feat(shell): add unset command (#6430)

## What's the problem this PR addresses?

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<!-- Link all issues that it closes. (Closes/Resolves #xxxx.) -->

Fixes #4447.

## How did you fix it?

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I fixed it by adding an `unset` built-in.

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---------

Co-authored-by: merceyz <merceyz@users.noreply.github.com>

.yarn/versions/2f8bb76f.yml

@@ -0,0 +1,24 @@
+releases:
+  "@yarnpkg/cli": minor
+  "@yarnpkg/shell": minor
+
+declined:
+  - "@yarnpkg/plugin-compat"
+  - "@yarnpkg/plugin-constraints"
+  - "@yarnpkg/plugin-dlx"
+  - "@yarnpkg/plugin-essentials"
+  - "@yarnpkg/plugin-init"
+  - "@yarnpkg/plugin-interactive-tools"
+  - "@yarnpkg/plugin-nm"
+  - "@yarnpkg/plugin-npm-cli"
+  - "@yarnpkg/plugin-pack"
+  - "@yarnpkg/plugin-patch"
+  - "@yarnpkg/plugin-pnp"
+  - "@yarnpkg/plugin-pnpm"
+  - "@yarnpkg/plugin-stage"
+  - "@yarnpkg/plugin-typescript"
+  - "@yarnpkg/plugin-version"
+  - "@yarnpkg/plugin-workspace-tools"
+  - "@yarnpkg/builder"
+  - "@yarnpkg/core"
+  - "@yarnpkg/doctor"

packages/yarnpkg-shell/sources/index.ts

@@ -166,6 +166,15 @@ const BUILTINS = new Map<string, ShellBuiltin>([
     return await setTimeout(1000 * seconds, 0);
   }],
 
+  [`unset`, async (args: Array<string>, opts: ShellOptions, state: ShellState) => {
+    for (const name of args) {
+      delete state.environment[name];
+      delete state.variables[name];
+    }
+
+    return 0;
+  }],
+
   [`__ysh_run_procedure`, async (args: Array<string>, opts: ShellOptions, state: ShellState) => {
     const procedure = state.procedures[args[0]];
 

packages/yarnpkg-shell/tests/shell.test.ts

@@ -2094,5 +2094,25 @@ describe(`Shell`, () => {
         });
       });
     });
+
+    describe(`unset`, () => {
+      it(`should unset one variable`, async () => {
+        await expectResult(bufferResult(
+          `FOO=bar; unset FOO; echo $FOO`,
+        ), {
+          exitCode: 1,
+          stderr: `Unbound variable "FOO"\n`,
+        });
+      });
+
+      it(`should unset multiple variables`, async () => {
+        await expectResult(bufferResult(
+          `A=1 B=2; unset A B; echo $A; echo $B`,
+        ), {
+          exitCode: 1,
+          stderr: `Unbound variable "A"\nUnbound variable "B"\n`,
+        });
+      });
+    });
   });
 });