Usage of envsubst combined with tee randomly results in an empty file - linux

Question

I'm looking to understand what is going on in a simple script, that seems to produce random results.
What I am trying to do:
Replace variables in pre-existing files from the values defined in the environment.
This is done inside a Docker container with a bash script, that runs the command:
envsubst '$VAR1 $VAR2' < $FILE | tee $FILE
What happens:
Sometimes a $FILE in question has contents before the command, but contains nothing after the command.
How to reproduce the issue:
Dockerfile:
FROM debian:stretch
RUN apt-get update -qy
RUN apt-get install -qy gettext
COPY main-script /main-script
RUN chmod +x /main-script
ENTRYPOINT [ "/main-script" ]
Bash script:
#!/bin/bash
mkdir -p /test
export TEST1=1
export TEST2=2
export TEST3=3
for I in {1..300} ; do
echo '$TEST1 $TEST2 $TEST3' > /test/file-$I
done
for FILE in /test/file-* ; do
envsubst < $FILE | tee $FILE
done
for FILE in /test/file-* ; do
if [[ -z "$(cat $FILE)" ]]; then
echo "$FILE is empty!"
FAIL=1
fi
done
if [[ -n "$FAIL" ]]; then
exit 2
fi
Output looks something like this:
...
/test/file-11 is empty!
/test/file-180 is empty!
/test/file-183 is empty!
/test/file-295 is empty!

Answer 1

Pipes are asynchronous, and you've introduced a race condition. You can't predict if envsubst reads from $FILE before or after tee truncates it.
The correct approach is to write the changes to a temporary file, then replace the original with the temporary file after that has succeeded.
tmp=$(mktemp)
envsubst < "$FILE" > "$tmp" && mv "$tmp" "$FILE"