OPS235 Lab 5 - CentOS7

LAB PREPARATION

Purpose / Objectives of Lab 5

The purpose of this lab is to discuss how a Linux sys admin can manage partitions including adjusting the size of their Linux systems if space is required. Various topics will be discussed including:

• Connecting and Disconnecting Directories to existing partitions (mount, umount).
• Monitoring Disk Space (df -h).
• Using LVM to resize partitions graphically and via commands.
• Create, partition and format virtual hard disks to increase the size of a file system.
• Create a Bash Shell Script to monitor and report low disk size (run periodically in crontab).

Minimum Required Materials

Removable Hard Disk Pack (SATA)

USB key (for backups)

Lab5 Log Book

My Toolkit (CLI Reference)

LVM Information: vgs lvs pvs vgdisplay lvdisplay pvdisplay

LVM Management system-config-lvm lvextend lvcreate lvreduce pvcreate vgextend

Miscellaneous mount umount df -h fdisk mkfs

INVESTIGATION 1: Adjusting File System Sizes with LVM

Monitoring and ensuring adequate space for a Linux file-system is considered to be an important task for a sys admin. An application called LVM is a very useful tool for Linux system adminstrators.

LVM (Logical Volume Management) is used to manage hard disk drives / partitions for Unix/Linux systems. LVM provides more flexibility than just working with hard disks / hard disk partitions. Volume Groups are areas used to define Physical Volumes (i.e. hard disks, disk partitions, or other forms of storage devices). Logical Volumes are then used to relate directories (mount points) to a specific physical volume or for a "range" or "span" of physical volumes.

Therefore, LVM allows more flexibility and growth potential for Linux systems (for example, having Logical volumes span multiple hard disks). CentOS uses LVM by default upon installation. Other Linux distributions may provide the capacity to install LVM, or later install and then use Logical Volume Management.

Part 1: Manage LVM Graphically

The system-config-lvm application allows the Linux administrator to manage LVMs Graphically.

1. Let's learn to administer (manage) our LVM graphically for our centos2 Virtual Machine.
2. CentOS provides a tool called system-config-lvm to graphically administer LVM. Install the system-config-lvm application by issuing the command: yum install system-config-lvm

3. Open a shell as root and run the command: system-config-lvm
4. On the left-hand side, you can click on the Volume Group, Physical Volume and Logical Volumes and view their properties on the on the right-hand side.
5. Determine the current LVM configuration by clicking on the appropriate element and reading the properties in the right-hand panel -- write down the answers:
   1. What are the names and sizes of the Volume Group?
   2. What is the name and size of the Physical Volumes?s
   3. What are the names and sizes of the Logical Volumess?
   4. Is there any space in the VG which is not allocated to a LV?
6. Perform the following steps in this application to increase the size of the home file-system to 4 GB:
   1. On the left-hand side, click on the Logical Volume containing the home file-system.
   2. Click on Edit Properties.
   3. Change the size to 4 GB and click Ok.
   4. Verify that the home file-system has increased in size.
7. Create a new 3G LV (LV Properties: linear) containing an ext4 filesystem named: lv_archive and mount it at: /archive
8. Let's see what happens when we copy data over to lv_archive, and then reduce the size of lv_archive. Complete the following steps below:
   1. Issue the following command: cp -R /etc/* /archive
   2. Shrink the size of lv_archive to 1 GB. What happens?
   3. If you could not shrink the size of lv_archive, what do you think is the cause for the problem?

Answer Part 1 observations / questions in your lab log book.

Part 2: Managing LVM Via Command Line

Let's say that you have run out of disk space on your centos3 VM, you need more space on the root file-system, perhaps to host more webpages or a larger database or new software. What are your options? Getting a replacement hard-drive would probably require re-installation of the operating system and backup/restore of the data.

Because we're using LVM we can avoid this problem. We can add a new hard-drive (which will serve as a physical volume) to the volume group, and extend the root logical volume to make use of the new available space.

Perform the following operations to increase the size of lv_root in centos3:

1. Perform this Part in your centos3 VM.

2. Run the following commands and make note of the output:

 ls /dev/vd*
 pvs
 vgs
 lvs
 df -h

3. Open the centos3 virtual machine console
4. Go to the hardware details view
5. Click "Add Hardware" and add a new storage device of 2GBs, make sure it's a VirtIO disk.
6. Go back to the console view
7. Run the same ls command (performed in step 1), what's changed?
8. Now we'll make the new device as a physical volume, add it to the volume group, and extend lv_root:

 pvcreate /dev/vdb

 vgextend vg_centos3 /dev/vdb

 lvextend -L +2G -r vg_centos3/lv_root

9. Now rerun the ls/pvs/vgs/lvs/df commands. What has changed and what caused those changes?
10. Among the changes, note that your root filesystem is now 2GB bigger - and you have not even rebooted your machine!

Answer Part 2 observations / questions in your lab log book.

INVESTIGATION 2: CREATING VIRTUAL HARD DRIVES

Part 1: Adding Virtual Hard Disks and Managing with LVM

x

1. x

Answer Part 1 observations / questions in your lab log book.

INVESTIGATION 3: Mounting / Un-Mounting File-systems & Monitoring Disk Space

We take for granted that a file-system must be mounted (for example the root partition) in order for a Linux system to be usable upon system start-up. The /etc/fstab (file system table) contains entries to mount various file systems automatically upon start-up of the Linux system.

The Linux sys admin also has the ability to manually mount (connect) and un-mount (disconnect) partitions in order to perform maintenance on the file system (for example un-mounting the /home partition to install software and prevent users from logging in during that process).

We will now learn how to perform these operations (including monitoring of disk space usage) in Part 1.

Part 1: Mounting and Un-mounting Partitions

1. x

Answer the Part 1 observations / questions in your lab log book.

Part 2: Monitoring Disk Space

1. x

Answer the Part 2 observations / questions in your lab log book.

INVESTIGATION 4: LOOKING AHEAD

Automating Routine Tasks (Shell Scripting and Using Crontab)

Bash Shell Scripting Tips: The case statement: The case statement is a control-flow statement that works in a similar way as the if-elif-else statement (but is more concise). This statement presents scenerios or "cases" based on values or regular expressions (not ranges of values like if-elif-else statements). After action(s) are taken for a particular scenerio (or "case"), a break statement (;;) is used to "break-out" of the statement (and not perform other actions). A default case (*) is also used to catch exceptions. Examples (try in shell script): read -p "pick a door (1 or 2): " pick case $pick in   1) echo "You win a car!" ;;   2) echo "You win a bag of dirt!" ;;   *) echo "Not a valid entry"      exit 1 ;; esac read -p "enter a single digit: " digit case $digit in   [0-9]) echo "Your single digit is: $digit" ;;          *) echo "not a valid single digit"              exit 1 ;; esac The getopts function: The getopts function allows the shell scripter to create scripts that accept options (like options for Linux commands). This provides the Linux administrator with scripts that provide more flexibility and versatility. A built-in function called getopts (i.e. get command options) is used in conjunction with a while loop and a case statement to carry out actions based on if certain options are present when the shell script is run. The variable $OPTARG can be used if an option accepts text (denoted in the getopts function with an option letter followed by a colon. Case statement exceptions use the :) and \?) cases for error handling. Example of getopts (try in script and run with options) while getopts abc: name do   case $name in     a) echo "Action for option \"a\"" ;;     b) echo "Action for option \"b\"" ;;     c) echo "Action for option \"c\""         echo Value is: $OPTARG" ;;     :) echo "Error: You need text after -c option"         exit 1 ;;     \?) echo "Error: Incorrect option"         exit 1 ;; esac

We will now use shell scripting to help automate the task for a Linux adminstrator to create regular user accounts.

1. Download, study, and run the following shell script. Issue the command:
   wget https://scs.senecac.on.ca/~murray.saul/user-create.bash
2. Try to understand what these Bash Shell scripts do, and then run the script as root. After running the shell script, view the contents of the /home directory to confirm.

Although the zenity command is a "user-friendly" way to run shell scripts, Linux administrators usually create shell scripts that resemble common Linux commands. In this lab, you will learn to create a shell script using the getopts function to make your shell script behave more like actual Linux commands (including the use of options). Refer to the notes section on the right-hand-side for reference about the case statement and the getopts function.

3. Open a Bash shell terminal and login as root.
4. Use the wget command to download the input file called user-data.txt by issuing the command:
   wget https://scs.senecac.on.ca/~murray.saul/user-data.txt
5. View the contents on the user-data.txt file to confirm there are 3 fields (username, fullname, and e-mail address)which are separated by the colon (:) symbol.
7. Use a text editor (such as vi or nano) to create a Bash Shell script called: createUsers.bash in /root's home directory.
8. Enter the following text content into your text-editing session:


#!/bin/bash

# createUsers.bash
# Purpose: Generates a batch of user accounts (user data stored in a text file)
#
# USAGE:
#
# /root/createUsers.bash [-i {input-path}]
#
# Author: *** INSERT YOUR NAME ***
# Date: *** CURRENT DATE ***

if [ $PWD != "/root" ] # only runs if in root's home directory
then
 echo "You must be in root's home directory." >&2
 exit 1
fi
if [ "$#" -eq 0 ] # if no arguments after command
then
 echo "You must enter an argument" >&2
 echo "USAGE: $0 [-i {input-path}]" >&2
 exit 2
fi


6. Save your editing session, but remain in the text editor.
7. The code displayed below uses the getopt function set the input file pathname or check for invalid options or missing option text. Add the following code


outputFlag="n"
while getopts i: name
do
 case $name in
   i) inputFile=$OPTARG ;;
   :) echo "Error: You need text after options requiring text"
       exit 1 ;;
   \?) echo "Error: Incorrect option"
        exit 1 ;;
 esac
done


6. Save your editing session, but remain in the text editor.
7. The code displayed below uses logic to exit the script if the input file does not exist. Command substitution is used to store each line of the input file as a positional parameter. There is one subtle problem here: The full names of the users contain spaces which can create havoc when trying to set each line as a separate positional parameter. In this case the sed command is used to convert spaces to plus signs (+), which will be converted back later. Finally, a for loop is used to create each account (useradd) and mail the user their account information (mail). Add the following code:


if [ ! -f $inputFile ]
then
  echo "The file pathname \"$inputFile\" is empty or does not exist" >&2
  exit 2
fi

set $(sed 's/ /+/g' $inputFile) # temporarily convert spaces to + for storing lines as positional parameters

for x
do
    useradd -m -c "$(echo $x | cut -d":" -f2 | sed 's/+/ /g')" -p $(date | md5sum | cut -d" " -f1) $(echo $x | cut -d":" -f1)
    mail -s "Server Account Information" $(echo $x | cut -d":" -f3) <<+
    Here is your server account information:
    servername: myserver.senecac.on.ca
    username: $(echo $x | cut -d":" -f1)
    password: $(date | md5sum | cut -d" " -f1)
    Regards,
    IT Department
+
done

echo -e "\n\nAccounts have been created\n\n"
exit 0


8. Save, set permissions, and then run that shell script for the input text file user-data.txt. Did it work? Try running the script without an argument - What did it do?
9. You have completed lab4. Proceed to Completing The Lab, and follow the instructions for "lab sign-off".

Answer Investigation 3 observations / questions in your lab log book.

LAB 5 SIGN-OFF (SHOW INSTRUCTOR)

Time for a new backup!
If you have successfully completed this lab, make a new backup of your virtual machines. Remember to also make a backup of the new second virtual disk drive on centos1 -- you now have two virtual disks on centos1, and therefore two image files, and therefore will need two backup files.

Arrange proof of the following on the screen:

✓ x

✓ x

✓ x

✓ x

✓ x

Preparing for the Quizzes

1. What is a VG? PV? LV?
2. What is the total size of the "main" VG on your system?
3. How do you create a LV?
4. How do you delete an LV?
5. How would you add the disk partition /dev/sdb7 to your volume group "main"?
6. How would you increase the size of the root filesystem by 50 MB?
7. What is the purpose of /etc/fstab?