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

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_999 TILs and counting..._
+_1000 TILs and counting..._
 
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@@ -963,6 +963,7 @@ _999 TILs and counting..._
 - [PID Of The Current Shell](unix/pid-of-the-current-shell.md)
 - [Print A Range Of Lines For A File With Bat](unix/print-a-range-of-lines-for-a-file-with-bat.md)
 - [Print Out Files In Reverse](unix/print-out-files-in-reverse.md)
+- [Provide A Fallback Value For Unset Parameter](unix/provide-a-fallback-value-for-unset-parameter.md)
 - [Repeat Yourself](unix/repeat-yourself.md)
 - [Saying Yes](unix/saying-yes.md)
 - [Search Files Specific To A Language](unix/search-files-specific-to-a-language.md)

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

@@ -0,0 +1,29 @@
+# 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)