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Running into an issue that is likely simple for people that know how. I am writing an .sh script that reads in a file that has variable names in it. I am trying to use the file following this with the values of the variables in place for the names in the file.

My Attempted .sh Script:

HELLOVAR = "hello"
cat infilename.json > outfilename.json

I then attempt to use outfilename.json, but it has not changed from the original infile.

File In Example:

I would like this to be ${HELLOVAR}.

File Out Wanted:

I would like this to be hello.
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    Surely you get an error of the form HELLOVAR: command not found. If not, show us your actual code. Commented Dec 6, 2022 at 16:26
  • Do you need to worry about HELLOVAR='hello "cruel" world' (which, to be included in a JSON string, would need to be changed to hello \"cruel\" world), or is your data pre-escaped to be JSON-safe? (If it's the former, you should use jq instead of envsubst; it's perfectly capable of looking up environment variables). Commented Dec 6, 2022 at 16:48

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You probably just want:

HELLOVAR=hello envsubst < infilename.json > outfilename.json

envsubst reads its input stream looking for simple variable expressions (eg, it will substitute $FOO and ${FOO}, but will not perform the expected substitution on expressions like ${FOO-bar}) and expands them according to the current setting in the environment. HELLOVAR=hello envsubst invokes envsubst with HELLOVAR set to hello in its environment.

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4 Comments

This is problematic if any of the values require escaping. I'd argue that this would be better resolved by the answers to Replacing variables in a JSON template using jq
(maybe not that one; I know we have a Q&A entry somewhere with jq code that searches a document's strings for ${varname} references and looks up the named variables in env)
Would there be a way to achieve this without actual saving the output file. Instead just have it be usable in the script itself?
@toren It depends on what you mean. There are a lot of options. You could pipe the output of envsubst into some command block, or you could save its output to a variable, or you could run envsubst inside a heredoc, or any number of other methods for making the data available to the script. But storing it on the filesystem is a pretty good method.

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