Shell Scripting

Sun, 23 Jul 2023 00:00:00 +0000

Shell provides an interface between the user and the kernel and executes programs called commands.

Different shells:

sh => Bourne shell
ksh => korn shell [–more–]
bash => Bourne again shell
csh => c shell
zsh = z shell

Most popular one is Bash and it is used in shell scripting.

Script File

Shell script file should end with .sh or it should have shebang #!.
```
#!/bin/bash

echo "Hello world"
```
Shebang defines the interpreter to use to interpret the script.
Script is exectued line by line.
It also have programming concepts like conditional statement, loops, etc.
To execute the script you can use either ways
1. Make it executable and execute like binary
```
chmod +x script.sh
./script.sh
```
2. Execute directly using interpreter
```
bash script.sh
```

Variables

Variable store some values
```
#!/bin/bash

name="tester"
echo "hello $name"

read -p "Enter you age: " age
echo "your age is $age"
```
- read: It is used for user input, age is variable in which we are storing that user input.
- $ is use to access variable content.

Arguments

Arguments are values which is provided at runtime
```
#!/bin/bash

echo "File name is: $0"
echo "Hello $1"
echo "Creating file $2" && touch $2 
```
Execute
```
./script tester myfile2
```
Output
```
File name is: /tmp/script
Hello tester
Creating file myfile2
```
- $1,$2,.. $n This is how we access arguments.
- $0 is the first argument, which is the name of file.
- $# This returns the count of arguments.
- $@ This represents all the arguments.

Loops

Execute until a specific condition is met.

Example:

#!/bin/bash

dir=/home/user/scripts/*.sh
for files in $dir
do
    echo $files
done

This script print all .sh files in that directory.

Example 2: Script to add multiple users.

#!/bin/bash

read -p "Enter the number of users: " user_num

for (( i=1; i<=$user_num; i++ ))
do
    read -p "Enter username: " user
    read -s -p "Enter pass: " pass
    sudo useradd -m $user
    echo "$user:$pass" | sudo chpasswd
    echo "Done"
done

cat /etc/passwd | grep $user

If else statement

Execute something on some condition

#!/bin/bash

name="tester"

if [ $name == "tester" ]
then
        echo "yes, tester logged in"
else 
        echo "Invalid user"
fi

Example script.

Install nginx and serve page using script.

# Update system
sudo apt update

# Install nginx
sudo apt install nginx -y

# Start & enable nginx service
sudo systemctl enable --now nginx

# Serve the page
sudo rm /var/www/html/*.html && echo "Hello world" | sudo tee /var/www/html/index.html

Backup script

#!/bin/bash

src=/home/user/devops/script
tgt=/home/user/backups
filename=$(date +'%d-%m-%Y').tar.gz
tar -cvzf $tgt/$filename $src 

echo "Completed!"

You can create crontab for this.

crontab -e

* * * * * /path/to/backup/script

That’s it.

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