[plug] Bash "Processing" Script

[Bernd Felsche]

On Fri, Jun 11, 2004 at 01:53:51PM +0800, Jay Turner wrote:
> > On Friday 11 June 2004 12:35, Jay Turner wrote:
> > > > Notwithstanding that this won't actually tell you that dd is
> > > > actually doing its thing:

> > > > 	( while : ; do sleep 3 ; echo -n "." ; done ) &
> > > > 	dotter=$!
> > > > 	dd if=/dev/the_ universe of=black_hole bs=1planet
> > > > 	kill $dotter
> Bernd said:
> > The shell's ":" is short-hand for "true", so
> > 	while :
> > will loop forever, at least until there's a break or exit inside the
> > loop.
> >
> > $! is the process ID of the last process put into background.
> >
> > > I have not encountered them before in my bash experience..

> I have successfully implemented your code and it works fine.

> Unfortunately, I am one of these people who needs to understand
> why something does what it does and not be happy just with the
> fact that "it does".

> Your use of "shorthand" bash is still a little confusing for me,
> in that I still code the long way
> while [ true ]

Incorrect syntax.
	while true
is correct
	while :
is quicker to type.

> do
> 	sleep 3
> 	echo -n "."
> done
> 
> Am I reading it correct that the 'while' is in the background
> printing dots, until we kill the 'while' process which won't be
> done until after dd has finished?

> If this is the case I am right in assuming that the brackets allow
> us to group those statements together and have them all put into
> the background?

The brackets are technically redundant because the statement starts
with "while" and is terminated by "done"; but the brackets add to
readability (believe it or not). Brackets are used in this context,
to create a sub-shell, which the & causes to run in background.
Additional statements can be run in the same sub-shell by placing
them before or after the while ... done statement; as required.

> Can this be expressed in the "long way" of coding?

I doubt it. The "&" is the only way of sending stuff into the
background. The "(" is the only way of creating a sub-shell.
I could expand the thing over several lines with appropriate
indents. For simple loops, that doesn't help with readability and
eats up too many display lines.

-- 
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