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Prefer a SHELL instruction over repeating pwsh or powershell wrappers in RUN.

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Description

This rule detects repeated shell-form RUN instructions that invoke PowerShell explicitly, for example:
  • RUN pwsh -Command ...
  • RUN powershell -Command ...
  • RUN @powershell -Command ...
When PowerShell is already the effective shell via a preceding SHELL [...] instruction, the rule does nothing. The recommendation applies on both Windows and Linux. A Linux image such as mcr.microsoft.com/powershell:ubuntu-22.04 still benefits from switching to a PowerShell SHELL once multiple PowerShell RUN commands appear.

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Why this matters

Repeating the full wrapper on every RUN line adds noise and makes PowerShell-specific defaults easy to forget. A dedicated SHELL instruction:
  • makes repeated PowerShell build steps easier to read
  • centralizes the shell choice instead of duplicating it across RUNs
  • lets tally inject sane build defaults once:
    • $ErrorActionPreference = 'Stop'
    • $PSNativeCommandUseErrorActionPreference = $true
    • $ProgressPreference = 'SilentlyContinue'

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Examples

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Before (violation)

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After (fixed with --fix --fix-unsafe)

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Windows example

becomes:

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Configuration

This rule has no rule-specific options today.

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Fix behavior

The fixer is intentionally conservative. It only rewrites repeated PowerShell wrappers when the repeated RUN instructions share the same executable and the same arguments before -Command (for example, repeated pwsh -NoProfile -Command ...). On Windows container stages, the fixer also collaborates with tally/prefer-run-heredoc:
  • it can qualify bare RUN powershell ... chains, not only explicit -Command wrappers
  • when a cmd stage is converted to a PowerShell SHELL, later PowerShell-safe RUN instructions can stay under that shell instead of forcing an immediate restore to cmd
  • after that rewrite, tally/prefer-run-heredoc can merge the resulting PowerShell RUN sequence into a PowerShell heredoc in the same fix pass

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References