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82 lines
4.6 KiB
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82 lines
4.6 KiB
Plaintext
---
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title: "Deprecating the bash rprompt"
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description: "Deprecating the bash rprompt"
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slug: deprecating-bash-rprompt
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authors:
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- name: Jan De Dobbeleer
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title: Maintainer
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url: https://github.com/jandedobbeleer
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image_url: https://avatars.githubusercontent.com/u/2492783?v=4
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tags: [breaking, ohmyposh]
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hide_table_of_contents: false
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---
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After careful consideration, it was decided to remove support for the [`rprompt`][rprompt] in bash. If you are
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interested in the reasons behind this decision, please read on.
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<!--truncate-->
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## Context
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[`rprompt`][rprompt] is a feature that allows you to display a prompt at the right side of the terminal, on
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the same line as your cursor. This feature is available natively on a lot of shells, but not on bash.
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To work around this limitation, Oh My Posh has a **custom implementation** we use for both bash and PowerShell.
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## History
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### First implementation
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There were two evolutions of bash' custom [`rprompt`][rprompt] in Oh My Posh. The first one printed the [`rprompt`][rprompt]
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together with the primany prompt in the `PS1` variable, so we only need one CLI call to visualize the prompt.
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The challenge is that, as the [`rprompt`][rprompt] is printed together with the primary prompt, we need to make sure
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bash' readline can interprete the prompt correctly. In bash, every character that isn't a printable character,
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like color codes, needs to be wrapped in `\[` and `\]`. This is necessary to make sure bash can **calculate the length**
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of the prompt and ultimately **position the cursor** correctly.
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It turned out that for [`rprompt`][rprompt], this _needed to be done differently_. The [`rprompt`][rprompt] is printed after what
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bash interpretes as the end of the prompt. We found out that we need to wrap the entire [`rprompt`][rprompt] in `\[` and `\]`,
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and not escape individual non-printable characters.
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We did that for a while, but apparently this approach was **not very robust**. It was easy to break readline's
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cursor position calculation, and it was hard to debug. The behaviour of this was also different cross platform,
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so we had to go back to the drawing board.
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### Second implementation
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The second implementation of the [`rprompt`][rprompt] was a lot more robust. We decided to print the [`rprompt`][rprompt] in a separate
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CLI call, so we could control the output more easily. It leveraged the use of the `PROMPT_COMMAND` variable in bash, which
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is a hook that is executed before the prompt is printed. We would first print the [`rprompt`][rprompt] in the `PROMPT_COMMAND`,
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and then set the primary prompt in the `PS1` variable so bash would not bother about the [`rprompt`][rprompt] being there.
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This approach was a lot **cleaner from an achitectural point of view**, but it came with its own challenges.
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The most obvious one is needing two CLI calls to print the prompt, which made rendering a bit slower than the first implementation.
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Additionally, as we print before `PS1` is evaluated, we noticed the following issues along the way:
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1. when the output of the previous command didn't end with a newline, the [`rprompt`][rprompt] would be printed on the same line
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as the output of the previous command
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2. when the prompt was at the bottom of the terminal buffer, the [`rprompt`][rprompt] would be printed on the same line as the
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prompt, which would break the prompt
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3. depending on the platform, it would still break command history navigation
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4. it required different logic for multiline prompts as we print before `PS1` and need to reposition the cursor correctly
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Of these issues, **only bullet 4 could be fixed**. Everything else was outside of our control.
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## Conclusion
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Oh My Posh is a tool that needs to be easy to use, maintain and be 100% reliable. One of the core principles of Oh My Posh is that it should
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**never break the shell**. The [`rprompt`][rprompt] feature in bash has never been reliable enough, and it was _hard to debug_ when it broke.
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I spent countless hours debugging issues with the [`rprompt`][rprompt] in bash, but it's **time to move on**. If you
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want to use the [`rprompt`][rprompt] feature, I would recommend using a shell that supports it natively, like [nushell], [zsh] or [fish].
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## What about Powershell?
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The [`rprompt`][rprompt] feature in PowerShell is **100% reliable**, even if it's not natively supported. PowerShell doesn't
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have the same challenges calculating the actual prompt width like bash, there's _no need to escape anything_.
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We can use the first implementation mentioned above, and it works like a charm.
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[rprompt]: /docs/configuration/block#type
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[nushell]: https://www.nushell.sh/
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[zsh]: https://www.zsh.org/
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[fish]: https://fishshell.com/
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