envsubst: The simplest bash variable substitution tool

🧐 𝗪𝗵𝗮𝘁 𝗶𝘀 𝗲𝗻𝘃𝘀𝘂𝗯𝘀𝘁?
envsubst (GNU gettext) streams stdin ➜ stdout, replacing any $VAR with their environment values. Think sed for env-vars, no regex.

🚀 𝗪𝗵𝗲𝗻 𝗱𝗼𝗲𝘀 𝗶𝘁 𝘀𝗵𝗶𝗻𝗲?
• Container start-up configs: inject secrets or hostnames into nginx.conf
• Quick local scripting: stamp today’s date or $HOME into a file in 2 lines
• CI/CD pipelines: render YAML/INI/JSON per-environment without Helm/Jinja

👨‍💻 𝗖𝗼𝗱𝗲:

export IP=$(curl -s https://api.ipify.org)
envsubst '$IP' < hostname.tmpl > hostname.conf

ℹ️ 𝗘𝘅𝗽𝗹𝗮𝗻𝗮𝘁𝗶𝗼𝗻:

1️⃣ curl fetches my public IP address and stores it in the IP environment variable.

2️⃣ envsubst reads the template file, replaces every instance of $IP with that value, and saves the finished file as hostname.conf.

🎞️ Check the 14-second demo below to watch it happen in real time.