On 13/3/18 8:46 pm, Patrick O'Callaghan wrote:
On Mon, 2018-03-12 at 18:25 -0700, Rick Stevens wrote:
On 03/12/2018 03:37 PM, Patrick O'Callaghan wrote:
On Tue, 2018-03-13 at 07:26 +1100, Stephen Morris wrote:
'du' with no parameters recursively lists all the subdirectories and
their sizes, along with the grand total. When applied to my home
directory, I get over 30,000 lines of output. That's almost never what
I want. My usual call is 'du -hs'.

poc
Thanks Patrick, taking this a step further, it seems to me that the only
parameter for du that, to me, provides the correct file size is -b as
shown below.
I think you have a misconception here. 'du' does not give file sizes,
it gives disk usage. A 1-byte file takes up at least 1 disk block, so
that's the size 'du' will give. I seem to remember that it also counts
indirect blocks and other housekeeping that corresponds to the file
without being included in the file's content, but I could be mistaken
(though I'm fairly sure early versions did do that).
du (with no flags) gives disk usage. As Patrick says, a 1-byte file
uses one disk block (which is generally 4KiB) and that's what du is
reporting (after all, "du" means "disk usage"). The "-b" flag means
"set the block size to 1 byte and show the apparent size", which is what
"ls -l" would report (there may be differences between du and ls if
sparse files are involved).
Good point about sparse files, which I'd forgotten in this context. A
sparse file can be 10TB in size yet occupy only 1 disk block. 'du' will
give the usage as 1 block and 'ls' will say it's 10TB.

Also, du walks down the entire current directory unless you give it
arguments to tell it what to look at. Note that the arguments you pass
it are interpreted by the _shell_, not "du" (even the man page says
"PATTERN is a shell pattern (not a regular expression)").

This is a common confusion point with many people. Unless you enclose
shell metacharacters in quotes (and dots and splats are metacharacters),
the shell WILL interpret them--sometimes in ways you aren't expecting!
By default, shell globbing does NOT expand filenames that start with a
dot (to the shell, a dot means "current directory").
Slight nit: '.' (on its own) means 'current directory' to the kernel,
not to the Shell, i.e. it's wired into the system and doesn't depend on
the command interpreter (in the same way that '/' is the path separator
and cannot be changed). The use of dot-files as a way of hiding certain
names is IIRC originally just a convention of 'ls' which the Shell
adopted for consistency.

poc

Thanks guys, I fully understood the -b parameter was giving an apparent size of the file (which approximately matches what ls -l provides), that sparse files can impact the apparent size, and that the physical size of a file is governed by the sector size, but my usage of du, especially with the -s parameter, is the amount of "apparent" space occupied by files and directories (where possible I usually use a sector size of 512) to identify space hogs. I'm also fully aware of the sector size trade off between efficient space utilization and efficient I/O and the apparent space used by files.

What I hadn't realized until Patrick mentioned it, was the significance of the * in the path specification. I hadn't realized that with using the * not only did it cause files prefixed with a '.' to be ignored but it also causes directories prefixed with a '.' to be ignored also.


regards,

Steve


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