Advanced Linux Shell Scripting for DevOps Engineers with User management

Task 5 of #90daysofdevops

Write a bash script createDirectories.sh that when the script is executed with three given arguments (one is the directory name and second is the start number of directories and third is the end number of directories ) it creates a specified number of directories with a dynamic directory name.

Example 1: When the script is executed as

./createDirectories.sh day 1 90

then it creates 90 directories as day1 day2 day3 .... day90

Example 2: When the script is executed as

./createDirectories.sh Movie 20 50 then it creates 50 directories as Movie20 Movie21 Movie23 ...Movie50

Answer:-
```
 #!/bin/bash
 for (( i=$2;i<=$3;i++))
 do
      mkdir $1:$i
 done
 ls
```
We just have to use 2 arguments while execution i.e.

./createDirectories.sh day 1 90

./createDirectories.sh Movie 20 50

Create a Script to back up all your work done till now.

 #!/bin/bash

 src_dir=/home/ubuntu/*
 dest_dir=/home/ubuntu/recovery

 curr_timestamp=$(date "+%Y-%m-%d-%H-%M-%S")

 backup_file=$dest_dir/$curr_timestamp.tgz

 tar -cvzf $backup_file --absolute-names $src_dir

 echo "Backup is taken on $curr_timestamp"
 echo "Backup Completed successfully."

About Cron and Crontab, to automate the backup Script

Cron

Cron is a service/process which allows scheduling or automating a particular task at a predefined interval.

Crontab

Crontab is a job scheduling utility used to schedule a particular script /Linux command written in a file for a specific time, day or period. It is configured in /etc/crontab. crond is a daemon utilised to manage cronjob. Always used for regular tasks which helps in reducing human efforts & errors.

Syntax: crontab <option> <user>

where options are

-e, -u, -l, -r

To create a new cronjob for the root user, use the following command and enter the new cron entry in a text editor

crontab -e

To list out the crontab list, use this command

crontab -l

To create a cronjob for the particular user, use this command

crontab -u

To remove created cronjob, use this command

crontab -r

Scheduling format :

* * * * * user-name <script/command path>

1)Minute(0-59)

2)Hour (0-23)

3)Day of Month (1-31)

4)Month (1-12) or (Jan, Feb, Mar,......., Dec)

5)Week (0-7) or (Sun, Mon, Tue, Wed, Thu, Fri, Sat, Sun)

where,

* means all/unknown

- means Range

, means list/separator

/ means interval

Example :

17 6 * * * echo "This is my first cron job" > /home/ubuntu/first_cronjob.txt
About User Management

A user is an entity, in a Linux operating system, that can manipulate files and perform several other operations. Each user is assigned an id that is unique for each user in the operating system.

id 0 is assigned to the root user and the id's 1 to 999 (both inclusive) are assigned to the system users hence the ids for local user begins from 1000 onwards. In a single directory, we can create 60,000 users.

Command to list all the users:
```
 awk -F':' '{print $1}' /etc/passwd
```
with the help of the awk command, we are printing the first column separated by “:”

Command to get the id of a user:
```
 id username
```
Command to add a user:
```
 sudo useradd username
```
Command to assign a password to a user:
```
 passwd username
```
Command to access user configuration:
```
 cat /etc/passwd
```
Command to change userid:
```
 usermod -u new_id username
```
Command to modify the group id of a user:
```
 usermod -g new_group_id username
```
Command to change login name for a user:
```
 usermod -l new_login_name old_login_name
```
Command to change the home directory of a user:
```
 usermod -d new_home_dir username
```
Command to delete a user:
```
 userdel -r username
```
Note: If a user is part of a group, then we have to remove the user from the group and then delete the user.
Create 2 users and just display their Usernames

sudo useradd Shawn

sudo useradd Mendis

To display their usernames, the following command is used:

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

Reference link: https://youtu.be/aolKiws4Joc

Thanks for reading.

See you next time.

Advanced Linux Shell Scripting for DevOps Engineers with User management

Table of contents

Task 5 of #90daysofdevops