Mastering the Shell: Unleashing the Power of Bash Scripting

Shell scripting is a powerful tool for automating tasks, managing systems, and enhancing your productivity within a Linux or Unix-like environment. Bash, the Bourne Again Shell, is the most common shell and provides a robust scripting language. This chapter will introduce you to the fundamentals of Bash scripting and demonstrate its capabilities through practical examples.

1. Introduction to Bash Scripting

A shell script is a plain text file containing a series of commands that the shell executes sequentially. These commands can include system utilities, shell built-ins, and even other scripts.

1.1. Creating and Executing Scripts

1. Create a script: Open a text editor and write your commands.
2. Save the script: Save the file with a .sh extension (e.g., my_script.sh).
3. Make it executable: Use chmod +x my_script.sh.
4. Run the script: Execute it from the terminal using ./my_script.sh.

1.2. The Shebang (#!)

The first line of a script, the shebang, specifies the interpreter to use. For Bash scripts, it’s #!/bin/bash or #!/bin/sh (for more basic shell compatibility).

2. Basic Scripting Concepts

2.1. Variables

Variables store data. Assign values using variable_name=value. Access them using $variable_name or ${variable_name}.

Bash

#!/bin/bash
name="John Doe"
echo "Hello, $name!"

2.2. Input/Output

• echo: Prints text to the console.
• read: Reads input from the user.

Bash

#!/bin/bash
read -p "Enter your name: " name
echo "Your name is: $name"

  2.3. Conditional Statements (if, elif, else)

Control the flow of execution based on conditions.

Bash

 #!/bin/bash 
 read -p "Enter a number: " num
 if [ $num -gt 0 ]; then
   echo "Positive number."
 elif [ $num -lt 0 ]; then
   echo "Negative number."
 else
   echo "Zero."
 fi

2.4. Loops (for, while)

Repeat a block of code.

• for loop: Iterates over a list of items.

Bash

 #!/bin/bash 
 for i in 1 2 3 4 5; do
   echo "Number: $i"
 done

• while loop: Executes as long as a condition is true.

Bash

#!/bin/bash
count=1
 while [ $count -le5 ]; do
   echo "Count: $count"
   count=$((count + 1))
 done

3. Practical Scripting Examples

Here are the scripts from your previous requests, integrated into this chapter:

3.1. Reversing a Number

Bash

 #!/bin/bash
 read -p "Enter a number: " num
 reversed=""
 original_num=$num
 while [ $num -gt 0 ]; do
   digit=$((num % 10))
   reversed="$reversed$digit"
   num=$((num / 10))
 done
 echo "Original number: $original_num"
 echo "Reversed number: $reversed"

3.2. Displaying Username as Banner

Bash

#!/bin/bash
username=$(whoami)
 banner=$(banner "$username")
 if [[ -n "$banner" ]]; then
   echo "$banner"
 else
   echo "Error: banner command not found or failed."
 fi

3.3. Listing Directories

Bash

#!/bin/bash
for item in *; do
   if [ -d "$item" ]; then
     echo "$item"
   fi
 done

3.4. Using ls and more Based on File Type

Bash

 #!/bin/bash
 if [ $# -ne 1 ]; then
   echo "Usage: $0 "
   exit 1
 fi
 filename="$1"
 if [ -d "$filename" ]; then
   ls -l "$filename"
 elif [ -f "$filename" ]; then
   more "$filename"
 else
   echo "Error: '$filename' is not a file or directory."
   exit 1
 fi

 3.5. Copying/Overwriting a File

Bash

 #!/bin/bash
 if [ $# -ne 2 ]; then
   echo "Usage: $0 "
   exit 1
 fi
 source_file="$1"
 destination_file="$2"
 if [ ! -f "$source_file" ]; then
   echo "Error: Source file '$source_file' does not exist."
   exit 1
 fi
 cp "$source_file" "$destination_file"
 if [ $? -eq 0 ]; then
   echo "File '$source_file' copied to '$destination_file' successfully."
 else
   echo "Error: Failed to copy file '$source_file' to '$destination_file'."
   exit 1
 fi

 3.6. Word Length

Bash

 #!/bin/bash
 read -p "Enter a word: " word
 length=${#word}
 echo "The word '$word' is $length characters long."

3.7. Appending Line to File

Bash

#!/bin/sh 
if [ $# -ne 2 ]; then
   echo "Usage: $0 "
   exit 1
 fi
 filename="$1"
 line_to_append="$2"
 if [ ! -f "$filename" ]; then
   echo "Error: File '$filename' does not exist."
   exit 1
 fi
 echo "$line_to_append" >> "$filename"
 if [ $? -ne 0 ]; then
   echo "Error: Failed to append line to '$filename'."
   exit 1
 fi
 line_count=$(wc -l < "$filename")
 echo "Line appended successfully. Total lines in '$filename': $line_count"

 3.8. User UID and Login Count

Bash

 #!/bin/bash
if [ $# -ne 1 ]; then
echo "Usage: $0 "
exit 1
fi
username="$1"
uid=$(id -u "$username" 2>/dev/null)
if [ -z "$uid" ]; then
echo "Error: User '$username' not found."
exit 1
fi
login_count=$(lastb "$username" | wc -l)
successful_logins=$(last "$username" | wc -l)
total_logins=$((login_count + successful_logins))
echo "Username: $username"
echo "UID: $uid"
echo "Total Logins (successful and failed): $total_logins"
echo "Successful Logins: $successful_logins"
echo "Failed Logins: $login_count"

3.9. User Information from UID

Bash

#!/bin/bash
if [ $# -ne 1 ]; then
  echo "Usage: $0 "
  exit 1
fi
uid="$1"
username=$(getent passwd "$uid" | cut -d: -f1)
if [ -z "$username" ]; then
  echo "Error: UID '$uid' not found."
  exit 1
fi
fullname=$(getent passwd "$uid" | cut -d: -f5 | cut -d, -f1)
home_dir=$(getent passwd "$uid" | cut -d: -f6)
shell=$(getent passwd "$uid" | cut -d: -f7)
primary_group=$(getent group "$(getent passwd "$uid" | cut -d: -f4)" | cut -d: -f1)
secondary_groups=$(getent group | awk -F: '$4 ~ /'"$uid"'/ {print $1}')
echo "Username: $username"
echo "Full Name: $fullname"
echo "Home Directory: $home_dir"
echo "Shell: $shell"
echo "Primary Group: $primary_group"
if [ -n "$secondary_groups" ]; then
  echo "Secondary Groups: $secondary_groups"
fi

3.10. Mail Merge Facility

Bash

#!/bin/bash

# Mail Merge Script
# Usage: mail_merge.sh

# Check for correct number of arguments
if [ $# -ne 4 ]; then
  echo "Usage: $0 "
  exit 1
fi

TEMPLATE_FILE="$1"
DATA_FILE="$2"
SUBJECT="$3"
OUTPUT_DIR="$4"

# Check if template and data files exist
if [ ! -f "$TEMPLATE_FILE" ]; then
  echo "Error: Template file '$TEMPLATE_FILE' not found."
  exit 1
fi

if [ ! -f "$DATA_FILE" ]; then
  echo "Error: Data file '$DATA_FILE' not found."
  exit 1
fi

# Check if output directory exists, if not, create it
if [ ! -d "$OUTPUT_DIR" ]; then
  mkdir -p "$OUTPUT_DIR"
  fi

#Read data file line by line
while IFS=',' read -r name email address; do
  # Create output file name
  OUTPUT_FILE="$OUTPUT_DIR/mail_to_$email.txt"
  # Replace placeholders in template file
  cat "$TEMPLATE_FILE" | sed \
    -e "s/{{NAME}}/$name/g" \
    -e "s/{{EMAIL}}/$email/g" \
    -e "s/{{ADDRESS}}/$address/g" > "$OUTPUT_FILE"

# Send email (optional - requires mail command or similar)
# echo "Mail content generated in $OUTPUT_FILE"
# mail -s "$SUBJECT" "$email" < "$OUTPUT_FILE"

done < "$DATA_FILE"
echo "Mail merge completed. Output files generated in '$OUTPUT_DIR'."
exit 0

Explanation and Usage:

1. Script Arguments:
   • <template_file>: The path to the template file containing placeholders like {{NAME}}, {{EMAIL}}, and {{ADDRESS}}.
   • <data_file>: The path to a CSV (comma-separated values) file containing the data to be merged. The first line of the CSV file should be name,email,address. Subsequent lines contain the data for each recipient.
   • <subject>: The subject line for the emails.
   • <output_dir>: The directory where the generated mail files will be saved.
2. Error Handling:
   • The script checks if the correct number of arguments are provided.
   • It verifies that the template and data files exist.
   • It creates the output directory if it doesn’t already exist.
3. Data Processing:
   • The while IFS=',' read -r name email address; do ... done < "$DATA_FILE" loop reads the data file line by line, splitting each line into the name, email, and address variables using commas as delimiters.
   • For each line, it creates an output file named mail_to_<email>.txt in the specified output directory.
   • The cat "$TEMPLATE_FILE" | sed ... > "$OUTPUT_FILE" command reads the template file and uses sed to replace the placeholders with the corresponding data from the current line.
4. Optional Email Sending:
   • The commented-out mail command demonstrates how to send the generated emails. You’ll need to have the mail command (or a similar mail utility) installed on your system.
   • Uncomment the lines to enable email sending.

Example Template File (template.txt):

Dear {{NAME}},

This is a personalized message for you.

Your email address is: {{EMAIL}}
Your address is: {{ADDRESS}}

Sincerely,
The Team

Example Data File (data.csv):

Code snippet

name,email,address
John Doe,john.doe@example.com,123 Main St
Jane Smith,jane.smith@example.com,456 Oak Ave
Peter Jones,peter.jones@example.com,789 Pine Ln

How to Run the Script:

1. Save the script to a file (e.g., mail_merge.sh).
2. Make the script executable: chmod +x mail_merge.sh.
3. Run the script: ./mail_merge.sh template.txt data.csv "Your Subject" output_mails.

This will generate three files in the output_mails directory: mail_to_john.doe@example.com.txt, mail_to_jane.smith@example.com.txt, and mail_to_peter.jones@example.com.txt.