feat: add -prefix flag for variable filtering#73
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Add --prefix flag to substitute only variables matching a given prefix.
Variables not matching the prefix are left as literal text in the output.
This enables use cases like:
envsubst --prefix ARGOCD_ENV_ < template.yaml
Which will only substitute $ARGOCD_ENV_* variables, leaving other
variable references (like $OTHER_VAR) intact.
Key behavior:
- Variables matching the prefix are processed normally
- Unset variables matching prefix use defaults (${VAR:-default})
- Variables not matching prefix are preserved as literal text
- Level 1 variable nesting in defaults works (${VAR:-$VAR2})
Known limitation:
- Deeply nested defaults like ${VAR:-${VAR2:-default}} have issues
with extra closing braces (pre-existing parser limitation)
Comprehensive tests added for:
- VarFilter prefix matching
- All substitution operators (:- - := :+)
- Mixed allowed/disallowed variables
- Variable defaults with different prefixes
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
nicholasklem
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Upstream a8m/envsubst treats $$ as a shell-style escape that collapses to $ (since 2018, commit e89c09c "parse: add support for escaping expressions"). That silently mangles inputs containing literal $$ — most notably the KEDA Helm chart, which embeds Kubernetes' own env-var expansion docs in CRD schema descriptions: "Double $$ are reduced to a single $". Result: ArgoCD shows perpetual $$ → $ drift on KEDA resources, since rendered manifests have $ but live cluster has $$ (or vice versa, depending on which side of the apply you read). This fork is scoped to ArgoCD CMP rendering with --prefix filtering, not generic shell templating. There is no use case here for shell-style $$ escapes; the only effect is unintended mangling of literal text. Drop the l.ignore() that throws away the first $ in the lexer's $$ branch, so $$ emits as literal text instead of collapsing. Also reframes the README: this fork is now documented as an envsubst variant tuned for ArgoCD CMP rendering, with two deliberate deviations from upstream (the existing --prefix flag and this $$ change). It is not a generic shell-templating tool; users who want shell-style escapes should use upstream a8m/envsubst. Tests: 4 existing escape tests in parse_test.go + 2 in lex_test.go updated to assert new semantics. Added 4 KEDA-flavored regression cases covering CRD descriptions, kubelet $$(VAR_NAME) escapes, and $$ mixed with prefix-allowed substitutions. Verified end-to-end against actual kustomize build --enable-helm output of keda/environments/production: 18 $$ occurrences in rendered chart, 18 preserved after envsubst -prefix ARGOCD_ENV_, IAM annotation substitution still works correctly.
…on tests Address self-review feedback on PR #1: - README "About this fork": clarify which shell operators are preserved vs dropped. List all the kept operators explicitly so readers don't have to guess. Only the $$ escape is removed. - README new section "Combining literal $$ with a substitution": document the four forms users are likely to try, with verified behavior. The only working forms are $$$VAR (triple-dollar) and $$ ${VAR} (space- separated). $$VAR and $${VAR} both fail to substitute because the $$ consumes both dollars, leaving the variable name as plain text. Initial draft of this section claimed $${VAR} would substitute — that was wrong, verified empirically before commit. - Tests: pin the four documented forms as regression cases in parseTests, so README and behavior can't drift apart silently. Also add two cases for $$ inside substitution operands (${VAR:-pre$$post}) — that path is unchanged by this fork but worth pinning so a future refactor of lexSubstitution doesn't regress it. All 6 new test cases pass.
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Summary
Add
-prefixflag to substitute only variables matching a given prefix, leaving other variable references as literal text.This is a simpler alternative to #72 (GNU SHELL-FORMAT compatibility), which I've converted to draft. The
-prefixapproach is easier to use in practice.Use Case
When templating files that contain variable references you want to preserve, you can use a prefix convention to distinguish which variables should be substituted:
Input:
Output:
Behavior
:-,-,:=,:+)${MY_X:-$OTHER}→ substitutesMY_X, preserves$OTHERif used as defaultTest Plan
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