Learn Shell Scripting: A Simple Guide to Loops, Functions, and Automation

Shell scripting is a powerful tool for automating repetitive tasks in Linux. Whether you're processing files, checking connectivity, or scheduling jobs, mastering loops, functions, and automation can make your scripts efficient and robust.

In this blog, we'll cover:

• Different types of loops (for, while, until, and infinite loops)

• Reading files with loops

• Functions and argument passing

• Useful scripting concepts (variables, redirection, logging, debugging)

• Automation using at and cron

Let’s dive in!

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1. For Loop in Shell Script

The for loop iterates over a list of values.

Syntax:

for var in value1 value2 value3; do  
    echo "$var"  
done

Example:

for i in 1 2 3 4 5; do  
    echo "Number: $i"  
done

---

2. For Loop to Get Values from a File

You can read lines from a file using a for loop.

Example:

for line in $(cat file.txt); do  
    echo "$line"  
done

Better Approach (handles spaces correctly):

while IFS= read -r line; do  
    echo "$line"  
done < file.txt

---

3. While Loop in Shell Script

The while loop runs as long as a condition is true.

Syntax:

while [ condition ]; do  
    commands  
done

Example (Countdown):

count=5  
while [ $count -gt 0 ]; do  
    echo "Countdown: $count"  
    ((count--))  
done

---

4. Until Loop in Shell Script

The until loop runs until a condition becomes true (opposite of while).

Syntax:

until [ condition ]; do  
    commands  
done

Example:

count=1  
until [ $count -gt 5 ]; do  
    echo "Count: $count"  
    ((count++))  
done

---

5. Infinite Loop in Shell Script

An infinite loop runs forever until manually stopped (Ctrl+C).

Using while:

while true; do  
    echo "Infinite loop running..."  
    sleep 1  
done

Using for:

for (( ; ; )); do  
    echo "Infinite loop"  
    sleep 1  
done

---

6. While Loop with File

You can read a file line-by-line using while.

Example:

while IFS= read -r line; do  
    echo "Line: $line"  
done < file.txt

---

7. Functions in Shell Script

Functions help in reusing code.

Syntax:

function_name() {  
    commands  
}

Example:

greet() {  
    echo "Hello, $1!"  
}  

greet "Alice"

---

8. Passing Arguments in Shell Script

You can pass arguments to scripts or functions.

Script Arguments:

echo "First arg: $1"  
echo "Second arg: $2"

Function Arguments:

sum() {  
    echo $(($1 + $2))  
}  

sum 5 10

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9. Other Useful Concepts

BASH Variables

name="John"  
echo "Hello, $name"

Redirection (>, >>, 2>, &>)

• > – Overwrite file

• >> – Append to file

• 2> – Redirect errors

• &> – Redirect both output and errors

/dev/null in Shell Script

Discard output:

command > /dev/null  # Suppress output  
command 2> /dev/null # Suppress errors

Logging in Shell Script

logfile="script.log"  
echo "$(date): Script started" >> "$logfile"

Debugging in Shell Script

#!/bin/bash -x  # Enable debug mode  
set -x          # Start debugging  
set +x          # Stop debugging

---

10. Connectivity Check Script

Check if a host is reachable:

ping -c 1 google.com &> /dev/null  
if [ $? -eq 0 ]; then  
    echo "Host is reachable."  
else  
    echo "Host is down."  
fi

---

11. Automate Script Using at Command

Run a script once at a specific time:

echo "/path/to/script.sh" | at 10:00 PM

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12. Automate Script Using Crontab

Schedule recurring tasks with cron:

Edit Crontab:

crontab -e

Example (Run daily at 5 AM):

0 5 * * * /path/to/script.sh >> /path/to/logfile.log

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Conclusion

Shell scripting is essential for automation in Linux. By mastering loops, functions, and scheduling tools like cron and at, you can streamline repetitive tasks and improve efficiency.

Try implementing these concepts in your scripts and automate your workflows today! 🚀