➜  ~ cd $(mktemp -d)
➜  tmp.PN7dEmBP vim simple.c
➜  tmp.PN7dEmBP vim simple_builder.sh
➜  tmp.PN7dEmBP nix-repl
Welcome to Nix version 1.11.15. Type :? for help.

nix-repl> :l <nixpkgs>
Added 8100 variables.

nix-repl> simple = derivation { name = "simple"; builder = "${bash}/bin/bash"; args = [ ./simple_builder.sh ]; gcc = gcc; coreutils = coreutils; src = ./simple.c; system = builtins.currentSystem; }

nix-repl> :b simple
these derivations will be built:
  /nix/store/ck6a571bvpgj4iw1afiwzp409law6ji5-simple.drv
building path(s) ‘/nix/store/68xymm5wq1zv6z3kr0qwplr354ckxq1q-simple’
/nix/store/9sll05wmn9b05f0pm1pwfmmq92fg0syh-simple.c: In function 'main':
/nix/store/9sll05wmn9b05f0pm1pwfmmq92fg0syh-simple.c:2:3: warning: implicit declaration of function 'puts' [-Wimplicit-function-declaration]
   puts("Simple!");
   ^~~~
/nix/store/wh6raqzph3kb75n7wqyp327wnwy5zcid-gcc-wrapper-6.4.0/bin/as: assembler (clang) not installed
builder for ‘/nix/store/ck6a571bvpgj4iw1afiwzp409law6ji5-simple.drv’ failed with exit code 1
error: build of ‘/nix/store/ck6a571bvpgj4iw1afiwzp409law6ji5-simple.drv’ failed

nix-repl>