Re: Fwd: Clearing RAM Caches

[Greg Wooledge <greg@wooledge.org>]

On Tue, Aug 16, 2022 at 08:25:12AM +0100, Tixy wrote:
> On Mon, 2022-08-15 at 21:05 -0400, Timothy M Butterworth wrote:
> > > 
> > Thanks for the clarification. `echo 1 > sudo /proc/sys/vm/drop_caches`
> > seems to work just fine.
> 
> It doesn't, as Tomas pointed out it creates a file called 'sudo' and
> puts a '1' in it.

The following two commands are equivalent:

echo 1 > sudo /proc/sys/vm/drop_caches

echo 1 /proc/sys/vm/drop_caches > sudo

The file "sudo" will have "1 /proc/sys/vm/drop_caches" in it, because
echo received two arguments.  Redirections may appear anywhere in a
"simple" command.  It's conventional to write them at the end of the
command, but the shell permits them to be anywhere.

This is a standard feature of all POSIX shells, by the way.  Not a
bash extension.

Redirections must appear at the end of "compound" commands, which are
basically anything with internal shell syntax -- if, case, while, and
so on.  It may be desirable to try to write something like:

< inputfile  while read -r line; do
  ...
done

but it's not allowed.  You have to write it as:

while read -r line; do
  ...
done < inputfile

Or, explicitly open the input file on a new FD (file descriptor), and
close it afterward:

exec 3< inputfile
while read -r line <&3; do
  ...
done
exec 3<&-