[Linux 常見問題] Assigning default values to shell variables with a single command in bash

Source From Here 
Question 
I have a whole bunch of tests on variables in a bash (3.00) shell script where if the variable is not set, then it assigns a default, e.g.: 

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1. if [ -z "${VARIABLE}" ]; then   
2.     FOO='default'  
3. else   
4.     FOO=${VARIABLE}  
5. fi  

I seem to recall there's some syntax to doing this in one line, something resembling a ternary operator, e.g.: 

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1. FOO=${ ${VARIABLE} : 'default' }  

(though I know that won't work...) 
Am I crazy, or does something like that exist? 

How-To 
There are two ways to carry out your request: 

1. ${variable:-'defaultValue'} : When variable is not defined or as None, temporally assign 'defaultValue' to it right in that line. Afterword, the variable is still kept as original state.
2. ${variable:='defaultValue'} : When variable doesn't exist or as None, then assign 'defaultValue' to it and the variable will carry the assigned default value among whole script.

For example, let's do experiment with a simple shell script: 
- test.sh 

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1. #!/bin/sh  
2.   
3. echo "variable=${variable:-'defaultValue1'}"  
4. echo "variable=${variable}"  
5. echo "varialbe=${variable:='defaultValue2'}"  
6. echo "variable=${variable}"  

Testing it this way: 

# ./test.sh
variable='defaultValue1'
variable=
varialbe='defaultValue2'
variable='defaultValue2'
# variable='test' ./test.sh
variable=test
variable=test
varialbe=test
variable=test

Supplement 
* [Linux 文章收集] 變數值代換 (variables substitution) 精簡筆記