The Linux file system hierarchy standard (FHS) defines the main directories and their contents.

• /bin: Essential user command binaries.
• /boot: Boot loader files and the Linux kernel.
• /dev: Device files.
• /etc: System-wide configuration files.
• /home: User home directories.
• /lib: Essential shared libraries and kernel modules.
• /lib64: Essential 64-bit shared libraries and kernel modules.
• /media: Mount point for removable media.
• /mnt: Temporary mount point for file systems.
• /opt: Optional application software packages.
• /proc: Virtual file system containing process and kernel information (lost on reboot).
• /root: Home directory for the root user.
• /run: Runtime variable data.
• /sbin: Essential system binaries.
• /srv: Data for services provided by the system.
• /sys: Virtual file system providing information about devices.
• /tmp: Temporary files (often cleared on reboot).
• /usr: User applications and data.
• /var: Variable datta files (e.g., logs).

Mounting File Systems

To mount a storage device, such as a CD-ROM drive, perform the following steps:

1. Ensure the device is accessible: For a CD-ROM, this means having a disc loaded.
2. Identify the device file: Use ls -l /dev/cdrom to find the device node.
3. Mount the device: Create a mount point directory (e.g., /mnt) if it doesn't exist, then use the mount command:

   mount /dev/cdrom /mnt
   

   To unmount, use umount:

   umount /mnt/
   

Network Configuration

Network interface configuration is typically found in /etc/sysconfig/network-scripts/. The ifcfg-eth0 file contains parameters like:

• TYPE: Network type (e.g., Ethernet).
• BOOTPROTO: IP address assignment method (none, static, dhcp).
• NAME: Logical interface name (e.g., eth0).
• DEVICE: Physical interface name (e.g., eth0).
• ONBOOT: Whether the interface is enabled on boot (yes or no).
• IPADDR: Static IP address.
• PREFIX: Subnet mask prefix length.
• GATEWAY: Default gateway address.
• DNS1: Primary DNS server address.

To apply changes:

• Restart the network service: systemctl restart network.
• Or, restart specific inetrface: ifdown eth0 && ifup eth0.

DNS resolution information is stored in /etc/resolv.conf.

Hostname Configuration

• CentOS 7:
  • Temporary: hostname new_hostname
  • Permanent: hostnamectl set-hostname new_hostname (also sets temporarily)
  • To view the file for permanent configuration (though hostnamectl is preferred): cat /etc/hostname.
• CentOS 6:
  • Temporary: hostname new_hostname
  • Permanent: Edit /etc/sysconfig/network.

To check the OS version, use cat /etc/redhat-release.

Host File Mapping

The /etc/hosts file provides local IP address to hostname mapping. This is checked before DNS resolution.

Automatic Mounts

The /etc/fstab file defines file systems to be mounted automatically at boot time.

Startup Scripts

The /etc/rc.local script is executed at the end of the boot process. Ensure it has execute permissions (chmod +x /etc/rc.d/rc.local). Commands placed here will run on startup.

System Runlevels

Runlevels define the system's operating state. In modern systems (systemd), these are referred to as 'targets'.

• CentOS 6 Runlevels:

  • 0: Halt
  • 1: Single-user mode (rescue)
  • 2: Multi-user mode, without networking
  • 3: Full multi-user mode, with networking (standard CLI)
  • 4: Unused
  • 5: Graphical multi-user mode
  • 6: Reboot

• CentOS 7 Targets:

  • poweroff.target (0)
  • rescue.target (1)
  • multi-user.target (2, 3, 4)
  • graphical.target (5)
  • reboot.target (6)

• View current runlevel: runlevel.

• Switch runlevel temporarily: init <runlevel>.

• Set default target permanently (CentOS 7): systemctl set-default <target_name>.target.

• Check default target: systemctl get-default.

Environment Variables

• Temporary variables: Set using VARIABLE_NAME=value. Access with echo $VARIABLE_NAME.
• Permanent variables: Add export VARIABLE_NAME=value to /etc/profile and run source /etc/profile.
• The PATH variable defines directories searched for executable commands. To modify PATH permanently, edit /etc/profile and add the desired directories using export PATH=....

Aliases

Aliases create shortcuts for commands. Syntax: alias new_name='original_command'. Example: alias catnet='cat /etc/sysconfig/network-scripts/ifcfg-eth0'. To remove an alias: unalias new_name.

Login Banners

• Message of the Day (MOTD): The /etc/motd file's content is displayed after a successful login.
• Pre-login Banners: The /etc/issue and /etc/issue.net files display messages before login. To clear a file, use echo "" > /path/to/file or > /path/to/file.