Fix warnings not being printed

[JoaquinTrinanes]

Since a few days ago (so I assume starting at direnv 2.36.0), the DIRENV_LOG_FORMAT environment variable is not set by default. This breaks nix-direnv every time it's called with strict_env due to an unbound variable error.

Even if the variable was accessed safely, warnings won't be rendered anymore because it's empty and we skip printing anything when that happens.

With this approach, we use the stdlib log_status, and just wrap it around ANSI escapes for the yellow color.

This has the advantage of delegating the logging format logic back to direnv, but it might be possible that at some point in the future direnv sanitizes the strings and breaks this solution. However, the alternative is to keep relying on DIRENV_LOG_FORMAT, which will break by default, and in doing so we would also be ignoring the new log_format option in direnv.toml.

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Other thing I tried to get colored logs "idiomatically" was to embed them inside the log_status arguments, not around it:

log_status "$(printf "%b" $color_warning)${_NIX_DIRENV_LOG_PREFIX}${msg}$(printf "%b" $color_normal)"

This makes the output look like this (note that the direnv: prefix is not colored):

But it would be a valid alternative if, for example, the behaviour at direnv/direnv#884 is restored.

[StabbarN]

I had to add

[global]
log_format = "direnv: %s"

in my direnv.toml to get the warning color.

I'm unsure why but I hope it helps someone.