nobe4 / Posts / Poor Man's devenv: Now with Caching _

In Poor Man’s devenv, I wrote a small bash script called use. It finds project files, maps them to Nix packages, and opens a shell with:

exec nix-shell -p "${packages[@]}"

It worked, but every shell still paid the nix-shell setup cost. It was just slow enough for me to change it.

The new version caches the built environment.

One cache per package set ¶

use sorts and deduplicates the package names, then hashes the result:

hash_input="$(printf '%s\n' "${shell_packages[@]}" | LC_ALL=C sort -u)"
package_key="$(printf '%s\n' "$hash_input" | md5sum)"

Order does not matter. use go jq and use jq go share the same cache.

The hash is only used as a file name. It is not used for security, so MD5 is enough.

Build once, reuse later ¶

nix print-dev-env evaluates the mkShell expression and prints a bash script that recreates its build environment. Unlike nix develop, it does not start a shell. This makes its output easy to save and source later:

nix print-dev-env \
    --impure \
    --expr "with import <nixpkgs> {}; mkShell { packages = [ ${shell_packages[*]} ]; }" \
    --profile "$HOME/.local/state/use/${package_key}.profile" \
    > "$HOME/.local/state/use/${package_key}.bash"

• <hash>.bash is the generated shell script. Sourcing it restores the build environment, including its variables and shell functions, without calling Nix again.

• <hash>.profile is the Nix profile created by --profile. It keeps the store paths used by that environment alive and available to <hash>.bash.

Later runs source the saved environment and start the user’s shell:

. "$HOME/.local/state/use/${package_key}.bash"
exec "$SHELL" -i

Conclusion ¶

This keeps use small and efficient:

• Detect packages.
• Build each package set once.
• Enter it fast.

See the full diff.