Nixos Command Not Found

NixOS Command Not Found Handler

Automatically handles "command not found" errors in NixOS environments by searching for packages and retrying with nix shell.

When to Use

Activate this skill when:

• Command fails with "command not found" or "No such file or directory"
• User is in NixOS environment (detected by /etc/nixos existence or NIX_STORE env var)
• Command appears to be a standard Unix tool or known package

Workflow

1. Detect NixOS environment: Check for /etc/nixos or NIX_STORE env var
2. Extract command name: Get the missing command from error output
3. Delegate everything to nixos agent: Agent finds package AND runs command
4. Return command output: Agent provides the actual command result

Implementation Steps

1. Environment Detection

Check if we're in NixOS:

test -d /etc/nixos || test -n "$NIX_STORE"

2. Delegate Complete Task

Use Task tool with nixos agent to handle everything:

Task(
  description="Run missing command with nix shell",
  prompt="The command '<original-command>' failed with 'command not found'. Please:
1. Search for the NixOS package that provides this command
2. Run the command using 'nix shell nixpkgs#<package> -c <original-command>'
3. Return the actual command output/result",
  subagent_type="nixos"
)

3. Return Results

The nixos agent returns the actual command execution result, not just package info.

Examples

Missing tree command

1. User runs: tree /some/path
2. Error: zsh: command not found: tree
3. Detect NixOS environment ✓
4. Delegate to nixos agent: "Run 'tree /some/path' with nix shell"
5. Agent finds tree package, runs nix shell nixpkgs#tree -c tree /some/path
6. Agent returns: actual directory tree output

Missing jq command

1. User runs: cat data.json | jq '.name'
2. Error: command not found: jq
3. Delegate to nixos agent: "Run 'jq '.name'' with nix shell"
4. Agent finds jq package, runs with stdin preserved
5. Agent returns: actual JSON parsing result

Missing cargo command

1. User runs: cargo build
2. Error: command not found: cargo
3. Delegate to nixos agent: "Run 'cargo build' with nix shell"
4. Agent finds cargo package, runs nix shell nixpkgs#cargo -c cargo build
5. Agent returns: actual build output or errors

Edge Cases

Multiple Package Options

Nixos agent handles all selection logic internally:

• Uses nixos MCP tools to find best match
• Considers exact name matches, package descriptions, etc.
• Picks most appropriate option or asks user if truly ambiguous

Complex Commands

Nixos agent handles command complexity:

• Preserves pipes, redirects, and shell syntax
• Uses appropriate shell escaping when needed
• Maintains stdin/stdout/stderr as expected

Already in Nix Shell

Nixos agent handles this case:

• Detects if already in nix shell
• Still proceeds (might need different package)
• Can inform user about current nix shell context

Error Handling

Package Not Found

Nixos agent handles all error scenarios:

• Tries broader search terms and fallbacks
• Searches programs, options, or suggests alternatives
• Returns helpful error message if no package found

Command Failures

Nixos agent returns actual command results:

• Shows real command output if successful
• Returns actual error messages if command fails
• Distinguishes between "package not found" vs "command failed"

Integration Notes

Detection Triggers

Monitor for these error patterns:

• command not found: <cmd>
• <cmd>: No such file or directory
• zsh: command not found: <cmd>
• bash: <cmd>: command not found

Agent Responsibilities

Nixos agent handles all technical details:

• Maintains current working directory
• Preserves environment variables
• Keeps stdin/stdout/stderr intact
• Shows what package was used
• Provides appropriate feedback

Main Agent Role

Main agent just:

• Detects NixOS environment
• Delegates to nixos agent
• Returns agent's command results to user

Configuration

Excluded Commands

Don't handle these (likely typos or not packages):

• Single letters: a, b, x
• Common typos: sl, gerp
• Shell builtins that failed: cd, export

Channel Preference

• Default to nixpkgs# (follows system channel)
• Could use nixpkgs/unstable# for newest versions
• Respect user's flake configuration if detected