Optimizing shell startup time

[twpayne]

@mono0x designed a really neat trick to speed up your startup scripts.

The post is in Japanese but you can read it with Google Translate.

[mono0x]

Thank you for developing such powerful software and considering my idea.

The idea is to cache scripts generated by specific commands.

For instance, Homebrew's brew shellenv command generates a script that shell rc files should load. This result can be cached using chezmoi's template feature.

{{ output "/opt/homebrew/bin/brew" "shellenv" | trim }}

This is equivalent to eval "$(/opt/homebrew/bin/brew shellenv)" as a result. However, the brew shellenv command is executed during the chezmoi apply phase. This eliminates the overhead of executing the brew shellenv command when starting the shell.

Moreover, the cached script can be easily refreshed by running chezmoi apply.

Thanks to chezmoi, we have introduced a handy and low-cost cache mechanism.

[mono0x]

does the template get evaluated everytime chezmoi apply is run?

Yes. output function executes the command every time. I think that such behavior helps to keep the cache up-to-date.

https://www.chezmoi.io/reference/templates/functions/output/#:~:text=The%20execution%20occurs%20every%20time%20that%20the%20template%20is%20executed.

[halostatue]

There may be some drawbacks depending on the tool. I’m specifically thinking of brew shellenv, which uses /bin/ps to select the currently running shell and cannot be overridden. If you generate shell caches for multiple types of shells, you will get unanticipated results.

I’ve opened Homebrew/brew#15358 as a suggestion to make this work for this (admittedly narrow) use case.

What’s nice is that you can write your shell scripts to be smart, something like:

if [[ -s "${HOME}/.cache/shell-cache/brew.sh" ]]; then # exists and non-empty
  source "${HOME}/.cache/shell-cache/brew.sh"
else
  command -v brew >/dev/null 2>/dev/null && eval "$(brew shellenv)"
fi

It would be necessary to use -s if you have conditional caches (e.g., caching starship shell zsh should only run if you have starship installed).

[mono0x]

To run brew shellenv in the intended shell, it seems that the following workaround can be used.

{{ output "fish" "-c" "/opt/homebrew/bin/brew shellenv" | trim }}

It would be necessary to use -s if you have conditional caches (e.g., caching starship shell zsh should only run if you have starship installed).

Yes, but you can also check if the app is installed using lookPath function.

{{- if lookPath "starship" -}}
{{ output "starship" "init" "zsh" "--print-full-init" | trim }}
{{- end -}}

[halostatue]

That does seem to work. I’m still hopeful that they will allow the extension to shellenv since I may want to build something that supports powershell without having to have powershell installed. (Yes, I know I could probably do some script shenanigans to get and transform the values as required, but…)

[halostatue]

I’ve opened Homebrew/brew#15363 to add this feature.