Shell Scripting Revision

Here are Shell Scripting Revision Notes for quick reference. 🚀

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1. Basics of Shell Scripting

What is a Shell Script?

• A shell script is a file containing a series of Linux commands.

• It automates tasks that you would normally run manually in a terminal.

Shebang (#!)

• The first line in a shell script specifies the interpreter:

    #!/bin/bash   # Bash shell
    #!/bin/sh     # POSIX shell
    #!/usr/bin/env python3  # Python script
  

• To make the script executable:

    chmod +x script.sh
  

• To run the script:

    ./script.sh
  

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2. Variables in Shell

Defining Variables

name="Alice"
age=25

Accessing Variables

echo "Name: $name"
echo "Age: ${age}"

Read Input from User

echo "Enter your name:"
read username
echo "Hello, $username!"

Special Variables

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3. Conditional Statements

If-Else Statement

if [ "$age" -ge 18 ]; then
  echo "You are an adult."
else
  echo "You are a minor."
fi

String Comparisons

if [ "$name" = "Alice" ]; then
  echo "Welcome, Alice!"
fi

File Checks

Example:

if [ -f "data.txt" ]; then
  echo "File exists!"
else
  echo "File not found!"
fi

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4. Loops in Shell Scripting

For Loop

for i in 1 2 3 4 5; do
  echo "Iteration $i"
done

While Loop

count=1
while [ $count -le 5 ]; do
  echo "Count: $count"
  count=$((count + 1))
done

Until Loop

n=1
until [ $n -gt 5 ]; do
  echo "Number: $n"
  n=$((n + 1))
done

Loop Through a File

while read line; do
  echo "Line: $line"
done < file.txt

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5. Functions in Shell Scripting

function greet() {
  echo "Hello, $1!"
}
greet "Alice"

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6. Case Statement

echo "Enter a choice: "
read option

case $option in
  1) echo "You selected One" ;;
  2) echo "You selected Two" ;;
  *) echo "Invalid option" ;;
esac

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7. Command Line Arguments

echo "Script Name: $0"
echo "First Argument: $1"
echo "Second Argument: $2"
echo "All Arguments: $@"
echo "Total Arguments: $#"

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8. Input and Output Redirection

Example:

ls > output.txt    # Save output to file
ls >> output.txt   # Append output to file
cat < input.txt    # Read from file
ls 2> error.log    # Redirect errors
ls &> all.log      # Redirect both stdout & stderr

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9. Background & Foreground Execution

• Run a script in the background:

    ./script.sh &
  

• Bring a process to the foreground:

    fg %1
  

• List background jobs:

    jobs
  

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10. Process Management

Example:

kill $(pgrep firefox)

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11. Scheduling Jobs (cron & at)

Cron Jobs (Repeated Execution)

• Edit cron jobs:

    crontab -e
  

• List cron jobs:

    crontab -l
  

• Cron Job Syntax:

    * * * * * command_to_run
    ┬ ┬ ┬ ┬ ┬
    │ │ │ │ │
    │ │ │ │ └── Day of the week (0 - 6) [Sunday = 0]
    │ │ │ └──── Month (1 - 12)
    │ │ └────── Day of the month (1 - 31)
    │ └──────── Hour (0 - 23)
    └────────── Minute (0 - 59)
  

Example:
Run a script every day at 5 AM:

0 5 * * * /path/to/script.sh

at Command (One-time Execution)

• Run a job at a specific time:

    echo "echo Hello" | at 14:30
  

• List pending jobs:

    atq
  

• Remove a job:

    atrm <job_id>
  

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12. Networking in Shell

• Ping a server:

    ping -c 4 google.com
  

• Fetch an external IP address:

    curl ifconfig.me
  

• Check open ports:

    netstat -tulnp
  

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13. Debugging Shell Scripts

• Run the script in debug mode:

    bash -x script.sh
  

• Debug specific lines:

    set -x   # Enable debugging
    set +x   # Disable debugging
  

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These revision notes cover the most important shell scripting concepts!