Nix Repl

Interactive Nix expression evaluator. Useful for exploring nixpkgs, testing derivations, and debugging Nix values.

Launch

nix repl                          # bare repl
nix repl '<nixpkgs>'              # load nixpkgs
nix repl /path/to/flake           # load a flake
nix repl --file default.nix       # load a nix file

Common commands

Command	Description
:?	Show help
:p <expr>	Evaluate and print expression
<name> = <expr>	Variable assignment
:env	Print environment
:a <expr>	Add expression attrset to variable scope
:l <expr>	Load and merge attribute set into scope
:lf <path>	Load a flake into scope
:doc <expr>	Show documentation for an attribute
:e <expr>	Open expression in $EDITOR
:b <drv>	Build a derivation
:sh <expr>	Load a devshell
:t <expr>	Show the type of an expression
:q	Quit
<Tab>	Autocomplete

Typical session

nix-repl> :l <nixpkgs>
Added 20000 variables.
 
nix-repl> pkgs.hello
«derivation /nix/store/...-hello-2.12.1.drv»
 
nix-repl> pkgs.hello.meta.description
"A program that produces a familiar, friendly greeting"
 
nix-repl> :b pkgs.hello
 
nix-repl> lib.strings.toUpper "hello"
"HELLO"
 
nix-repl> builtins.attrNames pkgs.python3.pkgs | lib.take 5
[ "Babel" "Flask" "Jinja2" "Markdown" "Pillow" ]

Loading a flake

nix-repl> :l .#            # load current flake outputs
 
nix-repl> packages.x86_64-linux.default
«derivation ...»
 
nix-repl> nixosConfigurations.myhost.config.networking.hostName
"myhost"

Tips

• Use <Tab> heavily — attribute set completion is very helpful
• :b builds to /tmp/nix-repl-* and symlinks result
• Multi-line: wrap in ( ... ) or use let ... in
• Access builtins directly: builtins.nixVersion, builtins.currentSystem