Reading environment variable in command alias causes evaluating it?

Question

I have simple alias for writing to log files (this is what I have in my .profile file):

logtee() {
    ${TEST_LOG_FOLDER:?"empty_log_folder"}
    tee -a $TEST_LOG_FOLDER/$1
}

I'm reading the folder from environment variable (so this variable holds value similar to 'd/my/logs'). However, when I try using this alias by executing echo 'this is reading path from env.variable!' | logtee whatever.log it works, but the output is: sh.exe": /d/my/log/path: is a directory.
Did I do something wrong? Why does reading environment variable evaluate it? I suspect it have something to do with a fact that it's an alias, because in usual .sh script similar code works without any issues
P.S. I'm using msysgit on Windows 7, may be that's the problem?

Answer 1

The first line of your function is indeed evaluating it. You could replace it by:

echo ${TEST_LOG_FOLDER:?"empty_log_folder"} > /dev/null

A better solution would be:

[ -d "${TEST_LOG_FOLDER:?'empty_log_folder'}" ] && \

The best solution is not to use a separate line, so the whole function becomes:-

logtee() {
    tee -a "${TEST_LOG_FOLDER:?'empty_log_folder'}/$1"
}

Note that I have changed the quoting, in case there is a space in $TEST_LOG_FOLDER or $1.