Add key mutation commands (add, set, unset, delete) for v0.0.4

- Add CLI commands: add, set, unset, delete for modifying .env files
- Add value auto-quoting for special characters (spaces, #, quotes, newlines)
- Add example file detection with warning for .env.example/.sample/.template
- Add library API: addEnvFileKey, setEnvFileKey, unsetEnvFileKey, deleteEnvFileKeys
- Export isExampleEnvFile utility
- Add tests for new mutation operations
- Update README and CHANGELOG

CHANGELOG.md

@@ -4,6 +4,27 @@ All notable changes to this project are documented in this file.
 
 This project follows [Semantic Versioning](https://semver.org/) and the format is loosely based on [Keep a Changelog](https://keepachangelog.com/).
 
+## 0.0.4 (2026-01-15)
+
+### Added
+
+- Key mutation commands (CLI): `add`, `set`, `unset`, `delete`.
+  - `add`: Add a new key-value pair (fails if key already exists).
+  - `set`: Create or update a key-value pair (idempotent).
+  - `unset`: Set a key's value to empty (`KEY=`).
+  - `delete`: Remove key(s) from the file entirely (supports `--keys` for multiple).
+- Value auto-quoting: Values with spaces, `#`, quotes, newlines, or other special characters are automatically double-quoted with proper escaping.
+- Example file detection: Warns when targeting `.env.example`, `.env.sample`, `.env.template`, `.env.dist`, `.env.default(s)` files. Suppressible with `--no-example-warning`.
+- Library API exports for key mutations: `addEnvFileKey`, `setEnvFileKey`, `unsetEnvFileKey`, `deleteEnvFileKeys`.
+- Utility export: `isExampleEnvFile` for detecting example/template env files.
+- Test coverage for new mutation operations.
+
+### Changed
+
+- Package version bumped to `0.0.4`.
+- Updated CLI help text to include new commands and notes about dry-run behavior.
+- Updated README with documentation for new commands and library API.
+
 ## 0.0.3 (2026-01-13)
 
 ### Added

README.md

@@ -61,11 +61,16 @@ Commands:
 - `format --file <path> [--mode sections|global] [--sort alpha|none] [--write]`
 - `reorder --file <path> [--mode sections|global] [--sort alpha|none] [--write]`
 - `annotate --file <path> --key <KEY> --comment <text> [--line <n>] [--write]`
+- `add --file <path> --key <KEY> [--value <v> | --value-stdin] [--write]`
+- `set --file <path> --key <KEY> [--value <v> | --value-stdin] [--write]`
+- `unset --file <path> --key <KEY> [--write]`
+- `delete --file <path> (--key <KEY> | --keys <K1,K2>) [--write]`
 
 Notes for file operations:
 
-- Commands that modify files (`copy`, `format`/`reorder`, `annotate`) are dry-run unless `--write` is provided.
+- Commands that modify files (`copy`, `format`/`reorder`, `annotate`, `add`, `set`, `unset`, `delete`) are dry-run unless `--write` is provided.
 - These commands never print secret values; output includes keys, booleans, and line numbers only.
+- When targeting example files (`.env.example`, `.env.sample`, `.env.template`), a warning is emitted. Use `--no-example-warning` to suppress.
 
 ### List keys
 
@@ -217,6 +222,38 @@ envsitter format --file .env --mode sections --sort alpha --write
 envsitter reorder --file .env --mode sections --sort alpha --write
 ```
 
+### Add a new key (fails if key exists)
+
+```bash
+envsitter add --file .env --key NEW_API_KEY --value "sk-xxx" --write
+# or via stdin (recommended to avoid shell history):
+node -e "process.stdout.write('sk-xxx')" | envsitter add --file .env --key NEW_API_KEY --value-stdin --write
+```
+
+### Set a key (creates or updates)
+
+```bash
+envsitter set --file .env --key API_KEY --value "new-value" --write
+# or via stdin:
+node -e "process.stdout.write('new-value')" | envsitter set --file .env --key API_KEY --value-stdin --write
+```
+
+### Unset a key (set to empty value)
+
+```bash
+envsitter unset --file .env --key OLD_KEY --write
+```
+
+### Delete keys
+
+```bash
+# Single key:
+envsitter delete --file .env --key DEPRECATED_KEY --write
+
+# Multiple keys:
+envsitter delete --file .env --keys OLD_KEY,UNUSED_KEY,LEGACY_KEY --write
+```
+
 ## Output contract (for LLMs)
 
 General rules:
@@ -241,6 +278,8 @@ JSON outputs:
 - `copy --json` -> `{ "from": string, "to": string, "onConflict": string, "willWrite": boolean, "wrote": boolean, "hasChanges": boolean, "issues": Array<...>, "plan": Array<...> }`
 - `format --json` / `reorder --json` -> `{ "file": string, "mode": string, "sort": string, "willWrite": boolean, "wrote": boolean, "hasChanges": boolean, "issues": Array<...> }`
 - `annotate --json` -> `{ "file": string, "willWrite": boolean, "wrote": boolean, "hasChanges": boolean, "issues": Array<...>, "plan": { ... } }`
+- `add --json` / `set --json` / `unset --json` -> `{ "file": string, "key": string, "willWrite": boolean, "wrote": boolean, "hasChanges": boolean, "issues": Array<...>, "plan": { "key": string, "action": "added"|"updated"|"unset"|"key_exists"|"not_found"|"no_change", "line"?: number } }`
+- `delete --json` -> `{ "file": string, "keys": string[], "willWrite": boolean, "wrote": boolean, "hasChanges": boolean, "issues": Array<...>, "plan": Array<{ "key": string, "action": "deleted"|"not_found", "line"?: number }> }`
 
 ## Library API
 
@@ -259,7 +298,16 @@ const match = await es.matchCandidate('OPENAI_API_KEY', 'candidate-secret');
 ### File operations via the library
 
 ```ts
-import { annotateEnvFile, copyEnvFileKeys, formatEnvFile, validateEnvFile } from 'envsitter';
+import {
+  addEnvFileKey,
+  annotateEnvFile,
+  copyEnvFileKeys,
+  deleteEnvFileKeys,
+  formatEnvFile,
+  setEnvFileKey,
+  unsetEnvFileKey,
+  validateEnvFile
+} from 'envsitter';
 
 await validateEnvFile('.env');
 
@@ -273,6 +321,18 @@ await copyEnvFileKeys({
 
 await annotateEnvFile({ file: '.env', key: 'DATABASE_URL', comment: 'prod only', write: true });
 await formatEnvFile({ file: '.env', mode: 'sections', sort: 'alpha', write: true });
+
+// Add a new key (fails if exists)
+await addEnvFileKey({ file: '.env', key: 'NEW_KEY', value: 'new_value', write: true });
+
+// Set a key (creates or updates)
+await setEnvFileKey({ file: '.env', key: 'API_KEY', value: 'updated_value', write: true });
+
+// Unset a key (set to empty)
+await unsetEnvFileKey({ file: '.env', key: 'OLD_KEY', write: true });
+
+// Delete keys
+await deleteEnvFileKeys({ file: '.env', keys: ['DEPRECATED', 'UNUSED'], write: true });
 ```
 
 ### Match operators via the library

package.json

@@ -1,6 +1,6 @@
 {
   "name": "envsitter",
-  "version": "0.0.3",
+  "version": "0.0.4",
   "private": false,
   "type": "module",
   "description": "Safely inspect and match .env secrets without exposing values",

src/cli.ts

@@ -1,8 +1,9 @@
 #!/usr/bin/env node
 import { readFile } from 'node:fs/promises';
 import { EnvSitter, type EnvSitterMatcher } from './envsitter.js';
-import { annotateDotenvKey, copyDotenvKeys, formatDotenv, validateDotenv } from './dotenv/edit.js';
+import { addDotenvKey, annotateDotenvKey, copyDotenvKeys, deleteDotenvKeys, formatDotenv, setDotenvKey, unsetDotenvKey, validateDotenv } from './dotenv/edit.js';
 import { readTextFileOrEmpty, writeTextFileAtomic } from './dotenv/io.js';
+import { isExampleEnvFile } from './dotenv/utils.js';
 
 
 function parseRegex(input: string): RegExp {
@@ -104,6 +105,12 @@ function jsonOut(value: unknown): void {
   process.stdout.write(`${JSON.stringify(value, null, 2)}\n`);
 }
 
+function warnIfExampleFile(file: string, noWarn: boolean): void {
+  if (!noWarn && isExampleEnvFile(file)) {
+    process.stderr.write(`Warning: ${file} appears to be an example/template file. Use --no-example-warning to suppress.\n`);
+  }
+}
+
 function printHelp(): void {
   process.stdout.write(
     [
@@ -120,13 +127,18 @@ function printHelp(): void {
       '  format --file <path> [--mode sections|global] [--sort alpha|none] [--write]',
       '  reorder --file <path> [--mode sections|global] [--sort alpha|none] [--write]',
       '  annotate --file <path> --key <KEY> --comment <text> [--line <n>] [--write]',
+      '  add --file <path> --key <KEY> [--value <v> | --value-stdin] [--write]',
+      '  set --file <path> --key <KEY> [--value <v> | --value-stdin] [--write]',
+      '  unset --file <path> --key <KEY> [--write]',
+      '  delete --file <path> (--key <KEY> | --keys <K1,K2>) [--write]',
       '',
       'Pepper options:',
       '  --pepper-file <path>   Defaults to .envsitter/pepper (auto-created)',
       '',
       'Notes:',
       '  match --op defaults to is_equal. Ops: exists,is_empty,is_equal,partial_match_regex,partial_match_prefix,partial_match_suffix,is_number,is_string,is_boolean',
-      '  Candidate values passed via argv may end up in shell history. Prefer --candidate-stdin.',
+      '  Values passed via argv may end up in shell history. Prefer --value-stdin or --candidate-stdin.',
+      '  Mutation commands (add, set, unset, delete) are dry-run unless --write is provided.',
       ''
     ].join('\n')
   );
@@ -273,6 +285,106 @@ async function run(): Promise<number> {
     return result.issues.length > 0 ? 2 : 0;
   }
 
+  if (cmd === 'add') {
+    const file = requireValue(typeof flags['file'] === 'string' ? flags['file'] : undefined, '--file is required');
+    const key = requireValue(typeof flags['key'] === 'string' ? flags['key'] : undefined, '--key is required');
+    const noExampleWarning = flags['no-example-warning'] === true;
+    warnIfExampleFile(file, noExampleWarning);
+
+    const valueArg = typeof flags['value'] === 'string' ? flags['value'] : undefined;
+    const valueStdin = flags['value-stdin'] === true ? (await readStdinText()).trimEnd() : undefined;
+    const value = valueStdin ?? valueArg ?? '';
+
+    const contents = await readTextFileOrEmpty(file);
+    const result = addDotenvKey({ contents, key, value });
+
+    const willWrite = flags['write'] === true;
+    if (willWrite && result.hasChanges) await writeTextFileAtomic(file, result.output);
+
+    if (json) {
+      jsonOut({ file, key, willWrite, wrote: willWrite && result.hasChanges, hasChanges: result.hasChanges, issues: result.issues, plan: result.plan });
+    } else {
+      process.stdout.write(`${result.plan.action}: ${result.plan.key}${result.plan.line ? ` L${result.plan.line}` : ''}\n`);
+    }
+
+    return result.plan.action === 'key_exists' ? 2 : 0;
+  }
+
+  if (cmd === 'set') {
+    const file = requireValue(typeof flags['file'] === 'string' ? flags['file'] : undefined, '--file is required');
+    const key = requireValue(typeof flags['key'] === 'string' ? flags['key'] : undefined, '--key is required');
+    const noExampleWarning = flags['no-example-warning'] === true;
+    warnIfExampleFile(file, noExampleWarning);
+
+    const valueArg = typeof flags['value'] === 'string' ? flags['value'] : undefined;
+    const valueStdin = flags['value-stdin'] === true ? (await readStdinText()).trimEnd() : undefined;
+    const value = valueStdin ?? valueArg ?? '';
+
+    const contents = await readTextFileOrEmpty(file);
+    const result = setDotenvKey({ contents, key, value });
+
+    const willWrite = flags['write'] === true;
+    if (willWrite && result.hasChanges) await writeTextFileAtomic(file, result.output);
+
+    if (json) {
+      jsonOut({ file, key, willWrite, wrote: willWrite && result.hasChanges, hasChanges: result.hasChanges, issues: result.issues, plan: result.plan });
+    } else {
+      process.stdout.write(`${result.plan.action}: ${result.plan.key}${result.plan.line ? ` L${result.plan.line}` : ''}\n`);
+    }
+
+    return 0;
+  }
+
+  if (cmd === 'unset') {
+    const file = requireValue(typeof flags['file'] === 'string' ? flags['file'] : undefined, '--file is required');
+    const key = requireValue(typeof flags['key'] === 'string' ? flags['key'] : undefined, '--key is required');
+    const noExampleWarning = flags['no-example-warning'] === true;
+    warnIfExampleFile(file, noExampleWarning);
+
+    const contents = await readFile(file, 'utf8');
+    const result = unsetDotenvKey({ contents, key });
+
+    const willWrite = flags['write'] === true;
+    if (willWrite && result.hasChanges) await writeTextFileAtomic(file, result.output);
+
+    if (json) {
+      jsonOut({ file, key, willWrite, wrote: willWrite && result.hasChanges, hasChanges: result.hasChanges, issues: result.issues, plan: result.plan });
+    } else {
+      process.stdout.write(`${result.plan.action}: ${result.plan.key}${result.plan.line ? ` L${result.plan.line}` : ''}\n`);
+    }
+
+    return result.plan.action === 'not_found' ? 2 : 0;
+  }
+
+  if (cmd === 'delete') {
+    const file = requireValue(typeof flags['file'] === 'string' ? flags['file'] : undefined, '--file is required');
+    const noExampleWarning = flags['no-example-warning'] === true;
+    warnIfExampleFile(file, noExampleWarning);
+
+    const keyArg = typeof flags['key'] === 'string' ? flags['key'] : undefined;
+    const keysArg = typeof flags['keys'] === 'string' ? flags['keys'] : undefined;
+
+    const keys = keyArg ? [keyArg] : keysArg ? parseList(keysArg) : undefined;
+    if (!keys || keys.length === 0) throw new Error('Provide --key or --keys');
+
+    const contents = await readFile(file, 'utf8');
+    const result = deleteDotenvKeys({ contents, keys });
+
+    const willWrite = flags['write'] === true;
+    if (willWrite && result.hasChanges) await writeTextFileAtomic(file, result.output);
+
+    if (json) {
+      jsonOut({ file, keys, willWrite, wrote: willWrite && result.hasChanges, hasChanges: result.hasChanges, issues: result.issues, plan: result.plan });
+    } else {
+      for (const p of result.plan) {
+        process.stdout.write(`${p.action}: ${p.key}${p.line ? ` L${p.line}` : ''}\n`);
+      }
+    }
+
+    const allNotFound = result.plan.every((p) => p.action === 'not_found');
+    return allNotFound ? 2 : 0;
+  }
+
   const file = requireValue(typeof flags['file'] === 'string' ? flags['file'] : undefined, '--file is required');
   const pepper = getPepperOptions(flags);
   const envsitter = EnvSitter.fromDotenvFile(file);

src/dotenv/edit.ts

@@ -1,4 +1,5 @@
 import { parseDotenvDocument, stringifyDotenvDocument, type DotenvParsedAssignment, type DotenvIssue } from './document.js';
+import { buildAssignmentLine } from './utils.js';
 
 export type DotenvWriteMode = 'dry-run' | 'write';
 
@@ -312,3 +313,219 @@ export function validateDotenv(contents: string): ValidateDotenvResult {
   const doc = parseDotenvDocument(contents);
   return { issues: doc.issues, ok: doc.issues.length === 0 };
 }
+
+export type KeyMutationAction = 'added' | 'updated' | 'unset' | 'deleted' | 'key_exists' | 'not_found' | 'no_change';
+
+export type KeyMutationPlanItem = {
+  key: string;
+  action: KeyMutationAction;
+  line?: number;
+};
+
+export type AddDotenvKeyResult = {
+  output: string;
+  issues: DotenvIssue[];
+  plan: KeyMutationPlanItem;
+  hasChanges: boolean;
+};
+
+export function addDotenvKey(options: { contents: string; key: string; value: string }): AddDotenvKeyResult {
+  const doc = parseDotenvDocument(options.contents);
+  const issues: DotenvIssue[] = [...doc.issues];
+  const assignments = listAssignments(doc.lines);
+
+  const existing = lastAssignmentForKey(assignments, options.key);
+  if (existing) {
+    return {
+      output: options.contents,
+      issues,
+      plan: { key: options.key, action: 'key_exists', line: existing.line },
+      hasChanges: false
+    };
+  }
+
+  const newLine = buildAssignmentLine(options.key, options.value);
+  const lastLine = doc.lines.length > 0 ? Math.max(...doc.lines.map((l) => l.line)) : 0;
+
+  doc.lines.push({
+    kind: 'assignment',
+    line: lastLine + 1,
+    raw: newLine,
+    leadingWhitespace: '',
+    exported: false,
+    key: options.key,
+    keyColumn: 1,
+    beforeEqWhitespace: '',
+    afterEqRaw: newLine.slice(options.key.length + 1),
+    quote: 'none',
+    value: options.value
+  });
+
+  return {
+    output: stringifyDotenvDocument(doc),
+    issues,
+    plan: { key: options.key, action: 'added', line: lastLine + 1 },
+    hasChanges: true
+  };
+}
+
+export type SetDotenvKeyResult = {
+  output: string;
+  issues: DotenvIssue[];
+  plan: KeyMutationPlanItem;
+  hasChanges: boolean;
+};
+
+export function setDotenvKey(options: { contents: string; key: string; value: string }): SetDotenvKeyResult {
+  const doc = parseDotenvDocument(options.contents);
+  const issues: DotenvIssue[] = [...doc.issues];
+  const assignments = listAssignments(doc.lines);
+
+  const existing = lastAssignmentForKey(assignments, options.key);
+
+  if (!existing) {
+    const newLine = buildAssignmentLine(options.key, options.value);
+    const lastLine = doc.lines.length > 0 ? Math.max(...doc.lines.map((l) => l.line)) : 0;
+
+    doc.lines.push({
+      kind: 'assignment',
+      line: lastLine + 1,
+      raw: newLine,
+      leadingWhitespace: '',
+      exported: false,
+      key: options.key,
+      keyColumn: 1,
+      beforeEqWhitespace: '',
+      afterEqRaw: newLine.slice(options.key.length + 1),
+      quote: 'none',
+      value: options.value
+    });
+
+    return {
+      output: stringifyDotenvDocument(doc),
+      issues,
+      plan: { key: options.key, action: 'added', line: lastLine + 1 },
+      hasChanges: true
+    };
+  }
+
+  if (existing.value === options.value) {
+    return {
+      output: options.contents,
+      issues,
+      plan: { key: options.key, action: 'no_change', line: existing.line },
+      hasChanges: false
+    };
+  }
+
+  const newRaw = `${existing.leadingWhitespace}${existing.exported ? 'export ' : ''}${options.key}${existing.beforeEqWhitespace}=${buildAssignmentLine('', options.value).slice(1)}`;
+
+  for (let i = doc.lines.length - 1; i >= 0; i--) {
+    const l = doc.lines[i];
+    if (l?.kind === 'assignment' && l.key === options.key && l.line === existing.line) {
+      doc.lines[i] = { ...l, raw: newRaw, value: options.value, afterEqRaw: newRaw.slice(newRaw.indexOf('=') + 1) };
+      break;
+    }
+  }
+
+  return {
+    output: stringifyDotenvDocument(doc),
+    issues,
+    plan: { key: options.key, action: 'updated', line: existing.line },
+    hasChanges: true
+  };
+}
+
+export type UnsetDotenvKeyResult = {
+  output: string;
+  issues: DotenvIssue[];
+  plan: KeyMutationPlanItem;
+  hasChanges: boolean;
+};
+
+export function unsetDotenvKey(options: { contents: string; key: string }): UnsetDotenvKeyResult {
+  const doc = parseDotenvDocument(options.contents);
+  const issues: DotenvIssue[] = [...doc.issues];
+  const assignments = listAssignments(doc.lines);
+
+  const existing = lastAssignmentForKey(assignments, options.key);
+
+  if (!existing) {
+    return {
+      output: options.contents,
+      issues,
+      plan: { key: options.key, action: 'not_found' },
+      hasChanges: false
+    };
+  }
+
+  if (existing.value === '') {
+    return {
+      output: options.contents,
+      issues,
+      plan: { key: options.key, action: 'no_change', line: existing.line },
+      hasChanges: false
+    };
+  }
+
+  const newRaw = `${existing.leadingWhitespace}${existing.exported ? 'export ' : ''}${options.key}${existing.beforeEqWhitespace}=`;
+
+  for (let i = doc.lines.length - 1; i >= 0; i--) {
+    const l = doc.lines[i];
+    if (l?.kind === 'assignment' && l.key === options.key && l.line === existing.line) {
+      doc.lines[i] = { ...l, raw: newRaw, value: '', afterEqRaw: '', quote: 'none' };
+      break;
+    }
+  }
+
+  return {
+    output: stringifyDotenvDocument(doc),
+    issues,
+    plan: { key: options.key, action: 'unset', line: existing.line },
+    hasChanges: true
+  };
+}
+
+export type DeleteDotenvKeysResult = {
+  output: string;
+  issues: DotenvIssue[];
+  plan: KeyMutationPlanItem[];
+  hasChanges: boolean;
+};
+
+export function deleteDotenvKeys(options: { contents: string; keys: readonly string[] }): DeleteDotenvKeysResult {
+  const doc = parseDotenvDocument(options.contents);
+  const issues: DotenvIssue[] = [...doc.issues];
+  const assignments = listAssignments(doc.lines);
+
+  const plan: KeyMutationPlanItem[] = [];
+  const keysToDelete = new Set<string>();
+  const linesToDelete = new Set<number>();
+
+  for (const key of options.keys) {
+    const existing = lastAssignmentForKey(assignments, key);
+    if (!existing) {
+      plan.push({ key, action: 'not_found' });
+      continue;
+    }
+    keysToDelete.add(key);
+    linesToDelete.add(existing.line);
+    plan.push({ key, action: 'deleted', line: existing.line });
+  }
+
+  if (linesToDelete.size === 0) {
+    return { output: options.contents, issues, plan, hasChanges: false };
+  }
+
+  doc.lines = doc.lines.filter((l) => {
+    if (l.kind !== 'assignment') return true;
+    return !linesToDelete.has(l.line);
+  });
+
+  return {
+    output: stringifyDotenvDocument(doc),
+    issues,
+    plan,
+    hasChanges: true
+  };
+}

src/dotenv/utils.ts

@@ -0,0 +1,31 @@
+const EXAMPLE_FILE_PATTERN = /\.env\.(example|sample|template|dist|defaults?)$/i;
+
+export function isExampleEnvFile(filePath: string): boolean {
+  return EXAMPLE_FILE_PATTERN.test(filePath);
+}
+
+export function quoteValue(value: string): string {
+  if (value === '') return '';
+
+  const hasWhitespace = /\s/.test(value);
+  const hasSpecialChars = /[#"'\\$`]/.test(value);
+  const hasControlChars = /[\n\r\t]/.test(value);
+  const hasEdgeSpaces = value.startsWith(' ') || value.endsWith(' ');
+
+  const needsQuoting = hasWhitespace || hasSpecialChars || hasControlChars || hasEdgeSpaces;
+
+  if (!needsQuoting) return value;
+
+  const escaped = value
+    .replace(/\\/g, '\\\\')
+    .replace(/"/g, '\\"')
+    .replace(/\n/g, '\\n')
+    .replace(/\r/g, '\\r')
+    .replace(/\t/g, '\\t');
+
+  return `"${escaped}"`;
+}
+
+export function buildAssignmentLine(key: string, value: string): string {
+  return `${key}=${quoteValue(value)}`;
+}

src/file-ops.ts

@@ -1,6 +1,17 @@
 import { readFile } from 'node:fs/promises';
 
-import { annotateDotenvKey, copyDotenvKeys, formatDotenv, validateDotenv } from './dotenv/edit.js';
+import {
+  addDotenvKey,
+  annotateDotenvKey,
+  copyDotenvKeys,
+  deleteDotenvKeys,
+  formatDotenv,
+  setDotenvKey,
+  unsetDotenvKey,
+  validateDotenv,
+  type KeyMutationAction,
+  type KeyMutationPlanItem
+} from './dotenv/edit.js';
 import { readTextFileOrEmpty, writeTextFileAtomic } from './dotenv/io.js';
 
 export type DotenvIssue = {
@@ -172,3 +183,143 @@ export async function validateEnvFile(file: string): Promise<ValidateEnvFileResu
   const result = validateDotenv(contents);
   return { file, ok: result.ok, issues: result.issues };
 }
+
+export type { KeyMutationAction, KeyMutationPlanItem };
+
+export type AddEnvFileKeyResult = {
+  file: string;
+  key: string;
+  willWrite: boolean;
+  wrote: boolean;
+  hasChanges: boolean;
+  issues: DotenvIssue[];
+  plan: KeyMutationPlanItem;
+};
+
+export async function addEnvFileKey(options: {
+  file: string;
+  key: string;
+  value: string;
+  write?: boolean;
+}): Promise<AddEnvFileKeyResult> {
+  const contents = await readTextFileOrEmpty(options.file);
+  const result = addDotenvKey({ contents, key: options.key, value: options.value });
+
+  const willWrite = options.write === true;
+  if (willWrite && result.hasChanges) {
+    await writeTextFileAtomic(options.file, result.output);
+  }
+
+  return {
+    file: options.file,
+    key: options.key,
+    willWrite,
+    wrote: willWrite && result.hasChanges,
+    hasChanges: result.hasChanges,
+    issues: result.issues,
+    plan: result.plan
+  };
+}
+
+export type SetEnvFileKeyResult = {
+  file: string;
+  key: string;
+  willWrite: boolean;
+  wrote: boolean;
+  hasChanges: boolean;
+  issues: DotenvIssue[];
+  plan: KeyMutationPlanItem;
+};
+
+export async function setEnvFileKey(options: {
+  file: string;
+  key: string;
+  value: string;
+  write?: boolean;
+}): Promise<SetEnvFileKeyResult> {
+  const contents = await readTextFileOrEmpty(options.file);
+  const result = setDotenvKey({ contents, key: options.key, value: options.value });
+
+  const willWrite = options.write === true;
+  if (willWrite && result.hasChanges) {
+    await writeTextFileAtomic(options.file, result.output);
+  }
+
+  return {
+    file: options.file,
+    key: options.key,
+    willWrite,
+    wrote: willWrite && result.hasChanges,
+    hasChanges: result.hasChanges,
+    issues: result.issues,
+    plan: result.plan
+  };
+}
+
+export type UnsetEnvFileKeyResult = {
+  file: string;
+  key: string;
+  willWrite: boolean;
+  wrote: boolean;
+  hasChanges: boolean;
+  issues: DotenvIssue[];
+  plan: KeyMutationPlanItem;
+};
+
+export async function unsetEnvFileKey(options: {
+  file: string;
+  key: string;
+  write?: boolean;
+}): Promise<UnsetEnvFileKeyResult> {
+  const contents = await readFile(options.file, 'utf8');
+  const result = unsetDotenvKey({ contents, key: options.key });
+
+  const willWrite = options.write === true;
+  if (willWrite && result.hasChanges) {
+    await writeTextFileAtomic(options.file, result.output);
+  }
+
+  return {
+    file: options.file,
+    key: options.key,
+    willWrite,
+    wrote: willWrite && result.hasChanges,
+    hasChanges: result.hasChanges,
+    issues: result.issues,
+    plan: result.plan
+  };
+}
+
+export type DeleteEnvFileKeysResult = {
+  file: string;
+  keys: string[];
+  willWrite: boolean;
+  wrote: boolean;
+  hasChanges: boolean;
+  issues: DotenvIssue[];
+  plan: KeyMutationPlanItem[];
+};
+
+export async function deleteEnvFileKeys(options: {
+  file: string;
+  keys: readonly string[];
+  write?: boolean;
+}): Promise<DeleteEnvFileKeysResult> {
+  const contents = await readFile(options.file, 'utf8');
+  const result = deleteDotenvKeys({ contents, keys: options.keys });
+
+  const willWrite = options.write === true;
+  if (willWrite && result.hasChanges) {
+    await writeTextFileAtomic(options.file, result.output);
+  }
+
+  return {
+    file: options.file,
+    keys: [...options.keys],
+    willWrite,
+    wrote: willWrite && result.hasChanges,
+    hasChanges: result.hasChanges,
+    issues: result.issues,
+    plan: result.plan
+  };
+}

src/index.ts

@@ -13,12 +13,24 @@ export {
 export { type PepperOptions, resolvePepper } from './pepper.js';
 
 export {
+  addEnvFileKey,
   annotateEnvFile,
   copyEnvFileKeys,
+  deleteEnvFileKeys,
   formatEnvFile,
+  setEnvFileKey,
+  unsetEnvFileKey,
   validateEnvFile,
+  type AddEnvFileKeyResult,
   type AnnotateEnvFileResult,
   type CopyEnvFilesResult,
+  type DeleteEnvFileKeysResult,
   type FormatEnvFileResult,
+  type KeyMutationAction,
+  type KeyMutationPlanItem,
+  type SetEnvFileKeyResult,
+  type UnsetEnvFileKeyResult,
   type ValidateEnvFileResult
 } from './file-ops.js';
+
+export { isExampleEnvFile } from './dotenv/utils.js';

src/test/dotenv-file-ops.test.ts

@@ -4,7 +4,7 @@ import { mkdtemp, readFile, writeFile } from 'node:fs/promises';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 
-import { annotateEnvFile, copyEnvFileKeys, formatEnvFile, validateEnvFile } from '../index.js';
+import { addEnvFileKey, annotateEnvFile, copyEnvFileKeys, deleteEnvFileKeys, formatEnvFile, isExampleEnvFile, setEnvFileKey, unsetEnvFileKey, validateEnvFile } from '../index.js';
 
 async function makeTempFile(fileName: string, contents: string): Promise<string> {
   const dir = await mkdtemp(join(tmpdir(), 'envsitter-ops-'));
@@ -75,3 +75,110 @@ test('formatEnvFile sorts assignments within sections', async () => {
   const expected = ['# section one', 'A=1', 'B=2', '', '# section two', 'Y=8', 'Z=9', ''].join('\n');
   assert.equal(out, expected);
 });
+
+test('addEnvFileKey adds new key and fails if key exists', async () => {
+  const file = await makeTempFile('.env', 'EXISTING=value\n');
+
+  const addNew = await addEnvFileKey({ file, key: 'NEW_KEY', value: 'new_value', write: true });
+  assert.equal(addNew.wrote, true);
+  assert.equal(addNew.plan.action, 'added');
+
+  const contents = await readFile(file, 'utf8');
+  assert.ok(contents.includes('NEW_KEY=new_value'));
+
+  const addExisting = await addEnvFileKey({ file, key: 'EXISTING', value: 'other', write: true });
+  assert.equal(addExisting.wrote, false);
+  assert.equal(addExisting.plan.action, 'key_exists');
+});
+
+test('addEnvFileKey auto-quotes values with special characters', async () => {
+  const file = await makeTempFile('.env', '');
+
+  await addEnvFileKey({ file, key: 'SIMPLE', value: 'simple', write: true });
+  await addEnvFileKey({ file, key: 'WITH_SPACE', value: 'has space', write: true });
+  await addEnvFileKey({ file, key: 'WITH_HASH', value: 'before#after', write: true });
+  await addEnvFileKey({ file, key: 'WITH_NEWLINE', value: 'line1\nline2', write: true });
+
+  const contents = await readFile(file, 'utf8');
+  assert.ok(contents.includes('SIMPLE=simple'));
+  assert.ok(contents.includes('WITH_SPACE="has space"'));
+  assert.ok(contents.includes('WITH_HASH="before#after"'));
+  assert.ok(contents.includes('WITH_NEWLINE="line1\\nline2"'));
+});
+
+test('setEnvFileKey creates or updates key', async () => {
+  const file = await makeTempFile('.env', 'A=1\n');
+
+  const setNew = await setEnvFileKey({ file, key: 'B', value: '2', write: true });
+  assert.equal(setNew.wrote, true);
+  assert.equal(setNew.plan.action, 'added');
+
+  const setExisting = await setEnvFileKey({ file, key: 'A', value: 'updated', write: true });
+  assert.equal(setExisting.wrote, true);
+  assert.equal(setExisting.plan.action, 'updated');
+
+  const contents = await readFile(file, 'utf8');
+  assert.ok(contents.includes('A=updated'));
+  assert.ok(contents.includes('B=2'));
+
+  const setSame = await setEnvFileKey({ file, key: 'A', value: 'updated', write: true });
+  assert.equal(setSame.wrote, false);
+  assert.equal(setSame.plan.action, 'no_change');
+});
+
+test('unsetEnvFileKey sets key to empty value', async () => {
+  const file = await makeTempFile('.env', 'A=value\nB=\n');
+
+  const unsetA = await unsetEnvFileKey({ file, key: 'A', write: true });
+  assert.equal(unsetA.wrote, true);
+  assert.equal(unsetA.plan.action, 'unset');
+
+  const contents = await readFile(file, 'utf8');
+  assert.ok(contents.includes('A=\n') || contents.includes('A='));
+  assert.ok(!contents.includes('A=value'));
+
+  const unsetB = await unsetEnvFileKey({ file, key: 'B', write: true });
+  assert.equal(unsetB.wrote, false);
+  assert.equal(unsetB.plan.action, 'no_change');
+
+  const unsetMissing = await unsetEnvFileKey({ file, key: 'MISSING', write: true });
+  assert.equal(unsetMissing.wrote, false);
+  assert.equal(unsetMissing.plan.action, 'not_found');
+});
+
+test('deleteEnvFileKeys removes keys from file', async () => {
+  const file = await makeTempFile('.env', 'A=1\nB=2\nC=3\n');
+
+  const deleteSingle = await deleteEnvFileKeys({ file, keys: ['B'], write: true });
+  assert.equal(deleteSingle.wrote, true);
+  assert.equal(deleteSingle.plan.length, 1);
+  assert.equal(deleteSingle.plan[0]?.action, 'deleted');
+
+  let contents = await readFile(file, 'utf8');
+  assert.ok(!contents.includes('B='));
+  assert.ok(contents.includes('A=1'));
+  assert.ok(contents.includes('C=3'));
+
+  const deleteMultiple = await deleteEnvFileKeys({ file, keys: ['A', 'C', 'MISSING'], write: true });
+  assert.equal(deleteMultiple.wrote, true);
+  assert.equal(deleteMultiple.plan.filter((p) => p.action === 'deleted').length, 2);
+  assert.equal(deleteMultiple.plan.filter((p) => p.action === 'not_found').length, 1);
+
+  contents = await readFile(file, 'utf8');
+  assert.ok(!contents.includes('A='));
+  assert.ok(!contents.includes('C='));
+});
+
+test('isExampleEnvFile detects example/template files', () => {
+  assert.equal(isExampleEnvFile('.env'), false);
+  assert.equal(isExampleEnvFile('.env.local'), false);
+  assert.equal(isExampleEnvFile('.env.production'), false);
+  assert.equal(isExampleEnvFile('.env.example'), true);
+  assert.equal(isExampleEnvFile('.env.sample'), true);
+  assert.equal(isExampleEnvFile('.env.template'), true);
+  assert.equal(isExampleEnvFile('.env.dist'), true);
+  assert.equal(isExampleEnvFile('.env.default'), true);
+  assert.equal(isExampleEnvFile('.env.defaults'), true);
+  assert.equal(isExampleEnvFile('/path/to/.env.example'), true);
+  assert.equal(isExampleEnvFile('.env.EXAMPLE'), true);
+});