Fossil has two commands that look like they do the same thing on initial examination, fossil update and fossil checkout, but there are several key differences:

1. fossil checkout aborts if there are changed files in the local directory unless you give the --force option, whereas fossil update merges upstream changes with your local changes. Since Fossil tends to follow the CVS command design, and CVS popularized the merge on update workflow, we expect that Fossil’s update behavior is more likely to be what you want.

2. Update triggers an autosync attempt; checkout does not.

3. Several features in fossil update do not exist in fossil checkout, so developing a habit to type fossil up means you’re more likely to have the features you want at hand.

4. Inversely, the fossil checkout --keep feature doesn’t exist in fossil update, but it’s a rarely-needed operation, so it doesn’t provide a good reason to develop a habit of using fossil checkout instead.

In summary, these are two separate commands; neither is an alias for the other. They overlap enough that they can be used interchangeably for some use cases, but update is more powerful and more broadly useful.