Add Provide A Fallback Value For Unset Parameter as a unix til

unix/provide-a-fallback-value-for-unset-parameter.md

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+# Provide A Fallback Value For Unset Parameter
+
+If you are using a value in a parameter expansion expression that isn't set,
+the result will be empty.
+
+For instance, the XDG paths are not defined for me on OSX.
+
+```bash
+$ echo "${XDG_CONFIG_HOME}"
+
+```
+
+To make a script more robust, you can provide a fallback value (i.e. [default
+value](https://wiki.bash-hackers.org/syntax/pe#use_a_default_value)). The
+parameter expansion will use the fallback value if the primary value is either
+unset or null.
+
+The syntax for this is to follow the primary parameter with `:-` and then the
+fallback parameter.
+
+```bash
+$ echo "${XDG_CONFIG_HOME:-$HOME/.local/share}"
+/Users/jbranchaud/.local/share
+```
+
+Because I'm on OSX, this expands to my `$HOME` directory with `/.local/share`
+appended.
+
+[source](https://unix.stackexchange.com/a/122848/5916)