Fun with envsubst

When using enbsubst to substitute environment variables, empty variables will be replaced with an empty string. This may not be the desired result. Even though, envsubst supports explcitly naming the variables to substitute, it is uncomfortable to use. This post demonstrated additional options to preserve environment variables that have no value set.

Prerequisites

Let’s consider the following file:

This line contains a variable that exists: $varexists
This line contains a variable that is not set: $varnotset

Well-known solution

The well-known solution is to explicitly name the variables that should be substituted. This will only substitute the variable that exists and leave the other variable untouched:

cat file | envsubst '${varexists}'

Obfuscating the dollar sign

The first solution is to obfuscate the dollar sign in front of the variable that is not set. This will prevent envsubst from substituting the variable:

This line contains a variable that exists: $varexists
This line contains a variable that is not set: §varnotset

The following command will process the above example:

cat file | envsubst | sed 's/§/$/g'

Inserting the dollar sign

This solution uses a dummy variable to insert the dollar sign in front of variables that are not set:

This line contains a variable that exists: $varexists
This line contains a variable that is not set: ${dollar}varnotset

The following command will process the above example:

cat file | dollar='$' envsubst

Restoring the dollar sign

This solution will work on the original file without any modifications:

This line contains a variable that exists: $varexists
This line contains a variable that is not set: $varnotset

The following command will process the above example:

cat file | varnotset='$varnotset' envsubst

Collecting existing variables

The last solution is based on an imaginary tool called envsubst-helper:

1. Collect all variables used in the file
2. Check which variables are set
3. Output all variables that are set

cat file | envsubst "$(envsubst-helper)"

Alternative tools

There are alternative tools that can be used to substitute existing environment variables but envsubst is readily available on most systems.