# Advanced Linux Shell Scripting

1. ### **Create Directories Using Shell Script**
    
    Q. Write a bash script [`createDirectories.sh`](http://createDirectories.sh) that, when executed with three arguments (directory name, start number of directories, and end number of directories), creates a specified number of directories with a dynamic directory name.
    
    Example 1: When executed as `./`[`createDirectories.sh`](http://createDirectories.sh) `day 1 90`, it creates 90 directories as `day1 day2 day3 ... day90`.
    
    Example 2: When executed as `./`[`createDirectories.sh`](http://createDirectories.sh) `Movie 20 50`, it creates 31 directories as `Movie20 Movie21 Movie22 ... Movie50`.
    

```bash
#!/bin/bash

# Taking input from user 
read -p "Enter Directory name: " dirName
read -p "Enter number from directory starts: " sNum
read -p "Enter number of directory end: " eNum

# for loop to create directories
for ((i=sNum; i<=eNum; i++))
do
        mkdir "${dirName}${i}"
done

echo "Directories created successfully."
```

```bash
#Output after running script
Enter Directory name: dir
Enter number from directory starts: 1
Enter number of directory end: 10
Directories created successfully.
```

```bash
# 10 Directories created 
$ ls -l
total 44
drwxr-xr-x 2 shuhari shuhari 4096 Jul  3 10:10 dir1
drwxr-xr-x 2 shuhari shuhari 4096 Jul  3 10:10 dir10
drwxr-xr-x 2 shuhari shuhari 4096 Jul  3 10:10 dir2
drwxr-xr-x 2 shuhari shuhari 4096 Jul  3 10:10 dir3
drwxr-xr-x 2 shuhari shuhari 4096 Jul  3 10:10 dir4
drwxr-xr-x 2 shuhari shuhari 4096 Jul  3 10:10 dir5
drwxr-xr-x 2 shuhari shuhari 4096 Jul  3 10:10 dir6
drwxr-xr-x 2 shuhari shuhari 4096 Jul  3 10:10 dir7
drwxr-xr-x 2 shuhari shuhari 4096 Jul  3 10:10 dir8
drwxr-xr-x 2 shuhari shuhari 4096 Jul  3 10:10 dir9
```

---

2. ### Create a Script to Backup All Your Work
    

```bash
#!/bin/bash

sDir=/home/shuhari/linux/demo/
dDir=/home/shuhari/backup

rsync -av --delete $sDir $dDir

echo "Backup successfully done."
```

```bash
#Output

$ ./backupScript.sh
sending incremental file list
created directory /home/shuhari/backup
./
file11.gif
file110.gif
file12.gif
file13.gif
file14.gif
file15.gif
file16.gif
file17.gif
file18.gif
file19.gif

sent 634 bytes  received 252 bytes  1,772.00 bytes/sec
Backup successfully done.
total size is 0  speedup is 0.00
```

---

3. ### **What is Cron?**
    

* In simple terms, cron is like an automated to-do list for your computer.
    
* It allows you to schedule tasks to run automatically at specific times or intervals.
    
* e.g. Think of it as setting an alarm or reminder for your computer to perform certain actions without you having to remember to do them manually.
    

---

4. ### **What is Crontab?**
    

* Crontab stands for "cron table" and refers to a file that contains a list of commands meant to be run at specified times.
    
* These files are read by the cron daemon to determine which tasks to execute and when to execute them.
    
* Each user on a Unix-like system can have their own crontab file, which they can edit to schedule their own tasks.
    

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1720022780987/69bd6a9d-7f99-444d-8fdd-34ef7c3c4f5b.webp align="center")

> To configure cron job

```bash
$ crontab -e
```

> To list down all crontab for current user

```bash
$ crontab -l
```

---

5. ### **User management in Linux**
    

* User management in Linux involves creating, modifying, and deleting user accounts, as well as managing user permissions and groups.
    
* Each user is assigned an ID that is unique within the system. IDs 0 to 999 are assigned to system users, and local user IDs start from 1000 onwards.
    

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1720023413322/380c1e27-4ee6-4fc8-8f1e-943c91dbf112.png align="center")

Step 1 : Create two users

```bash
$sudo useradd user1
$sudo useradd user2
```

Step 2 : Display usernames of users

```bash
$cut -d: -f1 /etc/passwd | grep "^user"

#Output
user1
user2
```

This command uses `cut` to extract the first field (username) from each line in `/etc/passwd`, then filters (`grep`) lines starting with "user". Adjust the `grep` pattern as needed based on your usernames or naming convention.