Bash scripting ??

Getting Started

• File Extension: Bash scripts are typically saved with a .sh extension.
• The Shebang: Every script must begin with a shebang (#!/bin/bash or #!/usr/bin/env bash) on the very first line. This tells the operating system's program loader to interpret and execute the file using Bash.

Core Concepts

Comments

Use # to write non-executing code. Developers use comments to document logic or write instructions.

# This is a comment. The system will ignore this line.

Variables

Variables store data for later use, such as strings, numbers, or characters. Note: Do not put spaces around the = sign.

country="Mexico"
age=25

Control Flow & Loops

If-Else Conditions

Used to execute code blocks based on specific conditions.

if [[ condition ]]; then 
  # Code to run if condition is true
elif [[ condition ]]; then
  # Code to run if first condition is false, but this one is true
else
  # Code to run by default if no conditions match
fi

While Loops

Repeats a block of code as long as the specified condition remains true.

while [[ condition ]]; do
  # Code statement
done

For Loops

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Iterates over a fixed sequence or range of items.

for i in {1..5}; do
  echo "Iteration $i"
done

Case Statements

An efficient alternative to deeply nested if-else blocks when matching a single variable against multiple patterns.

case $expression in
  pattern1)
    # Block of code
    ;;
  pattern2)
    # Block of code
    ;;
  *)
    # Default case (matches anything else)
    ;;
esac

Cron Expressions & Automation

Cron is a time-based job scheduler in Unix-like operating systems.

*   *   *   *   *   /path/to/command_or_script.sh
│   │   │   │   │
│   │   │   │   └─ Day of the week (0 - 6) (0 is Sunday)
│   │   │   └─ Month of the year (1 - 12)
│   │   └─ Day of the month (1 - 31)
│   └─ Hour of the day (0 - 23)
└─ Minute of the hour (0 - 59)

Example Schedules

• 0 0 * * * /home/user/backup.sh ➔ Runs every single day at exactly midnight.
• 30 8 * * 1 /home/user/report.sh ➔ Runs every Monday at 8:30 AM.
• */15 * * * * /home/user/check.sh ➔ Runs every 15 minutes (note the standard */15 syntax).
• 0 12 1 * * /home/user/rent.sh ➔ Runs at 12:00 PM on the 1st day of every month.
• 15 * * * * ➔ Runs at exactly minute 15 of every hour (e.g., 12:15, 1:15, 2:15).

Cron Best Practices

• Keep Logic in a Script: Cron executes tasks silently in the background without a visible terminal. Instead of jamming complex, multi-line commands inside your crontab -e file, write the logic in a .sh file and point Cron to that single file.
• Redirect Output to Log Files: Because Cron runs silently, you should redirect the output to a .log file to track success and troubleshoot failures:

0 0 * * * /home/user/run_all.sh >> /home/user/script_output.log 2>&1

Understanding 2>&1:

>> appends standard output (successful logs) to your file.

2>&1 ensures that standard error (critical failure messages) is redirected into the same log file, allowing you to debug issues later.