54.Variable_Substitution.md

Variable Substitution and Command Substitution

Variable Substitution

• Definition: Accessing the value of a variable using the $ symbol is called variable substitution.
• Syntax:

  $variablename  
  ${variablename}  

  It is recommended to use ${variablename} for clarity, especially in cases where additional text is appended to the variable value.

Example Script (test.sh):

#! /bin/bash
a=10
b=20
COURSE="linux"
ACTION="SLEEP"
echo "Values of a and b are: $a and $b"
echo "My Course is: ${COURSE}"
echo "My Favourite Action: $ACTIONING"
echo "My Favourite Action: ${ACTION}ING"

Output:

Values of a and b are: 10 and 20  
My Course is: linux  
My Favourite Action:  
My Favourite Action: SLEEPING  

Explanation:

• $ACTIONING does not substitute because there is no variable ACTIONING.
• ${ACTION}ING appends ING to the value of ACTION, resulting in SLEEPING.

Key Notes:

1. Using single quotes (') prevents variable substitution.
2. Using double quotes (") allows variable substitution.

Example:

NAME="durga"

echo NAME         # Output: NAME
echo $NAME        # Output: durga
echo '$NAME'      # Output: $NAME
echo "$NAME"      # Output: durga

Quiz:
Q: Consider the variable declaration NAME="durga". Which of the following prints the value of NAME?
A) echo NAME
B) echo $NAME
C) echo '$NAME'
D) echo "$NAME"
Answer: B, D

Command Substitution

• Definition: Execute a command and substitute its result within a script or command.
• Syntax:
  • Old style: `command` (backticks, not single quotes).
  • New style: $(command) (recommended for readability).

Examples:

1. Getting the current date:

   echo "Today Date is: `date +%D`"
   echo "Today Date is: $(date +%D)"

   Output:

   Today Date is: 12/06/19
   Today Date is: 12/06/19
   

2. Counting files in the current directory:

   echo "Number of files in Current Working Directory: `ls | wc -l`"
   echo "Number of files in Current Working Directory: $(ls | wc -l)"

   Output:

   Number of files in Current Working Directory: 5
   Number of files in Current Working Directory: 5
   

3. Counting lines in a file (test.sh):

   echo "The No of Lines in test.sh: `cat test.sh | wc -l`"
   echo "The No of Lines in test.sh: $(wc -l test.sh)"

   Output:

   The No of Lines in test.sh: 9
   The No of Lines in test.sh: 9 test.sh
   

4. Getting the current working directory:

   echo "My Current Working Directory: `pwd`"
   echo "My Current Working Directory: $(pwd)"

   Output:

   My Current Working Directory: /home/durgasoft/scripts
   My Current Working Directory: /home/durgasoft/scripts
   

Comparison of Substitutions

Type	Definition	Syntax	Example	Output
Variable Substitution	Replaces variable name with its value	$variablename or ${variablename}	echo $NAME or echo ${NAME}	Prints the value of NAME.
Command Substitution	Replaces command with the result of its execution	`command` or $(command)	echo $(date) or echo `date`	Prints the output of the command.

Case Study: Automating a System Monitoring Report Using Variable and Command Substitution

Scenario:
A company wants to generate a system monitoring report automatically and display the following details using a shell script:

1. Current Date and Time.
2. Logged-in User's Name.
3. Current Working Directory.
4. Total Number of Files in the Current Directory.
5. Free Disk Space.

The script should utilize Variable Substitution and Command Substitution to display this information dynamically.

Script: system_report.sh

#!/bin/bash

# Using Command Substitution to retrieve dynamic information
CURRENT_DATE=$(date '+%Y-%m-%d %H:%M:%S')
LOGGED_USER=$USER
WORKING_DIRECTORY=$(pwd)
TOTAL_FILES=$(ls | wc -l)
DISK_SPACE=$(df -h / | grep '/' | awk '{print $4}')

# Displaying the report using Variable Substitution
echo "-----------------------------------"
echo "       System Monitoring Report    "
echo "-----------------------------------"
echo "Date and Time          : $CURRENT_DATE"
echo "Logged-in User         : ${LOGGED_USER}"
echo "Current Working Directory : ${WORKING_DIRECTORY}"
echo "Total Number of Files  : ${TOTAL_FILES}"
echo "Available Disk Space   : ${DISK_SPACE}"
echo "-----------------------------------"

Explanation:

1. Variable Substitution:

   • $LOGGED_USER, ${WORKING_DIRECTORY}, ${TOTAL_FILES}, and ${DISK_SPACE} are used to substitute the values stored in variables.

2. Command Substitution:

   • $(date '+%Y-%m-%d %H:%M:%S'): Fetches the current date and time.
   • $(pwd): Retrieves the current working directory.
   • $(ls | wc -l): Counts the total number of files in the directory.
   • $(df -h / | grep '/' | awk '{print $4}'): Fetches the free disk space of the root partition.

Key Learning Points:

1. Dynamic Command Execution: Using command substitution ensures that the script fetches the latest system information each time it runs.
2. Readable Outputs: Variable substitution provides clear and formatted results, making the script output user-friendly.
3. Reusable Code: The use of variables like CURRENT_DATE and DISK_SPACE makes the script easy to maintain and modify.

---

Case Study: Backup Automation Script Using Variable and Command Substitution

Scenario:
An organization needs to back up important files daily to a specific directory with the current date in the backup folder name. The backup process should:

1. Create a directory named backup_<date>.
2. Copy all .txt files from a source directory to the backup directory.
3. Generate a log file containing the list of backed-up files.
4. Provide a summary at the end, including the total number of files backed up and the destination path.

Script: backup.sh

#!/bin/bash

# Variables and Command Substitution
SOURCE_DIR="/home/akshay/Desktop/operations/session54"
BACKUP_DIR="/home/akshay/Desktop/operations/session54"
CURRENT_DATE=$(date '+%Y-%m-%d')
BACKUP_FOLDER="${BACKUP_DIR}/backup_${CURRENT_DATE}"
LOG_FILE="${BACKUP_FOLDER}/backup_log.txt"

# Create the backup folder
mkdir -p "${BACKUP_FOLDER}"

# Start the log file
echo "Backup Log - Date: ${CURRENT_DATE}" > "${LOG_FILE}"
echo "-----------------------------------" >> "${LOG_FILE}"

# Copy .txt files to the backup folder and log their names
cp "${SOURCE_DIR}"/*.txt "${BACKUP_FOLDER}" 2>/dev/null && ls "${BACKUP_FOLDER}"/*.txt >> "${LOG_FILE}"

# Count the number of files backed up
TOTAL_FILES=$(ls "${BACKUP_FOLDER}"/*.txt 2>/dev/null | wc -l)

# Display summary
echo "-----------------------------------"
echo "         Backup Summary            "
echo "-----------------------------------"
echo "Date of Backup        : ${CURRENT_DATE}"
echo "Source Directory      : ${SOURCE_DIR}"
echo "Backup Directory      : ${BACKUP_FOLDER}"
echo "Total Files Backed Up : ${TOTAL_FILES}"
echo "Log File              : ${LOG_FILE}"
echo "-----------------------------------"

# Handle case of no files to backup
if [ "${TOTAL_FILES}" -eq 0 ]; then
    echo "No '.txt' files found in the source directory. Backup skipped."
    rm -rf "${BACKUP_FOLDER}"
else
    echo "Backup completed successfully! Check the log file for details."
fi

Explanation:

1. Variable Substitution:

   • ${SOURCE_DIR}, ${BACKUP_DIR}, ${CURRENT_DATE}, ${BACKUP_FOLDER}, and ${LOG_FILE} hold the source, destination, date, and log paths, respectively.
   • ${TOTAL_FILES} is used to count and display the number of files backed up.

2. Command Substitution:

   • $(date '+%Y-%m-%d'): Gets the current date for naming the backup folder.
   • $(ls "${BACKUP_FOLDER}"/*.txt | wc -l): Counts the number of .txt files in the backup directory.

3. Conditional Logic:

   • If no .txt files are found, the backup folder is removed, and the user is notified.

Key Learning Points:

1. Dynamic Folder Names: Using command substitution ensures each backup has a unique name with the current date.
2. Logging and Debugging: The log file records all backed-up files, which helps with future auditing and debugging.
3. Error Handling: The script gracefully handles cases where there are no files to back up.
4. Automating Repetitive Tasks: By integrating command and variable substitution, repetitive tasks like backups can be automated efficiently.