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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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