The `termux_step_host_build` runs only the first time package is built for its specific version, based on `TERMUX_HOSTBUILD_MARKER` in `termux_step_handle_hostbuild`. If `build-package.sh` is run twice, then second time, variables defined in `termux_step_host_build` won't be set in later functions since its not called, like `_PREFIX_FOR_BUILD` in `aapt/build.sh` would be unset inside `termux_step_pre_configure`, resulting in unbound variable error.
Even `termux_step_pre_configure` may not always run, depending on `$TERMUX_CONTINUE_BUILD`, so build scripts should not share variables between functions and redefine them again where necessary.
Also `_PREFIX_FOR_BUILD` should use a standard path `$TERMUX_PKG_HOSTBUILD_DIR/prefix` and not others like `$TERMUX_PKG_HOSTBUILD_DIR/_prefix`. Some build scripts use `$TERMUX_PKG_HOSTBUILD_DIR/src` for other stuff, so best use `prefix` sub directory.
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