AWS defines this option as:
```
--family-prefix (string)
The full family name with which to filter the ListTaskDefinitions
results. Specifying a familyPrefix limits the listed task
defini-tions to task definition revisions that belong to that
family.
```
This option behaves differently than ecs:ListTaskDefinitionFamilies.
Instead of doing a comparison like `startswith`, it does a full string
comparison by matching the entire task definition family to the prefix.
For example, let's say there exists a task definition with the family
`super-cool-task-def`.
ListTaskDefinitionFamilies would look like this:
```
aws ecs list-task-definition-families --family-prefix super-cool
{
"families": [
"super-cool-task-def"
]
}
```
ListTaskDefinitions would look like this:
```
aws ecs list-task-definitions --family-prefix super-cool
{
"taskDefinitionArns": []
}
```
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