變量替換

1. 變量替換並賦值
:= 變量已聲明，但取值爲空
= 變量未聲明

username=""
echo ${username:=$LOGNAME}      # usr_ins
echo $username                  # usr_ins

unset username
echo ${username=$LOGNAME}       # usr_ins
echo $username                  # usr_ins

2. 變量替換(空變量)
:- 變量已聲明，但取值爲空
- 變量未聲明

username=""   
echo ${username:-$LOGNAME}      # usr_ins
echo $username                  # NULL

unset username
echo ${username-$LOGNAME}       # usr_ins
echo $username                  # NULL

3. 變量檢測

username=""  
echo ${username:?"Empty variable"}      # ksh: username: Empty variable

unset username
echo ${username?"Undefined variable"}   # ksh: username: Undefined variable

4. 變量替換(非空變量)

username="eli"            
echo ${username:+$LOGNAME}      # usr_ins
echo $username                  # eli

username=""              
echo ${username+$LOGNAME}       # usr_ins

unset username
echo ${username+$LOGNAME}       # NULL