envsubst Command Reference for CI Scripts

envsubst replaces ${VAR} references in text with values from the environment.

envsubst renders config templates at deploy time, filling in environment-specific values. The danger is that by default it substitutes every variable it recognizes, including ones you meant to leave literal.

Common flags/usage

• envsubst < template > output: substitute all environment variables
• envsubst '$VAR1 $VAR2': restrict to a named list of variables
• reads stdin, writes stdout
• only ${VAR} and $VAR forms are replaced
• ships with gettext, sometimes a separate package

Example

export APP_PORT=8080 APP_ENV=production
envsubst '$APP_PORT $APP_ENV' < app.conf.tmpl > app.conf

In CI

Without the quoted variable-list argument, envsubst replaces every $-reference, so a literal $ in an nginx config or a shell snippet inside the template gets blanked out; always pass an explicit list like '$APP_PORT $APP_ENV'. envsubst comes from gettext and may need installing (apt-get install gettext-base) on slim images.

Key takeaways

• Pass an explicit "$VAR" list so envsubst does not blank out unrelated $ references.
• envsubst reads stdin and writes stdout; redirect a template through it.
• It ships with gettext and may need installing on minimal images.

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