As a follow-up to last week’s expiration of the certificate used to sign previously-released macOS installers, Apple has released re-signed macOS installers with the new certificate which is good until April 2029.
For those who archive older macOS installers, this means that the macOS installers in question will need to be re-downloaded. macOS Catalina has added some new functionality to the softwareupdate tool which can assist with this. For more details, please see below the jump.
To download the latest installer for macOS Catalina, which is currently at version 10.15.0, please run the command below with root privileges:
softwareupdate --fetch-full-installer --full-installer-version 10.15
Note: Normally the version is specified as 10.x.x, but Apple has historically listed a .0 release as 10.x. So in this case, the softwareupdate command uses 10.15 for macOS 10.15.0.
You should receive a message that the the installer is downloading and installing. In this context, the message refers to installing the macOS Installer app into the /Applications directory on the Mac.
Once the process has completed, you should receive a message that installation has completed successfully.
To verify, check to make sure there is now an Install macOS Catalina app in the /Applications directory.
To download the latest installer for macOS Mojave, please run the command below with root privileges:
softwareupdate --fetch-full-installer --full-installer-version 10.14.6
To download the latest installer for macOS High Sierra, please run the command below with root privileges:
softwareupdate --fetch-full-installer --full-installer-version 10.13.6
macOS Sierra is not available for download via the softwareupdate tool, but it is available via the Mac App Store (MAS). Apple has a KBase article, available via the link below, which shows how to access the macOS Sierra page in the MAS:
https://support.apple.com/HT208202
Update 10-28-2019:
Mike Brogan correctly points out that the KBase article is no longer linking to the MAS, but instead provides a download link to a disk image file which contains the Sierra installer.
It also appears the Apple KBase article for upgrading to OS X El Capitan and the Apple KBase article for upgrading to OS X Yosemite have similar download links to a disk image file which contains the OS X installer.
To access the macOS Sierra page directly, please click on the link below:
https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12
That link should open the MAS and take you to the macOS Sierra download page.
