Change:
Fish usually has available, but the aliases are now missing, when the shell is not a login shell (presumably because of ). Have Then launch as a not-login shell. Try to use . System: 17.09pre107265. (Hummingbird)
Agh, what a mess. Thanks for raising this issue.
I think this issue is a little subtler than one would think: aliases are sourced, but only when the parent shell has not already sourced the interactive configuration. This is out of line with the behavior of the other shells on NixOS. I have a fix which I've tested with your example here. I will post a link to the relevant PR momentarily (after I put it up).
So apparently outgoing SSH is blocked from the WiFi on this cruise ship, so I can't push commits with the remotes I have set up, and my sister is nagging me to get off the computer so we can go out. If you need a quick fix on your own machine (I will be back later today, while I still have internet access, to submit the new PR), just change the lines in the NixOS module for fish () to and your aliases will get sourced every time the shell starts, so changes will be reflected without a reboot or manually unsetting the relevant variable before starting a new shell (). (The latter will do the trick if you want a quick fix without any changes to your Nixpkgs tree.)
there's a fix in this commit:
Thanks!

Discussion:
<ins> {stdenv, fetchurl, yacc, flex, pkgconfig, glib, xz}: stdenv.mkDerivation rec { version = "0.15.2"; name = "vala-${version}"; src = fetchurl { url = "mirror://gnome/sources/vala/0.15/${name}.tar.xz"; sha256 = "0g71zq6dpqrw2f40wfzdf18fdw41ymr17laqniy2kr622hkxdi8w"; }; buildNativeInputs = [ yacc flex pkgconfig xz ]; buildInputs = [ glib ]; meta = { description = "Compiler for the GObject type system"; homepage = "http://live.gnome.org/Vala"; license = "free-copyleft"; platforms = stdenv.lib.platforms.all; maintainers = [ stdenv.lib.maintainers.antono ]; }; } </ins>

<ins> {stdenv, fetchurl, yacc, flex, pkgconfig, glib, xz}: stdenv.mkDerivation rec { version = "0.16.1"; name = "vala-${version}"; src = fetchurl { url = "mirror://gnome/sources/vala/0.16/${name}.tar.xz"; sha256 = "1n708n9ixyy9qrzyv1wf4ybvcclx43ib9ki028wwpvkz6kv8zqlb"; }; buildNativeInputs = [ yacc flex pkgconfig xz ]; buildInputs = [ glib ]; meta = { description = "Compiler for the GObject type system"; homepage = "http://live.gnome.org/Vala"; license = "free-copyleft"; platforms = stdenv.lib.platforms.all; maintainers = [ stdenv.lib.maintainers.antono ]; }; } </ins>

{stdenv, fetchurl, yacc, flex, pkgconfig, glib, xz}: stdenv.mkDerivation rec { <del> name = "vala-0.14.2"; </del> <ins> version = "0.17.2"; name = "vala-${version}"; </ins> src = fetchurl { <del> url = mirror://gnome/sources/vala/0.14/vala-0.14.2.tar.xz; sha256 = "1l5kllw9vpwv24lzv9fp64l3sad46wpxgvsgryrwlrjg91w6jzl0"; </del> <ins> url = "mirror://gnome/sources/vala/0.17/${name}.tar.xz"; sha256 = "09i2s0dwmrk147ind2dx7nq845g12fp6fsjqrphhrr0dbi0zzgh3"; </ins> }; buildNativeInputs = [ yacc flex pkgconfig xz ];

meta = { description = "Compiler for the GObject type system"; homepage = "http://live.gnome.org/Vala"; <ins> license = "free-copyleft"; platforms = stdenv.lib.platforms.all; maintainers = [ stdenv.lib.maintainers.antono ]; </ins> }; }

urweb = callPackage ../development/compilers/urweb { }; <del> vala = callPackage ../development/compilers/vala { }; </del> <ins> vala = vala17; vala15 = callPackage ../development/compilers/vala/15.2.nix { }; vala16 = callPackage ../development/compilers/vala/16.1.nix { }; vala17 = callPackage ../development/compilers/vala/default.nix { }; </ins> visualcpp = callPackage ../development/compilers/visual-c++ { };