shell - How does Bash parameter expansion work? -

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i'm trying understand bash script. stumbled upon this:

dir=${1:-"/tmp"}

what mean?

:- operator says if $1 (first argument script) not set or null use /tmp value of $dir , if it's set assign it's value $dir.

dir=${1:-"/tmp"}

is short for

if [ -z $1 ];         dir='/tmp' else         dir="$1" fi

it can used variables not positional parameters:

$ echo ${home:-/tmp} # since $home set displayed. /home/codaddict $ unset home   # unset $home. $ echo ${home:-/tmp} # since $home not set, /tmp displayed. /tmp $

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