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?modifier CLI Syntax

The ?modifier syntax is an alternative method of specifying CLI level modifiers, which allows users to specify modifiers for a target pattern by appending a ? to the pattern followed by the modifiers delimited by +.

buck2 build repo//foo:bar?debug+linux

Since the ?modifier syntax associates modifiers to target patterns rather than the command, it allows us to potentially condense multiple command invocations.

# Using --modifier flags
buck2 build --modifier debug repo//foo:bar
buck2 build --modifier debug --modifier linux repo//foo:bar

# Using ?modifier syntax
buck2 build repo//foo:bar?debug repo//foo:bar?debug+linux

Use in --target-universe flag​

For command invocations that specify the --target-universe flag, the ?modifier syntax must be used as the argument of the --target-universe flag.

# Use of `?modifier` syntax in `--target-universe` flag
buck2 build repo//foo:bar --target-universe repo//foo:baz?debug

Using the ?modifier syntax outside of the --target-universe flag when it is specified will throw an error.

# Command that would fail and throw an error
buck2 build repo//foo:bar?debug --target-universe repo//foo:baz

Incompatible with --modifier flag​

If the ?modifier syntax is used in a command invocation, the --modifier flag cannot be used.

# Commands that would fail and throw an error
buck2 build --modifier debug repo//foo:bar?linux
buck2 build --modifier debug repo//foo:bar --target-universe repo//foo:baz?linux

Commands that support ?modifier syntax​

Currently, the following commands support the ?modifier syntax:

  • build
  • cquery (only in --target-universe flag)
  • ctargets
  • run
  • install
  • audit providers
  • audit subtargets

Interaction with flags in build command​

To make it easier to see what output comes from which configuration when using the ?modifier syntax. The ?modifier syntax is also displayed in select flags in the build command.

?modifier syntax in --show-output flag​

# CLI input
buck2 build --show-output repo//foo:bar?debug repo//foo:bar?linux

# Example output
repo//foo:bar?debug buck-out/v2/gen/...
repo//foo:bar?linux buck-out/v2/gen/...

?modifier syntax in --build-report flag​

# CLI input
buck2 build repo//foo:bar?debug repo//foo:bar?linux --build-report repo/build_report.json
// Build report output in repo/build_report.json with ommited fields
{
   "results": {
       "repo//foo:bar?debug":  {
           "success": "SUCCESS",
           ...
       },
       "repo//foo:bar?linux":  {
           "success": "SUCCESS",
           ...
       },
   }
}