Git Bash - string parameter with '/' at start is being expanded to a file path. How to stop this?

The release notes to the Git Bash 2.21.0 update today mentioned this as a known issue. Fortunately, they also described two solutions to the problem:

If you specify command-line options starting with a slash, POSIX-to-Windows path conversion will kick in converting e.g. "/usr/bin/bash.exe" to "C:\Program Files\Git\usr\bin\bash.exe". When that is not desired -- e.g. "--upload-pack=/opt/git/bin/git-upload-pack" or "-L/regex/" -- you need to set the environment variable MSYS_NO_PATHCONV temporarily, like so:

MSYS_NO_PATHCONV=1 git blame -L/pathconv/ msys2_path_conv.cc

Alternatively, you can double the first slash to avoid POSIX-to-Windows path conversion, e.g. "//usr/bin/bash.exe".

Using MSYS_NO_PATHCONV=1 can be problematic if your script accesses files.

Prefixing with a double forward slash doesn't work for the specific case of OpenSSL, as it causes the first DN segment key to be read as "/C" instead of "C", so OpenSSL drops it, outputting:

req: Skipping unknown attribute "/C"

Instead, I used a function that detects if running on bash for Windows, and prefixes with a "dummy" segment if so:

# If running on bash for Windows, any argument starting with a forward slash is automatically
# interpreted as a drive path. To stop that, you can prefix with 2 forward slashes instead
# of 1 - but in the specific case of openssl, that causes the first CN segment key to be read as
# "/O" instead of "O", and is skipped. We work around that by prefixing with a spurious segment,
# which will be skipped by openssl
function fixup_cn_subject() {
    local result="${1}"
    case $OSTYPE in
        msys|win32) result="//XX=x${result}"
    esac
    echo "$result"
}

# Usage example
MY_SUBJECT=$(fixup_cn_subject "/C=GB/CN=foo")