Beginner's Guide to Advanced Shell Scripting

To create multiple directories in given range using script

#!/bin/bash  # Shebang to specify the script should be run using Bash

# Function to create directories
create_directories() {
    local prefix="$1"  # First argument: prefix for the directory names
    local start="$2"   # Second argument: starting number for the range
    local end="$3"     # Third argument: ending number for the range

    # Loop from start to end
    for (( i=$start; i<=$end; i++ )); do
        dir_name="${prefix}${i}"  # Construct the directory name
        mkdir "$dir_name"         # Create the directory
        echo "Directory '$dir_name' created."  # Print confirmation
    done
}

# Assign script arguments to variables
prefix="$1"  # First argument: prefix for the directory names
start="$2"   # Second argument: starting number for the range
end="$3"     # Third argument: ending number for the range

# Call the function with the arguments
create_directories "$prefix" "$start" "$end"  # Execute the function

day5.sh script

To execute the script run the command

./directiory_name.sh prefix start end

output after executing the scipt

What is Cron and Crontab?

Cron is service in Unix-like operating system to execute scripts or commands at specific times (like scheduling jobs).

Crontab is a list of commands to run each job to efficiently schedule and manage recurring tasks on the system.

Useful Cron Commands

To display the contents of the system-wide crontab file

cat /etc/crontab

To list scheduled cron jobs for the current user

crontab -l

To install crontab on On Red Hat-based systems (like CentOS or Fedora)

sudo yum install cronies

To edit the crontab file

crontab -e

After executing the command 'crontab -e' you'll be redirected to a text editor such as nano or vi to create cron jobs.

cron job

cron job created

Example Cron Job

To schedule the day5.sh script, edit your crontab:

39 20 * * * cd /home/ec2-user && bash /home/ec2-user/scripts/day5.sh dir 1 10 >> /home/ec2-user/scripts/cronjob.log 2>&1

cronjob output

Means that the cron job will run every day at 8:39 PM (20:39).

Breakdown of the Cron Job Schedule

39: The minute field (39th minute).
20: The hour field (20th hour, which is 8 PM in 24-hour format).
*: The day of the month field (every day of the month).
*: The month field (every month).
*: The day of the week field (every day of the week).

To Create Users in Unix-like Operating Systems

To create users in Unix like operating system

sudo useradd username

To assign a password to a user

sudo passwd username

To list Usernames of all users

list only the usernames, you can use awk to extract the first field from /etc/passwd:

awk -F: '{print $1}' /etc/passwd

output: user added

Summary of the cronjob Syntax:

cronjob syntax

To learn more about scheduling cronjobs visit:

https://www.freecodecamp.org/news/cron-jobs-in-linux/

Advance Shell Scripting Tutorial for beginners