When migrating Ubuntu servers, simply listing installed packages with dpkg -l doesn't reveal the original installation context. We need to distinguish between:

• Manually installed packages (explicit user commands)
• Automatically installed dependencies
• Default system packages

The simplest approach examines bash history:

cat ~/.bash_history | grep "apt-get install"
# Or for root commands:
sudo cat /root/.bash_history | grep "apt-get install"

Limitations:

• History gets rotated/truncated
• Doesn't capture installations via GUI tools
• Misses commands run in other shells

More reliable approach using APT's built-in logging:

grep "install " /var/log/apt/history.log
# For comprehensive output including timestamps:
zcat /var/log/apt/history.log.*.gz | grep "install "

Create an installation script from your history:

grep -h "Commandline: apt-get install" /var/log/apt/history.log* \
  | sed 's/Commandline: //' \
  | sort -u > install_history.sh

For understanding package relationships:

aptitude search '~i!~M' # Shows manually installed packages
aptitude why <package> # Shows why a package was installed

Complete solution for server migration:

#!/bin/bash
# Get all manually installed packages
apt-mark showmanual > manual-pkgs.txt

# Generate installation commands
echo "#!/bin/bash" > migrate.sh
awk '{print "sudo apt-get install -y " $0}' manual-pkgs.txt >> migrate.sh
chmod +x migrate.sh

For future reference, consider version controlling your package state:

apt list --installed > packages.list
git add packages.list
git commit -m "Snapshot of installed packages"

When migrating Ubuntu servers, one critical task is replicating the exact software environment from the old system to the new one. While dpkg -l lists installed packages, it doesn't distinguish between manually installed packages and automatically installed dependencies.

Ubuntu maintains detailed installation logs that can be accessed through:

cat /var/log/apt/history.log
# For compressed historical logs:
zcat /var/log/apt/history.log.*.gz

This will show entries like:

Commandline: apt-get install nginx
Install: libnginx-mod-http-geoip:amd64 (1.18.0-0ubuntu1), nginx-common:amd64 (1.18.0-0ubuntu1), nginx-core:amd64 (1.18.0-0ubuntu1)

To get specifically the packages you explicitly installed (excluding dependencies):

comm -23 <(apt-mark showmanual | sort -u) <(gzip -dc /var/log/installer/initial-status.gz | sed -n 's/^Package: //p' | sort -u)

For understanding which packages pulled in dependencies:

apt-cache depends --installed --recurse PACKAGE_NAME | grep -v " " | sort -u

Here's a complete bash script to generate an installation script for your new server:

#!/bin/bash
# Generate installation script from APT history
OUTFILE="migration_install.sh"

echo "#!/bin/bash" > $OUTFILE
echo "# Auto-generated package installation script" >> $OUTFILE
echo "sudo apt-get update" >> $OUTFILE

zgrep -h "Commandline: apt-get install" /var/log/apt/history.log* | \
  sed 's/Commandline: apt-get install/sudo apt-get install -y/' | \
  sort -u >> $OUTFILE

chmod +x $OUTFILE
echo "Generated installation script: $OUTFILE"

For more advanced scenarios, examine the package states:

grep -B1 "Automatically installed" /var/lib/apt/extended_states || \
  echo "No automatically installed packages found"

• Logs rotate and older entries may be compressed or deleted
• Consider using aptitude instead of apt-get for better tracking
• For production systems, consider using configuration management tools like Ansible