The Unappreciated Bourne Shell ":" Operator

Some people think that the Bourne shell's : is a comment character. It isn't, really. It evaluates its arguments and returns a zero exit status (). Here are a few places to use it:

• Replace the UNIX true command to make an endless while loop (). This is more efficient because the shell doesn't have to start a new process each time around the loop (as it does when you use while true):
  
  

  while : do commands done
  

  (Of course, one of the commands will probably be break, to end the loop eventually.)

• When you want to use the else in an if (), but leave the then empty, the : makes a nice "do-nothing" place filler:
  
  

  if something then : else commands fi
  

  
  
  
• If your Bourne shell doesn't have a true # comment character, you can use : to "fake it." It's safest to use quotes so the shell won't try to interpret characters like > or | in your "comment":
  
  

  : 'read answer and branch if < 3 or > 6'
  

  
  
  
• Finally, it's useful with parameter substitution () like ${var?} or ${var=default}. For instance, using this line in your script will print an error and exit if either the USER or HOME variables aren't set:
  
  

  : ${USER?} ${HOME?}
  

  
  
  

- JP