In modern Debian systems (including Squeeze and later), systemd manages service startup. To inspect the boot sequence, we'll primarily use systemd-analyze and related tools.

systemctl list-unit-files --state=enabled

This command shows all enabled services that start at boot. For more detailed information including dependencies:

systemd-analyze dot | dot -Tsvg > boot_sequence.svg

Note: You'll need Graphviz installed for the dot command.

To see the exact boot sequence with timing information:

systemd-analyze critical-chain

For a specific service:

systemd-analyze critical-chain apache2.service

For systems still using SysVinit, check the traditional boot sequence:

ls -l /etc/rc*.d/

To see the exact order for a specific runlevel (e.g., 2):

ls -l /etc/rc2.d/

When debugging why service A starts before service B:

systemd-analyze verify A.service B.service
systemd-analyze dump | grep -A 10 "Unit A.service"
systemd-analyze plot > boot_plot.svg

To modify the startup sequence, create or edit service dependencies:

[Unit]
Description=My Custom Service
After=network.target
Requires=postgresql.service

For graphical analysis:

sudo apt install bootchart
bootchart

This provides a visual timeline of all processes during boot.

Modern Debian systems (since Jessie) use systemd, while older releases like Squeeze use SysV init. Let's cover both approaches.

# List all enabled services with their dependencies
systemd-analyze dot | dot -Tsvg > boot_sequence.svg

# View boot performance metrics
systemd-analyze critical-chain

# List all units and their activation time
systemd-analyze plot > boot_timeline.svg

Check the traditional init.d scripts:

# View boot order in /etc/rc*
ls -l /etc/rc*.d/

# Alternative method using readlink
find /etc/rc*.d/ -name "S*" -exec readlink -f {} \; | sort

# For detailed execution order
cat /var/log/boot.log

For deeper inspection in Squeeze:

# Install bootchart if not present
apt-get install bootchart

# After reboot, analyze the chart
bootchart /var/log/bootchart.tgz

# Alternative using sysv-rc-conf
sysv-rc-conf --list

Suppose you need to verify Apache starts after networking:

# In systemd:
systemctl list-dependencies apache2.service

# In SysV init:
grep -r "Required-Start" /etc/init.d/apache2

For automated reporting:

#!/bin/bash
# Generate boot service report
{
  echo "=== Boot Services Report ==="
  date
  echo -e "\nEnabled Services:"
  if which systemctl >/dev/null; then
    systemctl list-unit-files --state=enabled | grep service
  else
    find /etc/rc*.d/ -name "S*" | sort
  fi
} > boot_services_report.txt