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---
title: Constants
eleventyNavigation:
key: 🟰 Constants
parent: ⚙️ Configuration
---
<span class="minilink minilink-addedin">New in version 1.7.10</span> borgmatic
supports defining custom configuration constants. This is similar to the
[variable interpolation
feature](https://torsion.org/borgmatic/reference/configuration/command-hooks/#variable-interpolation)
for command hooks, but the constants feature lets you substitute your own custom
values into any option values in the entire configuration file.
Here's an example usage:
```yaml
constants:
user: foo
archive_prefix: bar
source_directories:
- /home/{user}/.config
- /home/{user}/.ssh
...
archive_name_format: '{archive_prefix}-{now}'
```
<span class="minilink minilink-addedin">Prior to version 1.8.0</span> Don't
forget to specify the section (like `location:` or `storage:`) that any option
is in.
In this example, when borgmatic runs, all instances of `{user}` get replaced
with `foo` and all instances of `{archive_prefix}` get replaced with `bar`.
And `{now}` doesn't get replaced with anything, but gets passed directly to
Borg, which has its own
[placeholders](https://borgbackup.readthedocs.io/en/stable/usage/help.html#borg-help-placeholders)
using the same syntax as borgmatic constants. So borgmatic options like
`archive_name_format` that get passed directly to Borg can use either Borg
placeholders or borgmatic constants or both!
After substitution, the logical result looks something like this:
```yaml
source_directories:
- /home/foo/.config
- /home/foo/.ssh
...
archive_name_format: 'bar-{now}'
```
Note that if you'd like to interpolate a constant into the beginning of a
value, you'll need to quote it. For instance, this won't work:
```yaml
source_directories:
- {my_home_directory}/.config # This will error!
```
Instead, do this:
```yaml
source_directories:
- "{my_home_directory}/.config"
```
<span class="minilink minilink-addedin">New in version 1.8.5</span> Constants
work across
[includes](https://torsion.org/borgmatic/reference/configuration/includes/),
meaning you can define a constant and then include a separate configuration file
that uses that constant.
An alternate to constants is passing in your values via [environment
variables](https://torsion.org/borgmatic/reference/configuration/environment-variables/).
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