examples: fix process_stdin_trick example (#13953)

examples/process/process_stdin_trick.v

@@ -17,24 +17,17 @@ fn exec(cmd string) (string, int) {
 	p.set_redirect_stdio()
 	p.run()
 
-	if !cmd2.ends_with('\n') {
-		cmd2 += '\n'
-	}
+	p.stdin_write('$cmd2 && echo **OK**')
+	os.fd_close(p.stdio_fd[0]) // important: close stdin so cmd can end by itself
 
-	p.stdin_write(cmd2)
-	p.stdin_write('\necho **OK**\n')
-
-	for {
-		if !p.is_alive() {
-			break
-		}
+	for p.is_alive() {
 		line = p.stdout_read()
 		println(line)
 		// line_err = p.stderr_read() //IF WE CALL STDERR_READ will block
 		// we need a mechanism which allows us to check if stderr/stdout has data or it should never block
 		// is not a good way, need to use a string buffer, is slow like this
 		out += line
-		if out.ends_with('**OK**\n') {
+		if line.ends_with('**OK**\n') {
 			out = out[0..(out.len - 7)]
 			break
 		}
@@ -42,19 +35,18 @@ fn exec(cmd string) (string, int) {
 
 	// println("read from stdout, should not block")
 	// is not really needed but good test to see behaviour
-	// out += p.stdout_read()
-	// println("read done")
-
-	// println(cmd.stderr_read())
+	out += p.stdout_read()
+	println('read done')
 
+	println(p.stderr_read())
+	p.close()
+	p.wait()
 	if p.code > 0 {
 		rc = 1
 		println('ERROR:')
 		println(cmd2)
 		print(out)
 	}
-	// documentation says we need to call p.wait(), but this does not seem to work, will be process stop or become zombie?
-	// p.wait()
 
 	return out, rc
 }
@@ -63,10 +55,9 @@ fn main() {
 	mut out := ''
 	mut rc := 0
 
-	// the following does not work, not sure why not
-	// out,rc = exec("find /tmp/ && echo '******'")
+	// find files from /tmp excluding files unlistable by current user
 
-	out, rc = exec("find /tmp/ ; echo '******'")
+	out, rc = exec("find /tmp/ -user \$UID; echo '******'")
 	println(out)
 	assert out.ends_with('******\n')