bash - Can envsubst not do in-place substitution?

Question

I have a config file which contains some ENV_VARIABLE styled variables.

This is my file.
It might contain $EXAMPLES of text.

Now I want that variable replaced with a value which is saved in my actual environment variables. So I'm trying this:

export EXAMPLES=lots
envsubst < file.txt > file.txt

But it doesn't work when the input file and output file are identical. The result is an empty file of size 0.

There must be a good reason for this, some bash basics that I'm not aware of? How do I achieve what I want to do, ideally without first outputting to a different file and then replacing the original file with it?

I know that I can do it easily enough with sed, but when I discovered the envsubst command I thought that it should be perfect for my use case, so I'd like to use that.

Answer 1

Here is the solution that I use:

originalfile="file.txt"
tmpfile=$(mktemp)
cp --attributes-only --preserve $originalfile $tmpfile
cat $originalfile | envsubst > $tmpfile && mv $tmpfile $originalfile

Be careful with other solutions that do not use a temporary file. Pipes are asynchronous, so the file will occasionally be read after it has already been truncated.