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What does this do: ${pid:-}
http://bashscripts.org/forum/viewtopic.php?f=16&t=574
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Author:  magoo [ Fri Oct 10, 2008 11:55 pm ]
Post subject:  What does this do: ${pid:-}

As i understand it, this means
if pid is
a) unset, or
b) set to null
then replace with the value on the right of the minus sign--which is null

This confuses me because
a) I thought if a variable isn't set to anything it's automatically null. If not then what is an uninitiated variable set to?
b) if the variable is already set to null, then why replace it with another null?

I see this on line 74 in the file /lib/lsb/init-functions (I'm using hardy heron ubuntu)
Here's an excerpt from the file:
Code:
if [ -z "${pidfile:-}" ]; then
    pidfile=/var/run/${1##*/}.pid
fi

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