question about disk_var.sh

[andrew bezella]

On Fri, 2014-08-01 at 15:58 +0200, Thomas Neumann wrote:
[...] 
> SWAPLIST=${SWAPLIST:-"/dev/sda2"}
> BOOT_DEVICE=${BOOT_DEVICE:-"/dev/sda"}
> ROOT_PARTITION=${ROOT_PARTITION:-/dev/sda1}
> BOOT_PARTITION=${BOOT_PARTITION:-/dev/sda1}
> 
> (Same output for setup_storage 1.5 and 1.6.)
> 
> Sourcing the file and printing the variables does indeed yield the expected 
> values.
> 
> a) can someone confirm his setup_storage generates the same strange values?

still using setup-storage 1.3, doesn't generate that output.

> b) can someone explain what kind of bash trickery is going on here?

it's returning default values for the variables if they're unset/empty.
the ${PARAMETER:-WORD} syntax says "if PARAMETER is unset or empty,
return WORD."

note that there is a difference between ":-" and ":=", and that the
latter might be better in this case.  the shorter ${PARAMETER:=WORD}
would assign the value of WORD to PARAMETER in the unset/empty case.  in
my head ":-" is for returning defaults, ":=" is for assigning them.  but
please correct me if i'm off-base, as i've only recently grokked the
difference.

ex: 
$ unset MYVAR
$ echo "${MYVAR:-test}"
test
$ echo $MYVAR
                                           <- intentionally empty lines
$ ${MYVAR:-test}                             |
$ echo $MYVAR                                |
                                           <-
$ ${MYVAR:=test}
$ echo $MYVAR
test

see the "Parameter Expansion" portion of BASH(1) or
http://wiki.bash-hackers.org/syntax/pe for add'l info

hth

andy

-- 
andrew bezella <abezella at archive.org>
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