The Unappreciated Bourne Shell ":" Operator (Unix Power Tools, 3rd Edition)

36.6. The Unappreciated Bourne Shell ":" Operator

Replace the Unix true command to make an endless while loop (Section 35.15). 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. This presumes that it is actually a savings to have the break test inside the loop body rather than at the top, but it may well be clearer under certain circumstances to do it that way.)
When you want to use the else in an if ( Section 35.13) 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 (but nearly all of them do nowadays), 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 (Section 35.7) 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


36.5. The exec Command		36.7. Parameter Substitution