Add a relative path to $PATH on fish startup

The best way I have found to persistently add a path to your $PATH is

set -U fish_user_paths $fish_user_paths ~/path/name

This prepends to $PATH. And since it's persistent, the path stays in $PATH on shell restarts.

It's more efficient than putting a command in your config.fish to modify your $PATH, because it only runs once compared to running on every shell restart.

The variable fish_user_paths is intended to be set by the user¹, as stated by ridiculousfish, the maintainer of fish.

Consider creating a fish function for convenience: ²

# ~/.config/fish/functions/add_to_path.fishfunction add_to_path --description 'Persistently prepends paths to your PATH'  set --universal fish_user_paths $fish_user_paths $argvend

And use it as:

$ add_to_path foo bar  # Adds foo/ and bar/ to your PATH

Notes

1. On that page the author gives the example set -U fish_user_paths ~/bin. This overwrites fish_user_paths with a single value of ~/bin. To avoid losing existing paths set in fish_user_paths, be sure to include $fish_user_paths in addition to any new paths being added (as seen in my answer).

2. My dotfiles contain a slightly more advanced version that skips adding duplicates https://github.com/dideler/dotfiles/blob/master/.config/fish/functions/add_to_user_path.fish

I'd never heard of fish before this. I just installed it so I could try it out (and deleted a few paragraphs I had written here before realizing that fish is a shell).

It looks like set PATH dir-name $PATH is the right syntax to prepend a directory to $PATH.

But adding a relative directory name to $PATH is almost certainly a bad idea, and your shell is doing you a favor by warning you when the directory doesn't exist. (fish is designed to be user-friendly.)

Use an absolute path instead:

set PATH $PWD/bin $PATH

and first check whether $PWD/bin exists, printing an error message if it doesn't.

As for the "set: Value too large to be stored in data type" message, could you be adding the directory to your $PATH multiple times? There should be some way to check whether a directory is already in $PATH before adding it.

I think the answer is that using set -U is a red herring. Instead, add the following to ~/.config/fish/config.fish:

if status --is-interactive    set PATH $PATH ~/.local/bin;end