what is the use of "<<=" operator? I recently read this piece of code and I dont know what is the use of "&lt;&lt;=" in the for loop:

for (mask = 0x01; mask&gt;0; mask &lt;&lt;= 1) {
    if (data &amp; mask){ // choose bit


This is the entire code:

//Created August 23 2006
//Heather Dewey-Hagborg
//http://www.arduino.cc

#include &lt;ctype.h&gt;

#define bit9600Delay 100  
#define halfBit9600Delay 500
#define bit4800Delay 188 
#define halfBit4800Delay 94 

byte rx = 6;
byte tx = 7;
byte SWval;

void setup() {
  pinMode(rx,INPUT);
  pinMode(tx,OUTPUT);
  digitalWrite(tx,HIGH);
  delay(2);
  digitalWrite(13,HIGH); //turn on debugging LED
  SWprint('h');  //debugging hello
  SWprint('i');
  SWprint(10); //carriage return
}

void SWprint(int data)
{
  byte mask;
  //startbit
  digitalWrite(tx,LOW);
  delayMicroseconds(bit9600Delay);
  for (mask = 0x01; mask&gt;0; mask &lt;&lt;= 1) {
    if (data &amp; mask){ // choose bit
     digitalWrite(tx,HIGH); // send 1
    }
    else{
     digitalWrite(tx,LOW); // send 0
    }
    delayMicroseconds(bit9600Delay);
  }
  //stop bit
  digitalWrite(tx, HIGH);
  delayMicroseconds(bit9600Delay);
}

int SWread()
{
  byte val = 0;
  while (digitalRead(rx));
  //wait for start bit
  if (digitalRead(rx) == LOW) {
    delayMicroseconds(halfBit9600Delay);
    for (int offset = 0; offset &lt; 8; offset++) {
     delayMicroseconds(bit9600Delay);
     val |= digitalRead(rx) &lt;&lt; offset;
    }
    //wait for stop bit + extra
    delayMicroseconds(bit9600Delay); 
    delayMicroseconds(bit9600Delay);
    return val;
  }
}

void loop()
{
    SWval = SWread(); 
    SWprint(toupper(SWval));
}
In simpler terms mask &lt;&lt;= 1 is shorthand for mask = mask &lt;&lt; 1

In general 

variable operator= operand


is the augmented assignment operator which means 

variable = variable operator operand