Bash assign a default value to a variable if it is empty

CodeTryout 2012 Jan 15.

How to assign a default value to a variable if it is undefined or empty. This is useful when processing a user input or if the variable value came from another command which is failing or working as unexpected.

Syntax:

FINE_VARIABLE=${FINE_VARIABLE:-DEFAULT-value}

Here are some examples.

Case #1 – without assigning a value (this case the variable is empty):

[email protected]:~$ FINE_VARIABLE=
[email protected]:~$ 
[email protected]:~$ echo $FINE_VARIABLE 

[email protected]:~$ FINE_VARIABLE=${FINE_VARIABLE:-DEFAULT-value}
[email protected]:~$ 
[email protected]:~$ 
[email protected]:~$ echo $FINE_VARIABLE 
DEFAULT-value
[email protected]:~$

Case #2 – with assigning a value (variable has some value assigned):

[email protected]:~$ FINE_VARIABLE=LetMeAssignAValue
[email protected]:~$ 
[email protected]:~$ FINE_VARIABLE=${FINE_VARIABLE:-DEFAULT-value}
[email protected]:~$ 
[email protected]:~$ echo $FINE_VARIABLE 
LetMeAssignAValue
[email protected]:~$