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 (Section 35.12). Here are a few places to use it:
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:
ifsomethingthen : elsecommandsfi
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?}