Shell Scripting: If Variable Is Not Defined, Set Default Variable

To set a default value for a variable in a Bash shell script, you can use the ${VARIABLE:-DEFAULT_VALUE} syntax. This syntax checks if the VARIABLE is defined and not empty, and if it is not, it will expand to DEFAULT_VALUE. Otherwise, it will expand to the value of VARIABLE.

Here is an example of how to use the ${VARIABLE:-DEFAULT_VALUE} syntax to set a default value for a variable:

# Define the VARIABLE variable with an empty value
VARIABLE=""

# Set a default value for the VARIABLE variable
DEFAULT_VALUE="hello"

# Use the ${VARIABLE:-DEFAULT_VALUE} syntax to expand the VARIABLE variable
# with the default value if it is not defined or empty
echo "${VARIABLE:-DEFAULT_VALUE}"

In this example, the VARIABLE variable is defined with an empty value. Then, the DEFAULT_VALUE variable is defined with the string "hello". Finally, the ${VARIABLE:-DEFAULT_VALUE} syntax is used to expand the VARIABLE variable, and it will expand to DEFAULT_VALUE because VARIABLE is not defined or empty. The echo command is used to print the expanded value of VARIABLE, which will be the string "hello".