45.9 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 (44.7). Here are a few places to use it:

• Replace the UNIX true command to make an endless while loop (44.10). 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 (44.8), 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 (45.12) 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?}