Comma

Whenever a program lives somewhere in nixpkgs, Comma will find it there and run it, and the command for that is the program name with a comma in front. A line like , cowsay neato works without the package being added to your environment first.

Under the surface it wraps nix shell -c together with nix-index. The index has to come from somewhere: download the prebuilt database from the nix-index-database project, or enable Comma through that project's module, and keep the database up to date.

Installation follows the usual Nix paths. Adding comma to systemPackages in a NixOS configuration is the preferred route, while installing it into a Nix environment works but is discouraged. If the nix-index-database module enables Comma for you, do not add the package yourself as well.

• Level 0: caching is off and every command is resolved from scratch.
• Level 1: the derivation you picked for a command is remembered, so the same one comes back next time.
• Level 2: the evaluated path is cached as well, the default, and repeat runs become far faster.

The level is chosen with a cache-level flag or the COMMA_CACHING variable. Caching a path also means you may not run the newest version of a command, especially when the garbage collector rarely runs, so step back to level 1 when freshness matters. Running under sudo needs the index variable preserved if the database sits outside its default location.