Errors for bat cache --build when cache already exists and BAT_CACHE_PATH is specified #1726

Describe the bug you encountered:

I am attempting to distribute a custom bat theme with my tool to use. I want to avoid the possibility of messing with the user's installed bat themes and theme cache.

Notably, the bat cache --build --blank command does not work when the destination path already exists, and it throws an unusual error message.

Note that it works the first time:

$ mkdir -p /tmp/bat-cache-bug

$ cd /tmp/bat-cache-bug

$ XDG_CONFIG_HOME=$PWD BAT_CACHE_PATH=$PWD/cache bat cache --build
No themes were found in '/tmp/bat-cache-bug/bat/themes', using the default set
No syntaxes were found in '/tmp/bat-cache-bug/bat/syntaxes', using the default set.
Writing theme set to /tmp/bat-cache-bug/cache/themes.bin ... okay
Writing syntax set to /tmp/bat-cache-bug/cache/syntaxes.bin ... okay
Writing metadata to folder /tmp/bat-cache-bug/cache ... okay

But fails the second time, with the exact same command:

$ XDG_CONFIG_HOME=$PWD BAT_CACHE_PATH=$PWD/cache bat cache --build
error: Found argument '--build' which wasn't expected, or isn't valid in this context

USAGE:
    bat [OPTIONS] [FILE]...
    bat <SUBCOMMAND>

For more information try --help

What did you expect to happen instead?

Rebuilding the cache would also be nice, as that's the expected behavior.

A better error message would be helpful.

How did you install bat?

brew install bat

bat version and environment

$ XDG_CONFIG_HOME=$PWD BAT_CACHE_PATH=$PWD/cache bat --diagnostic

Software version

bat 0.18.1 ()

Operating system

macOS 12.0 (Darwin 21.0.0)

Command-line

bat --diagnostic

Environment variables

SHELL=/bin/zsh
PAGER=less
LESS='-F -i -M -R -S -w -X -z-4'
BAT_PAGER=<not set>
BAT_CACHE_PATH=/tmp/bat-cache-bug/cache
BAT_CONFIG_PATH=<not set>
BAT_OPTS=<not set>
BAT_STYLE=plain
BAT_TABS=<not set>
BAT_THEME=<not set>
XDG_CONFIG_HOME=/tmp/bat-cache-bug
XDG_CACHE_HOME=<not set>
COLORTERM=truecolor
NO_COLOR=<not set>
MANPAGER='sh -c '\''col -bx | bat -l man -p'\'''

Config file

Could not read contents of '/tmp/bat-cache-bug/bat/config': No such file or directory (os error 2).

Compile time information

• Profile: release
• Target triple: x86_64-apple-darwin
• Family: unix
• OS: macos
• Architecture: x86_64
• Pointer width: 64
• Endian: little
• CPU features: fxsr,sse,sse2,sse3,ssse3
• Host: x86_64-apple-darwin

Less version

> less --version
less 487 (POSIX regular expressions)
Copyright (C) 1984-2016  Mark Nudelman

less comes with NO WARRANTY, to the extent permitted by law.
For information about the terms of redistribution,
see the file named README in the less distribution.
Homepage: http://www.greenwoodsoftware.com/less