feat: implement $_ special parameter for last command argument

[lu-zero]

Add support for the $_ special parameter, which expands to the last argument of the previous command.

• Add LastCommandArgument variant to SpecialParameter enum
• Update PEG parser to match $_ only when not followed by variable-name chars (so $__phase is correctly parsed as variable reference)
• Add last_command_argument field to Shell struct
• Set last command argument before each command execution
• Add test cases for $_ behavior

[github-actions]

Public API changes for crate: brush-core

Added items

+pub fn brush_core::Shell<SE>::update_last_arg_variable(&mut self, last_arg: core::option::Option<&str>)
+pub fn brush_core::Shell<SE>::update_last_arg_variable(&mut self, last_arg: core::option::Option<&str>)
+pub fn brush_core::ShellState::update_last_arg_variable(&mut self, last_arg: core::option::Option<&str>)

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	17.15 μs	17.28 μs	0.13 μs	⚪ Unchanged
eval_arithmetic	0.15 μs	0.14 μs	-0.00 μs	⚪ Unchanged
expand_one_string	1.52 μs	1.67 μs	0.15 μs	🟠 +10.07%
for_loop	25.51 μs	29.04 μs	3.52 μs	🟠 +13.81%
full_peg_complex	58.44 μs	58.07 μs	-0.37 μs	⚪ Unchanged
full_peg_for_loop	6.20 μs	6.23 μs	0.03 μs	⚪ Unchanged
full_peg_nested_expansions	16.73 μs	16.88 μs	0.15 μs	🟠 +0.89%
full_peg_pipeline	4.33 μs	4.23 μs	-0.10 μs	⚪ Unchanged
full_peg_simple	1.83 μs	1.77 μs	-0.06 μs	🟢 -3.17%
function_call	2.70 μs	3.00 μs	0.30 μs	🟠 +10.98%
instantiate_shell	54.50 μs	54.37 μs	-0.13 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	25811.02 μs	26574.85 μs	763.83 μs	🟠 +2.96%
parse_peg_bash_completion	2057.07 μs	2061.50 μs	4.43 μs	⚪ Unchanged
parse_peg_complex	19.01 μs	18.75 μs	-0.26 μs	⚪ Unchanged
parse_peg_for_loop	1.98 μs	1.95 μs	-0.03 μs	⚪ Unchanged
parse_peg_pipeline	2.10 μs	2.12 μs	0.02 μs	⚪ Unchanged
parse_peg_simple	1.09 μs	1.07 μs	-0.02 μs	⚪ Unchanged
run_echo_builtin_command	17.95 μs	18.62 μs	0.67 μs	⚪ Unchanged
tokenize_sample_script	3.49 μs	3.44 μs	-0.06 μs	🟢 -1.58%

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/commands.rs	🟢 91.74%	🟢 91.91%	🟢 0.17%
brush-core/src/results.rs	🟢 83.33%	🟢 80.16%	🔴 -3.17%
brush-core/src/shell/state.rs	🔴 0%	🟢 100%	🟢 100%
brush-core/src/wellknownvars.rs	🟢 84.63%	🟢 84.67%	🟢 0.04%
brush-parser/src/ast.rs	🔴 45.21%	🔴 46.44%	🟢 1.23%
Overall Coverage	🟢 75.01%	🟢 75.06%	🟢 0.05%

Minimum allowed coverage is 70%, this run produced 75.06%

Test Summary: bash-completion test suite

Outcome	Count	Percentage
✅ Pass	1580	74.92
❗️ Error	17	0.81
❌ Fail	158	7.49
⏩ Skip	339	16.07
❎ Expected Fail	13	0.62
✔️ Unexpected Pass	2	0.09
📊 Total	2109	100.00

[lu-zero]

Now it should be cleaner

[Copilot]

Pull request overview

Adds bash-compatible handling for $_ by initializing and updating the _ shell variable to track the last argument of the previous command, and updates compatibility tests to cover/enable this behavior (notably #479).

Changes:

• Initialize _ to $0 during well-known variable setup and update _ from command execution paths.
• Set child-process _ env var for external commands to the resolved executable path (bash-like behavior).
• Add/adjust compat tests for $_ and remove the known-failure flag for the existing dollar-_ case.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
xtask/src/test.rs	Comment wrapping (no functional change).
brush-shell/tests/cases/compat/well_known_vars.yaml	Removes known_failure for the dollar-_ compat case.
brush-shell/tests/cases/compat/special_parameters.yaml	Adds a basic compat case covering $_ updates across sequential commands.
brush-parser/src/tokenizer.rs	Doc comment wrapping (no functional change).
brush-core/src/wellknownvars.rs	Initializes _ to the current shell name ($0).
brush-core/src/shell/state.rs	Adds ShellState::update_last_arg_variable helper to update _.
brush-core/src/shell.rs	Doc comment wrapping (no functional change).
brush-core/src/interp.rs	Comment wrapping/formatting (no functional change).
brush-core/src/commands.rs	Updates _ after builtin/function/external execution; sets child _ env var for external commands.
brush-builtins/src/read.rs	Doc comment wrapping (no functional change).

[Copilot]

$_ (via _ variable) is updated only after invoke_shell_function returns, which means the function body will see the previous command’s value for $_ instead of the last argument of the function invocation (bash updates _ for the duration of the command being executed, i.e., after argument expansion but before running the builtin/function body). Consider moving the _ update to just before invoking the function (and likewise for the builtin paths) so $_ is correct inside the executed command, not just for the next command.

[Copilot]

PR description mentions adding a SpecialParameter variant / PEG changes for parsing $_, but in the current codebase $_ is still parsed as a normal named variable (_) (no SpecialParameter addition found). If the intent is to implement this via the regular _ variable (as this PR does by setting _ in the environment), the PR description should be updated to reflect that; otherwise the missing parser/AST changes need to be added.

[Copilot]

The new $_ case covers basic sequential commands, but it doesn’t exercise the tricky scoping/update boundaries where bash behavior tends to differ (e.g., inside a function body immediately after invocation, inside a subshell/command substitution, and across pipelines/backgrounded commands). Adding a few compat cases covering those would help prevent regressions and confirm the update timing matches bash.

[reubeno]

Looking pretty good! Just a few comments.

[reubeno]

In each of these cases, we have an owned String of the last argument (or at least an Option of one). Couldn't we save a string clone by having the setter in Shell require the owned String?

[reubeno]

Could you please add a few test cases that cover other scenarios? e.g.:

• Pipeline with multiple commands
• Commands with && / ||
• Command substitution / subshell
• Builtin invocation
• Function invocation
• Commands that fail

We typically don't need to go so broad but because there are multiple code paths for handling different kinds of commands here, I think these will be important to ensure compatibility (and prevent us from regressing it in the future).