fix(shellenv): only export env vars Devbox actually modifies

[mikeland73]

Summary

Fixes #2826.

devbox shellenv and devbox global shellenv emit an export statement for every variable in the computed environment — including inherited variables that Devbox never touched, such as HOSTNAME, LANG, and PROFILEREAD. Re-exporting an unchanged variable is redundant, and it breaks in shells that mark some inherited variables as read-only. On openSUSE, eval "$(devbox global shellenv)" failed on every prompt with:

(eval):55: read-only variable: PROFILEREAD

Root cause

Devbox.computeEnv (internal/devbox/devbox.go) builds the environment by copying the entire parent environment (os.Environ()) and then layering Nix- and Devbox-specific variables on top. EnvExports then runs exportify over that whole map, so unchanged variables like PROFILEREAD are re-emitted as export PROFILEREAD="true";. When that output is eval'd in a shell that has marked PROFILEREAD read-only, the assignment errors out.

Fix

Filter the computed environment down to just the variables whose value differs from the current environment (or that are new) before formatting the export statements:

• New helper onlyModifiedEnvs(envs, baseEnv) in internal/devbox/envvars.go returns only entries that are absent from, or changed relative to, baseEnv.
• EnvExports now calls onlyModifiedEnvs(envs, envir.PairsToMap(os.Environ())) before choosing the bash/nushell export format.

The output of EnvExports is meant to be eval'd in the caller's shell (devbox shellenv, and devbox shell --print-env used by direnv), so dropping variables that already hold the same value is a no-op in the normal case and avoids the read-only error. Variables Devbox actually changes — PATH, XDG_DATA_DIRS, DEVBOX_*, Nix and plugin/config variables, the shellenv hash — all differ from the parent value and are still exported.

The interactive devbox shell path (which sources a generated shellrc in a child shell via shell.go) is intentionally unchanged; only EnvExports is affected.

How was it tested?

• Added internal/devbox/envvars_test.go::TestOnlyModifiedEnvs, covering unchanged variables being dropped, changed/new/emptied variables being kept, and empty-base-env behavior.
• go test ./internal/devbox/ -run TestOnlyModifiedEnvs — passes.
• go build ./internal/devbox/, go vet ./internal/devbox/, and gofmt — all clean.
• Verified the existing testscripts/shell/shellenv.test.txt flow is unaffected: it sources the export PATH= line from devbox shellenv, and PATH is always modified by Devbox, so it is still emitted.

cc @gasuketsu (issue reporter) — thanks for the clear write-up and pinpointing the read-only variable failure.

Community Contribution License

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maintainers under the terms of the
Apache 2 License.

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https://claude.ai/code/session_01WiTxPixVaWVbpBsZf1DLeG

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Generated by Claude Code

[Copilot]

Pull request overview

This PR updates devbox shellenv/devbox global shellenv to emit export statements only for environment variables that Devbox actually changes relative to the current environment, avoiding redundant re-exports that can fail on shells with read-only inherited variables (e.g. PROFILEREAD on openSUSE).

Changes:

• Added onlyModifiedEnvs(envs, baseEnv) helper to filter env maps down to new/changed variables.
• Updated Devbox.EnvExports to filter computed envs against os.Environ() before formatting bash/nushell output.
• Added unit tests for onlyModifiedEnvs covering unchanged, changed/new, empty-value, and empty-base behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/devbox/envvars.go	Adds onlyModifiedEnvs helper to filter environment variables by diff vs. a base env map.
internal/devbox/envvars_test.go	Introduces focused unit tests validating the filtering behavior.
internal/devbox/devbox.go	Applies the filtering in EnvExports so shellenv output avoids re-exporting unchanged inherited vars.

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