You can use this syntax to set default values for environment variables while allowing them to be overridden when the script is called:
# Default env var values which can be overridden by the user
# See: https://emmer.dev/blog/bash-environment-variable-defaults/
: "${CONFIGOME_ENV:=default}"
export CONFIGOME_ENV="${CONFIGOME_ENV}"
# ...The : part expands arguments but otherwise does nothing.
The ${variable:=value} part assigns value to variable only if variable is unset or null, which is exactly what we want (i.e. don’t override them if the user set them, but give them a default value if currently empty).
Use case, a script which can optionally receive options as environment variables (which can be an easier way to pass config options in some environments, e.g. docker).
Credit for all of this: Bash Environment Variable Defaults | Christian Emmer
Confirmation of using bash parameter expansion for this: Assigning default values to shell variables with a single command in bash - Stack Overflow
Bash docs for shell parameter expansion feature: Shell Parameter Expansion (Bash Reference Manual)
Bash docs for what “shell parameter” means: Shell Parameters (Bash Reference Manual)