ci: add format script and use in CI pipeline

The format script can be used to automatically format the Nix source
files and also verify that the files are formatted using the `-c`
command argument.

At the moment some files are exempt from the formatting to avoid
causing merge conflicts in active pull requests.

Finally, update the contribution guidelines to note that `nixfmt`
should be used.

1 files changed, 70 insertions, 0 deletions

diff --git a/format b/format
new file mode 100755
index 00000000000..539ae60de22
--- /dev/null
+++ b/format
@@ -0,0 +1,70 @@
+#! /usr/bin/env nix-shell
+#! nix-shell -i bash -p findutils nixfmt
+
+CHECK_ARG=
+
+case $1 in
+    -h)
+        echo "$0 [-c]"
+        ;;
+    -c)
+        CHECK_ARG=-c
+        ;;
+esac
+
+# The first block of excludes are files where nixfmt does a poor job,
+# IMHO. The second block of excludes are files touched by open pull
+# requests and we want to avoid merge conflicts.
+find . -name '*.nix' \
+  ! -path ./modules/programs/irssi.nix \
+  \
+  ! -path ./home-manager/default.nix \
+  ! -path ./home-manager/home-manager.nix \
+  ! -path ./modules/accounts/email.nix \
+  ! -path ./modules/default.nix \
+  ! -path ./modules/files.nix \
+  ! -path ./modules/home-environment.nix \
+  ! -path ./modules/lib/default.nix \
+  ! -path ./modules/lib/file-type.nix \
+  ! -path ./modules/lib/types.nix \
+  ! -path ./modules/manual.nix \
+  ! -path ./modules/misc/dconf.nix \
+  ! -path ./modules/misc/gtk.nix \
+  ! -path ./modules/misc/news.nix \
+  ! -path ./modules/misc/nixpkgs.nix \
+  ! -path ./modules/misc/xdg.nix \
+  ! -path ./modules/modules.nix \
+  ! -path ./modules/programs/afew.nix \
+  ! -path ./modules/programs/alot.nix \
+  ! -path ./modules/programs/bash.nix \
+  ! -path ./modules/programs/emacs.nix \
+  ! -path ./modules/programs/firefox.nix \
+  ! -path ./modules/programs/fish.nix \
+  ! -path ./modules/programs/gpg.nix \
+  ! -path ./modules/programs/lesspipe.nix \
+  ! -path ./modules/programs/neovim.nix \
+  ! -path ./modules/programs/ssh.nix \
+  ! -path ./modules/programs/tmux.nix \
+  ! -path ./modules/programs/vscode.nix \
+  ! -path ./modules/programs/zsh.nix \
+  ! -path ./modules/services/gpg-agent.nix \
+  ! -path ./modules/services/kbfs.nix \
+  ! -path ./modules/services/keybase.nix \
+  ! -path ./modules/services/mpd.nix \
+  ! -path ./modules/services/sxhkd.nix \
+  ! -path ./modules/services/window-managers/i3.nix \
+  ! -path ./modules/systemd.nix \
+  ! -path ./nix-darwin/default.nix \
+  ! -path ./tests/default.nix \
+  ! -path ./tests/modules/home-environment/default.nix \
+  ! -path ./tests/modules/home-environment/session-variables.nix \
+  ! -path ./tests/modules/programs/gpg/override-defaults.nix \
+  ! -path ./tests/modules/programs/tmux/default.nix \
+  ! -path ./tests/modules/programs/tmux/disable-confirmation-prompt.nix \
+  ! -path ./tests/modules/programs/tmux/secure-socket-enabled.nix \
+  ! -path ./tests/modules/programs/zsh/session-variables.nix \
+  ! -path ./tests/modules/services/sxhkd/service.nix \
+  ! -path ./tests/modules/systemd/default.nix \
+  ! -path ./tests/modules/systemd/services.nix \
+  ! -path ./tests/modules/systemd/session-variables.nix \
+  -exec nixfmt $CHECK_ARG {} +