When trying to determine how old a Linux system is, many administrators instinctively look at file timestamps. However, this approach quickly reveals limitations:

ls -l /bin/bash
-rwxr-xr-x 1 root root 1099016 Apr  1  2020 /bin/bash

The timestamp shows 2020, but this only reflects when that particular binary was compiled - not when your system was installed.

Here are several more accurate approaches:

1. Checking Package Manager History

For Debian/Ubuntu systems:

ls -lt /var/log/apt/history.log
zgrep 'install ' /var/log/apt/history.log* | head -n 1

For RHEL/CentOS systems:

rpm -qi basesystem | grep Install

2. Examining System Logs

System logs often contain the earliest records:

journalctl --list-boots | tail -n 1
zgrep 'Linux version' /var/log/kern.log* | head -n 1

3. Analyzing Filesystem Creation

dumpe2fs /dev/sda1 | grep 'Filesystem created'
tune2fs -l /dev/sda1 | grep 'Filesystem created'

For systems using GRUB:

grep -m 1 'Linux version' /var/log/dmesg*

For cloud instances or virtual machines, you might need to check provider-specific metadata:

# For AWS:
curl http://169.254.169.254/latest/meta-data/ami-launch-index

# For Azure:
curl -H Metadata:true "http://169.254.169.254/metadata/instance?api-version=2021-02-01"

Here's a bash function that tries multiple methods:

function get_install_date() {
  # Try package manager first
  if [ -f /var/log/apt/history.log ]; then
    date -d "$(zgrep -m 1 'install ' /var/log/apt/history.log* | \
      head -n 1 | awk '{print $1" "$2}')"
  elif command -v rpm >/dev/null; then
    rpm -qi basesystem | grep 'Install Date' | awk -F': ' '{print $2}'
  else
    # Fall back to filesystem creation date
    tune2fs -l $(df / --output=source | tail -n 1) 2>/dev/null | \
      grep 'Filesystem created' | awk -F': ' '{print $2}'
  fi
}

---

Determining the actual installation date of a Linux system can be surprisingly tricky. While checking file timestamps seems like an obvious solution, you'll quickly encounter files with creation dates from decades ago (like the 1991 example), as these originate from the original package creation dates rather than system installation.

Here are the most accurate approaches to find your Linux system's installation date:

1. Checking Package Manager History

For Debian/Ubuntu systems:

grep "install " /var/log/dpkg.log | sort -n

For RHEL/CentOS systems:

rpm -q --queryformat '%{INSTALLTIME:date} %{NAME}\n' -a | sort -n

2. Examining Root Filesystem Creation

sudo tune2fs -l /dev/sda1 | grep 'Filesystem created'

Note: Replace /dev/sda1 with your actual root partition.

3. Checking Systemd First Boot

journalctl --list-boots | head -n 1

For systems without the above logs, try these fallback approaches:

1. Oldest Modified System File

sudo find / -xdev -type f -printf '%T+ %p\n' | sort | head -n 10

2. Checking Bash History

grep -a "install" ~/.bash_history | sort -n

Here's a script that combines multiple methods for better accuracy:

#!/bin/bash

echo "Checking installation date via various methods..."
echo

echo "1. Oldest package installation:"
if [ -f /var/log/dpkg.log ]; then
    grep "install " /var/log/dpkg.log | sort -n | head -n 3
elif command -v rpm &>/dev/null; then
    rpm -q --queryformat '%{INSTALLTIME:date} %{NAME}\n' -a | sort -n | head -n 3
else
    echo "No package manager logs found"
fi

echo
echo "2. Filesystem creation date:"
if command -v tune2fs &>/dev/null; then
    ROOT_DEV=$(df / --output=source | tail -n 1)
    sudo tune2fs -l $ROOT_DEV | grep 'Filesystem created'
else
    echo "tune2fs not available"
fi

echo
echo "3. Systemd first boot:"
if command -v journalctl &>/dev/null; then
    journalctl --list-boots | head -n 1
else
    echo "journalctl not available"
fi

• Filesystem timestamps can be affected by disk cloning or imaging
• Cloud instances might show provider-specific creation dates
• System upgrades might reset some timestamps

For the most accurate results, combine multiple methods and look for consistent patterns in the output. The package manager logs typically provide the most reliable installation date, while filesystem creation dates can serve as a good secondary reference.