Introduction To Nix (Part II)

August 2018

by Michael Fellinger

In this Talk

We will explore Nix

What you will learn

• Using nix repl
• Doing our first nix build
• Managing environments using nix-env
• Developing in nix-shell

Getting started

For our first session, we’ll simply run Nix in a Docker container, so we don’t spend too much time on setup due to our different system configurations.

docker volume create nix_store
docker run -v nix_store:/nix/store --rm -it lnl7/nix

More information about this image are at https://github.com/LnL7/nix-docker

Starting the Nix REPL

This makes sure you’ll have access to all packages in the nixpkgs repository.

nix repl '<nixpkgs>'

Getting some help

bash-4.4# nix repl '<nixpkgs>'
Welcome to Nix version 2.0.4. Type :? for help.

Loading '<nixpkgs>'...
Added 9189 variables.

nix-repl> :?
The following commands are available:

  <expr>        Evaluate and print expression
  <x> = <expr>  Bind expression to variable
  :a <expr>     Add attributes from resulting set to scope
  :b <expr>     Build derivation
  :i <expr>     Build derivation, then install result into current profile
  :l <path>     Load Nix expression and add it to scope
  :p <expr>     Evaluate and print expression recursively
  :q            Exit nix-repl
  :r            Reload all files
  :s <expr>     Build dependencies of derivation, then start nix-shell
  :t <expr>     Describe result of evaluation
  :u <expr>     Build derivation, then start nix-shell

What does this mean?

Let me guide you through each command with an example

Evaluate and print expression

nix-repl> 1
1

nix-repl> "Hello, World!"
"Hello, World!"

nix-repl> ["lists"]
[ "lists" ]

nix-repl> { this = "set"; }
{ this = "set"; }

Bind expression to variable

nix-repl> a = 1

nix-repl> b = 2

nix-repl> a + b
3

Add attributes from resulting set to scope

nix-repl> :a builtins
Added 95 variables.

nix-repl> attrNames { this = "set"; }
[ "this" ]

nix-repl> length [1]
1

Build derivation

nix-repl> :b hello
these paths will be fetched (0.04 MiB download, 0.19 MiB unpacked):
  /nix/store/aq3byv8z9ax5zqbqi5b73q06yq8gfl99-hello-2.10
copying path '/nix/store/aq3byv8z9ax5zqbqi5b73q06yq8gfl99-hello-2.10' from 'https://cache.nixos.org'...
warning: you did not specify '--add-root'; the result might be removed by the garbage collector
/nix/store/aq3byv8z9ax5zqbqi5b73q06yq8gfl99-hello-2.10

this derivation produced the following outputs:
  out -> /nix/store/aq3byv8z9ax5zqbqi5b73q06yq8gfl99-hello-2.10



bash-4.4# /nix/store/aq3by*-hello-2.10/bin/hello 
Hello, world!

Install result into current profile

nix-repl> :i hello
installing 'hello-2.10.drv'
these paths will be fetched (0.04 MiB download, 0.19 MiB unpacked):
  /nix/store/aq3byv8z9ax5zqbqi5b73q06yq8gfl99-hello-2.10
copying path '/nix/store/aq3byv8z9ax5zqbqi5b73q06yq8gfl99-hello-2.10' from 'https://cache.nixos.org'...
building '/nix/store/ar8sjvpp0i0v8m2fn77fmw0kj1808w51-user-environment.drv'...
created 2 symlinks in user environment

nix-repl> 
bash-4.4# hello
Hello, world!

Load Nix expression and add it to scope

In a fresh nix repl:

bash-4.4# nix repl            
Welcome to Nix version 2.0.4. Type :? for help.

nix-repl> :l <nixpkgs>
Added 9189 variables.

nix-repl> hello
«derivation /nix/store/m5chcjbj7p08sy8adqpkr2rzf6rmdfh6-hello-2.10.drv»

Evaluate and print expression recursively

nix-repl> x = { a = { b = 2; }; }

nix-repl> x
{ a = { ... }; }

nix-repl> :p x
{ a = { b = 2; }; }

Build derivation, then start nix-shell

nix-repl> :u hello

[nix-shell:/]# type -f hello
hello is hashed (/nix/store/aq3byv8z9ax5zqbqi5b73q06yq8gfl99-hello-2.10/bin/hello)

[nix-shell:/]# type -f gcc
bash: type: gcc: not found

In this shell we have access to gcc, glibc, and binutils

Build dependencies of derivation, then start nix-shell

nix-repl> :s hello

[nix-shell:/]# type -f hello
bash: type: hello: not found

[nix-shell:/]# type -f gcc
gcc is /nix/store/38picpf5cl6d7lj38h8191vmxlxjbgq3-gcc-wrapper-7.3.0/bin/gcc

In this shell we have access to the built hello, but not its dependencies anymore.

Describe result of evaluation

nix-repl> :t hello.outPath
a string with context

On to nix-shell

Let’s run a command only once without installing it

nix-shell -p nix-info --run 'nix-info -m'
....
 - system: `"x86_64-linux"`
 - host os: `Linux 4.14.55`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.0.4`
 - nixpkgs: `/nix/store/1a7blwzdwsafkw1x5fql61ddbv1v3r25-nixpkgs-unstable-2018-07-17`

Development environment

cd /root
mkdir first
cd first
vim shell.nix

Our first shell.nix

with import <nixpkgs> {};
mkShell {
  buildInputs = [ tcl ];
}

Now invoke the file

nix-shell will automatically look for a shell.nix or default.nix in the current directory and put you at the first of its “phases”.

bash-4.4# nix-shell 

[nix-shell:~/first]# echo $phases
nobuildPhase

[nix-shell:~/first]# which tclsh
/nix/store/z6jc7mvhiziwy4mf86lsm0wlq1aaqdzz-tcl-8.6.6/bin/tclsh

[nix-shell:~/first]# tclsh
% puts "Hello, World!"
Hello, World!