How to fix E: Package 'package-name' is a virtual package provided by multiple packages in APT

Run apt-cache showpkg package-name to see which real packages provide the virtual package:

apt-cache showpkg libxaw-dev

Or use apt-cache search --names-only "^package-name$" to check if it is indeed virtual:

apt-cache search --names-only "^libxaw-dev$"

Look for lines that say "Provides:" - these are the real packages you can install instead.

From the list of packages that provide the virtual package, select the one that best matches your needs. For example:

- For libxaw-dev: Choose libxaw7-dev for newer systems or libxaw6-dev for older ones
- For linux-headers: Choose linux-headers-5.15.0-generic (matching your kernel version)
- For mail-transport-agent: Choose postfix, exim4, or sendmail based on your preference

If unsure, use apt-cache show package-name to read the description of each option:

apt-cache show libxaw7-dev
apt-cache show libxaw6-dev

Replace the virtual package name with the real package name in your apt install command:

# Instead of this (fails):
sudo apt install libxaw-dev

# Do this (works):
sudo apt install libxaw7-dev

For linux-headers, first check your kernel version:

uname -r

Then install the matching headers package:

sudo apt install linux-headers-5.15.0-generic

If this error appears in automated scripts, update them to specify the concrete package name. In Dockerfiles, you can check the system version first:

RUN apt-get update && apt-get install -y ca-certificates
# For package variants, specify explicitly
RUN apt-get install -y libxaw7-dev

# For linux-headers, use a specific version
RUN apt-get install -y linux-headers-$(uname -r)

Or use the dpkg approach to detect and install:

DEBIAN_FRONTEND=noninteractive apt-get install -y build-essential linux-headers-generic

If you need a specific file or library, use apt-file to find which concrete package provides it:

# Install apt-file if needed
sudo apt install apt-file
sudo apt-file update

# Search for files
apt-file search /usr/include/X11/Xaw/Command.h

This tells you exactly which package provides the file you need, removing all ambiguity.