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. If error output already suggests a nix shell: Run that exact command
4. Else search locally: Use nix search nixpkgs <cmd> and pick best match
5. Run via nix shell: nix shell nixpkgs#<package> -c <original-command>
6. Delegate only if needed: If no match or ambiguous, delegate to nixos agent

Implementation Steps

1. Environment Detection

Check if we're in NixOS:

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

2. Use Suggested nix shell (if present)

If the error output already includes a recommended nix shell nixpkgs#... -c ..., run it as-is.

3. Local Search + Run

Search locally and pick best match from plain text output:

nix search nixpkgs <cmd>

Selection order:

1. Exact attr path equals cmd
2. Attr path contains cmd
3. Description contains cmd

Then run:

nix shell nixpkgs#<package> -c <original-command>

4. Delegate When Needed

If no match or too many ambiguous results, delegate to nixos agent to resolve.

Examples

Missing tree command

1. User runs: tree /some/path
2. Error: zsh: command not found: tree
3. Detect NixOS environment ✓
4. Run nix search nixpkgs tree
5. Pick tree package, run nix shell nixpkgs#tree -c tree /some/path
6. Return: actual directory tree output

Missing jq command

1. User runs: cat data.json | jq '.name'
2. Error: command not found: jq
3. Run nix search nixpkgs jq
4. Pick jq package, run with stdin preserved
5. Return: actual JSON parsing result

Missing cargo command

1. User runs: cargo build
2. Error: command not found: cargo
3. Run nix search nixpkgs cargo
4. Pick cargo package, run nix shell nixpkgs#cargo -c cargo build
5. Return: actual build output or errors

Suggested nix shell already provided

1. Error output includes: use nix shell nixpkgs#ripgrep -c rg ...
2. Run that exact command
3. Return command output

Edge Cases

Multiple Package Options

Try local selection order first. If still ambiguous, delegate to nixos agent.

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

Still proceed (might need different package). Inform user about current nix shell context.

Error Handling

Package Not Found

If local search fails, delegate to nixos agent for broader search/fallbacks.

Command Failures

Return actual command results. Distinguish 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

Only delegate when local search fails or ambiguous. When delegated, nixos agent handles:

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

Main Agent Role

• Detects NixOS environment
• Runs suggested nix shell if present
• Else runs local nix search
• Delegates only if needed
• Returns 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