From ba1cc59cba0e9139e5a81b78ded95332cf6b063e Mon Sep 17 00:00:00 2001
From: jbranchaud <jbranchaud@gmail.com>
Date: Fri, 8 Jan 2021 15:29:22 -0600
Subject: [PATCH] Add Provide A Fallback Value For Unset Parameter as a unix
 til

---
 README.md                                     |  3 +-
 ...de-a-fallback-value-for-unset-parameter.md | 29 +++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 unix/provide-a-fallback-value-for-unset-parameter.md

diff --git a/README.md b/README.md
index 3efdee4..875fae2 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_999 TILs and counting..._
+_1000 TILs and counting..._
 
 ---
 
@@ -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)
diff --git a/unix/provide-a-fallback-value-for-unset-parameter.md b/unix/provide-a-fallback-value-for-unset-parameter.md
new file mode 100644
index 0000000..41e5436
--- /dev/null
+++ b/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)