With Imagemagick it is pretty straightforward to convert HEIC files. Version 7 has HEIC support built-in.
Install ImageMagick from Homebrew.
brew install imagemagick
For older versions you had to install Imagemagick with
brew install --with-libheif imagemagick on MacOS or compile from source on Linux. Because it now has HEIC enabled by default, that’s not needed anymore.
The –with-libheif option was needed to tell Homebrew to compile with HEIF support, which allows it to handle .heic images.
libheif is an ISO/IEC 23008-12:2017 HEIF and AVIF (AV1 Image File Format) file format decoder and encoder.
Convert HEIC to JPEG
mogrify -format jpg *.heic