Bash variable injection

Question

I'm trying to come up with a way of parameterizing some config files for a docker image using env vars. A sample input file could look something like:

<database>
  <host>${DB_HOST}</host>
  <name>${DB_NAME}</name>
  <user>${DB_USER}</user>
  <pass>${DB_PASSWORD}</pass>
</database>

and I'd want the output to basically be exactly the same, but with the fields filled in with the corresponding environment variable value.

Is there a simple way to do this directly in bash? The system I'm using doesn't natively support env vars, so I need to write these config files by hand, but I need to inject different vars for each environment I'm in so I don't particularly want to actually maintain real files for each env.

Gilles Quénot · Accepted Answer · 2018-03-13 00:47:42Z

2

This is that simple as :

cat<<EOF > new_config_file
<database>
  <host>${DB_HOST}</host>
  <name>${DB_NAME}</name>
  <user>${DB_USER}</user>
  <pass>${DB_PASSWORD}</pass>
</database>
EOF

ls -l new_config_file

edited Mar 13, 2018 at 0:47

answered Mar 13, 2018 at 0:41

Gilles Quénot

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1 Comment

brad Over a year ago

while I prefer that your solution doesn't have any package dependencies, technically I was asking about an existing file, not a heredoc, but this is also a great answer, thanks so much!

Gilles Quénot · Accepted Answer · 2018-03-13 00:44:51Z

1

you're looking for envsubst

$ envsubst < config_template > config_instance

edited Mar 13, 2018 at 0:44

Gilles Quénot

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answered Mar 13, 2018 at 0:42

karakfa

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Comments

SkyWriter · Accepted Answer · 2018-03-16 05:36:14Z

Just another option.

My generated config files often require just a bit of logic + embedding environment variable contents. At some point I got tired of reinventing the wheel every time and stitched together a simple tool in Go called Templater (https://github.com/reertech/templater).

It's just a standalone zero dependency binary for most of the systems, so you can just download it and use Go templating language (https://golang.org/pkg/text/template/) like that:

#!/bin/bash

YEAR=1997 \
NAME=John \
SURNAME=Connor \
./templater -t - -o /tmp/hello.txt <<EOF
Hello, {{env "NAME"}} {{env "SURNAME"}}!
{{ if env "YEAR" | parseInt | eq 1997 }}
Have a nice Judgement Day!
{{ end }}
EOF

In case you're not comfortable with prebuilt binaries you can use Go and build one yourself.

Hope this helps you or someone else who cannot cut it with just environment substitution and doesn't feel like Bashing.

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