06-22-2017
You can create an account with a user name that does not contain any lowercase letters, but that poor user will curse you every time they try to login if you do so. When you login to a UNIX system (such as Solaris release 10) with a user name that doesn't contain any lowercase letters, the system will assume that you are logging in from a device (such as a TTY Model 33 teletype) that only has uppercase letters. To login as BG0001, they will need to type in the string \B\G001 and any uppercase characters in their password will also have to be escaped. After they have successfully logged in (if they are able to do so), they'll need to use stty to tell the TTY subsystem that the terminal emulator they're using does understand both uppercase and lowercase letters. Then that user will need to remember the different possible modes they might be working in whenever they try to change their password, enter their user name, type commands into their shell, etc.
The automated scripts that set up new user accounts complain about uppercase only user names for a reason. I strongly encourage you to heed the warning provided by the useradd warnings.
This User Gave Thanks to Don Cragun For This Post:
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LEARN ABOUT DEBIAN
bup-margin
bup-margin(1) General Commands Manual bup-margin(1)
NAME
bup-margin - figure out your deduplication safety margin
SYNOPSIS
bup margin [options...]
DESCRIPTION
bup margin iterates through all objects in your bup repository, calculating the largest number of prefix bits shared between any two
entries. This number, n, identifies the longest subset of SHA-1 you could use and still encounter a collision between your object ids.
For example, one system that was tested had a collection of 11 million objects (70 GB), and bup margin returned 45. That means a 46-bit
hash would be sufficient to avoid all collisions among that set of objects; each object in that repository could be uniquely identified by
its first 46 bits.
The number of bits needed seems to increase by about 1 or 2 for every doubling of the number of objects. Since SHA-1 hashes have 160 bits,
that leaves 115 bits of margin. Of course, because SHA-1 hashes are essentially random, it's theoretically possible to use many more bits
with far fewer objects.
If you're paranoid about the possibility of SHA-1 collisions, you can monitor your repository by running bup margin occasionally to see if
you're getting dangerously close to 160 bits.
OPTIONS
--predict
Guess the offset into each index file where a particular object will appear, and report the maximum deviation of the correct answer
from the guess. This is potentially useful for tuning an interpolation search algorithm.
--ignore-midx
don't use .midx files, use only .idx files. This is only really useful when used with --predict.
EXAMPLE
$ bup margin
Reading indexes: 100.00% (1612581/1612581), done.
40
40 matching prefix bits
1.94 bits per doubling
120 bits (61.86 doublings) remaining
4.19338e+18 times larger is possible
Everyone on earth could have 625878182 data sets
like yours, all in one repository, and we would
expect 1 object collision.
$ bup margin --predict
PackIdxList: using 1 index.
Reading indexes: 100.00% (1612581/1612581), done.
915 of 1612581 (0.057%)
SEE ALSO
bup-midx(1), bup-save(1)
BUP
Part of the bup(1) suite.
AUTHORS
Avery Pennarun <apenwarr@gmail.com>.
Bup unknown- bup-margin(1)