basic scripts

✅ Shell Basics

echo $SHELL    # Check the default shell for the user (e.g., /bin/bash, /bin/zsh)

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✅ Shebang

#!/bin/bash    # Shebang line to specify the script should run using the bash shell

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✅ Script: exp1.sh

#!/bin/bash

# Script to print static welcome message

echo -e "Hi\nThis is DevOps class"
echo "Started on Nov"

🔧 Execution Methods

./exp1.sh        # Executes script directly (make sure it's executable)
bash exp1.sh     # Executes using bash explicitly
sh exp1.sh       # Executes using default shell interpreter

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✅ Script: exp2.sh

#!/bin/bash

# Script to demonstrate passing arguments to a shell script

echo "This is $1 class"
echo "Started on $2"

🔧 Execution Example

bashCopyEdit./exp2.sh DevOps Nov

# Output:

# This is DevOps class

# Started on Nov

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✅ Input Arguments in Shell Script

• $0 → Script name

• $1 to $9 → 1st to 9th arguments

• ${10}, ${11} → 10th argument onward

🧠 Example:

#!/bin/bash
echo "Script Name: $0"
echo "First Arg : $1"
echo "Tenth Arg: ${10}"

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✅ Script: exp3.sh

#!/bin/bash

# Script to print predefined variables

name="Test"
month="Nov"
num="2022"

echo "This is $name platform and month is $month"
echo "The number will be $num"

✅ Conditional Statement Syntax

if [ condition ]; then
    # commands if true
else
    # commands if false
fi

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✅ Script: exp4.sh – Check if Number is 5

#!/bin/bash

if [ "$1" -eq 5 ]; then
    echo "$1 is Five"
else
    echo "$1 is not Five"
fi

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✅ Numeric Comparison Operators in Bash

Operator	Meaning
-eq	Equal to
-ne	Not equal to
-lt	Less than
-le	Less than or equal to
-gt	Greater than
-ge	Greater than or equal to

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✅ Script: exp5.sh – Biggest of Two Numbers

#!/bin/bash

if [ "$1" -gt "$2" ]; then
    echo "$1 is Big"
else
    echo "$2 is Big"
fi

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✅ Script: exp6.sh – Biggest of Two Numbers with Argument Check

#!/bin/bash

if [ "$#" -ne 2 ]; then
    echo "Enter only two arguments"
    exit 1
fi

if [ "$1" -gt "$2" ]; then
    echo "$1 is Big"
else
    echo "$2 is Big"
fi

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✅ Special Shell Variables

Variable	Meaning
$?	Exit status of last command (0 = success, non-zero = failure)
$$	Process ID of the current shell script
$!	PID of the last background command
$*	All arguments as a single string
$@	All arguments as individual strings (useful in loops)
$#	Total number of input arguments

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✅ Loop Syntax

🔁 for Loop

for i in list; do
    # commands
done

🔁 while Loop

while [ condition ]; do
    # commands
done

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✅ Script: Check File, Directory, Link, or Not Exist

#!/bin/bash

echo "Enter the name to check:"
read name

if [ -f "$name" ]; then
    echo "$name is a file"
elif [ -d "$name" ]; then
    echo "$name is a directory"
elif [ -L "$name" ]; then
    echo "$name is a symbolic link"
else
    echo "$name does not exist"
fi

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✅ Script : Disk Usage Alert if More Than 90%

bashCopyEdit#!/bin/bash

# Check disk usage and send alert if usage exceeds 90%

usage=$(df -h . | tail -1 | awk '{print $5}' | tr -d '%')

if [ "$usage" -ge 90 ]; then
    echo "Disk is full. Please take appropriate action." \
    | mail -s "Alert: Disk Usage Full" -c "[email protected]" "[email protected]"
fi

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✅ Cron Job Syntax Reference

Field	Range	Description
Minute	0–59	Minute of the hour
Hour	0–23	Hour of the day
Day of Month	1–31	Day of the month
Month	1–12	Month of the year
Day of Week	0–6 (Sun–Sat)	Day of the week

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✅ Useful Cron Commands

crontab -l    # List current user's cron jobs
crontab -e    # Edit cron jobs

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✅ Cron Job Examples

* * * * * /home/ubuntu/shell.sh       # Every minute
0 * * * * /home/ubuntu/shell.sh       # Every hour at 0 minutes

# Run on 28th December 2022 at 1:30 PM on a Wednesday
30 13 28 12 3 /home/ubuntu/shell.sh

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✅ Display Employees Over Age 40

📄 input File: employees.txt

Name    Emp-ID    Age
ABC     202201    35
EFG     202202    40
HIJ     202203    38
LMN     202204    45
XYZ     202205    43
GHY     202206    42

✅ Script: filter_employees.sh

#!/bin/bash

# Display employees older than 40

awk 'NR==1 || $3 > 40' employees.txt