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+---
+date: 2025-07-08
+title: Writing robust bash scripts with Nix
+permalink: /daily/2025/07/08/writing-robust-bash-scripts-nix
+---
+
+I have a lot of custom Bash scripts I use - some to perform simple commands like `git log` with some additional arguments, to more complex ones that mount and unmount USB devices plugged into my laptop.
+
+A lot of people will post their scripts online along with their dotfiles for others to read and take inspiration from.
+
+Mine are in my [nix config][0] directory and each script is added as a custom package I can install.
+
+Here's an example of a script written as a Nix package:
+
+```nix
+{ pkgs }:
+
+pkgs.writeShellApplication {
+  name = "get-tags";
+
+  runtimeInputs = with pkgs; [ git ];
+
+  text = ''
+    if [[ "$#" -gt 0 ]]; then
+      git tag | grep "$*"
+      exit 0
+    fi
+
+    git tag
+  '';
+}
+```
+
+It gets a filtered list of Git tags within a repository using the `git tag` and `grep` commands.
+
+Nix automatically adds the shebang line and sets some default shell options, so those aren't included within the script text.
+
+It also automatically runs `shellcheck` to ensure the script is correct.
+
+## Injecting Dependencies
+
+This script depends on `git`. Without it, it would not run successfully.
+
+Instead of assuming it is installed, `runtimeInputs` ensures any dependencies are present and the script can be executed, even if the dependencies aren't enabled globally.
+
+Scripts can also rely on other scripts.
+
+`get-tags` has a corresponding `count-tags` script that counts the returned tags:
+
+```nix
+pkgs.writeShellApplication {
+  name = "count-tags";
+
+  runtimeInputs = with pkgs; [
+    coreutils
+    get-tags
+  ];
+
+  text = ''
+    get-tags "''${1:-}" | wc -l
+  '';
+}
+```
+
+`get-tags` is declared as a dependency, as well as `coreutils` to add the `wc` command that counts the lines.
+
+## Here's the thing
+
+Using this approach gives me more robust scripts that are checked and verified, and will always have the required dependencies.
+
+And, because they are added as custom Nix packages in my configuration, I am able to decide which scripts to install on which computer, giving me the most flexibility.
+
+[0]: https://code.oliverdavies.uk/opdavies/nix-config