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+---
+tags:
+  - bash
+---
+[[Bash]] [[Process substition (tmp file)]] [[Parameter subsitution]]
+
+---
+[This](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02) page contains a table with [[Parameter subsitution]] 
+
+When we want to expand a variable into an argument to a command but the variable is empty it will expand to **an empty string**. This is often unwanted, we just want `null` which  is removed from the command-line.
+
+*Example*
+```
+openstack server delete server "${waitFlag}"
+```
+if `waitFlag` is empty this will become:
+```
+openstack server delete server ''
+```
+which results in an error.
+
+This is where we can use `null substitution`:
+```
+openstack server delete server "${waitFlag:+$waitFlag}"
+```
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