        }       
+
+        outBytes := bytes.ToUpper(inBytes)  // `bytes.ToUpper` works directly on bytes, so we don't
+                                            // need to convert to a string for `strings.ToUpper`.
+
+        _, err = out.Write(outBytes)        // Write out the upercased bytes, checking for an error
+        if err != nil {                     // here as well.
+            panic(err)
+        }
+
+        if !pfx {                           // Unless this read was for a prefix (not the full
+            out.Write(newline)              // line), we need to add our own newline.
+        }        
+    }
+}
+
+/*
+$ cat > lines                               // Make a file with a few lowercase lines.
+hello
+filter
+$ cat lines | go run line-filter.go         // Use the line filter to get upercase lines.
+HELLO
+FILTER
+*/