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

CodeTryout 2012 Jan 15, Update:2023 Jul 19.

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.

Default setting syntax:

FINE_VARIABLE=${FINE_VARIABLE:-DEFAULT-value}

Here are some examples.

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

dev@codetryout:~$ FINE_VARIABLE=
dev@codetryout:~$ 
dev@codetryout:~$ echo $FINE_VARIABLE 

dev@codetryout:~$ FINE_VARIABLE=${FINE_VARIABLE:-DEFAULT-value}
dev@codetryout:~$ 
dev@codetryout:~$ 
dev@codetryout:~$ echo $FINE_VARIABLE 
DEFAULT-value
dev@codetryout:~$

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

dev@codetryout:~$ FINE_VARIABLE=LetMeAssignAValue
dev@codetryout:~$ 
dev@codetryout:~$ FINE_VARIABLE=${FINE_VARIABLE:-DEFAULT-value}
dev@codetryout:~$ 
dev@codetryout:~$ echo $FINE_VARIABLE 
LetMeAssignAValue
dev@codetryout:~$

Demo:

How to set a bash variable default value if empty