shell 中"${b2}" and "${b:2}"

在看 学习shell中练习的代码

#!/bin/bash
# User can keyin filename to touch 3 new files.
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
export PATH

echo "I will use 'touch' command to create 3 files."
read -p "Please input the filename what you want: " fileuser

filename=${fileuser:+"filename"}

date1=`date --date='2 days ago'  +%Y%m%d`
date2=`date --date='1 days ago'  +%Y%m%d`
date3=`date +%Y%m%d`

file1="$filename""$date1"
file2="$filename""$date2"
file3="$filename""$date3"

touch $file1
touch $file2
touch $file3

中的 filename=${fileuser:+"filename"}对于：+不是很明白，然后查了下用法。

lx@ubuntu:~/bsh/scripts$ b=""
lx@ubuntu:~/bsh/scripts$ c="Z"
lx@ubuntu:~/bsh/scripts$ echo a=${a-1}, b=${b-2}, c=${c-3}
a=1, b=, c=Z
lx@ubuntu:~/bsh/scripts$ echo a=${a:-1}, b=${b:-2}, c=${c:-3}
a=1, b=2, c=Z
lx@ubuntu:~/bsh/scripts$ 

+----------------------------------------------------------------------+
|Form		     Meaning					       
+----------------------------------------------------------------------+
|${variable:?word}   Complain if undefined or null		       
|${variable:-word}   Use new value if undefined or null		       
|${variable:+word}   Opposite of the above			       
|${variable:=word}   Use new value if undefined or null, and redefine. 
+----------------------------------------------------------------------+