Shell Scripting Concepts and Syntax — Variables, Conditions, and Loops (Part 2)

In the previous article, you learned what shell scripting is and how to write and execute a basic Bash script.
Now we take the next step and learn how Bash scripts actually become powerful.

In this part, we’ll cover:

• Variables

• Command substitution

• Conditionals (if / else)

• File, number, and string checks

• Script parameters

• Loops (for and while)

• Reading user input

These concepts allow you to write flexible, reusable, and intelligent scripts.

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Variables: Store Once, Use Everywhere

Variables allow you to store a value once and reuse it many times.

Example:

file_name="config.yaml"

Variable naming styles:

• file_name (underscore – common)

• fileName (camelCase)

Both are valid. Naming is a convention, not a rule.

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Using Variables

To reference a variable:

echo "Using file $file_name to configure server"

• $ tells Bash to read the variable value

• Double quotes are recommended

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Command Substitution: Save Command Output in a Variable

You can assign the output of a command to a variable.

Example:

config_files=$(ls config)

Now config_files contains whatever ls config outputs.

Using it:

echo "Here are all configuration files: $config_files"

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When Commands Fail

If the directory doesn’t exist:

• Bash prints an error

• The variable becomes empty

This happens often in real environments and that’s why we need conditionals.

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Conditionals: Preparing for Different Scenarios

Before running risky commands, we check conditions first.

Basic structure:

if [ condition ]; then
  commands
else
  other_commands
fi

---

File and Directory Conditions

Check if a directory exists:

if [ -d config ]; then
  echo "Reading config directory contents"
else
  echo "Config directory not found, creating one"
  mkdir config
fi

• -d → directory exists

• -f → file exists

• -r → readable

• -w → writable

• -x → executable

• -s → file is not empty

These checks protect your script from failures.

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Number Comparisons

Used when working with counts, sizes, or metrics.

Examples:

• -lt → less than

• -gt → greater than

• -le → less than or equal

• -ge → greater than or equal

• -ne → not equal

Example:

if [ "$file_count" -lt 10 ]; then
  echo "Safe to process files"
fi

---

String Comparisons

Used to compare text values.

Example:

if [ "$user_group" = "admin" ]; then
  echo "Admin access granted"
else
  echo "No permission"
fi

• Single = → POSIX-compatible

• Double == → Bash-specific

---

Multiple Conditions with elif

if [ "$user_group" = "nana" ]; then
  echo "Configure server"
elif [ "$user_group" = "admin" ]; then
  echo "Administer server"
else
  echo "Wrong user group"
fi

---

Script Parameters: Passing Values from Outside

Scripts can accept parameters at execution time.

Example execution:

./setup.sh admin config

Inside the script:

• $1 → first parameter

• $2 → second parameter

Example:

config_dir="$1"
user_group="$2"

This makes scripts configurable instead of hard-coded.

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Using Parameters in Conditions

if [ -d "$config_dir" ]; then
  echo "Reading config directory"
  ls "$config_dir"
else
  echo "Creating directory"
  mkdir "$config_dir"
fi

---

Reading User Input

Instead of parameters, you can ask the user directly.

Example:

read -p "Please enter your password: " user_password
echo "Thanks for your password: $user_password"

Useful when:

• Input is sensitive

• Parameters are unknown

• Interaction is required

---

Special Parameter Variables

• $* → all parameters

• $# → number of parameters

Example:

echo "Total params: $#"
echo "All params: $*"

---

Loops: Repeating Logic

When working with lists, you use loops.

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for Loop — Iterate Over a List

Example:

for param in $*; do
  echo "$param"
done

How it works:

• Each parameter is assigned to param

• Loop runs once per parameter

---

Practical for Loop Example

for param in $*; do
  if [ -d "$param" ]; then
    echo "Directory found: $param"
    ls "$param"
  else
    echo "Not a directory: $param"
  fi
done

Perfect for:

• Lists of files

• Lists of servers

• Lists of programs

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while Loop — Run Until a Condition Changes

Unlike for, while loops run as long as a condition is true.

---

Infinite Loop Example

sum=0

while true; do
  read -p "Enter a score (Q to quit): " score

  if [ "$score" = "Q" ]; then
    break
  fi

  sum=$((sum + score))
  echo "Total score: $sum"
done

Key concepts:

• while true → infinite loop

• break → exit loop

• $(( )) → numeric calculation

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Arithmetic in Bash

Without $(( )), Bash treats values as strings.

Correct numeric addition:

sum=$((sum + score))

---

Single vs Double Brackets

• [ condition ] → standard

• [[ condition ]] → Bash-enhanced

With [[ ]]:

• Quotes are optional

• Safer string comparisons

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Bash vs Other Automation Tools

Bash is powerful—but:

• Syntax is complex

• Hard to maintain at scale

Alternatives:

• Python (more readable)

• Configuration tools (like Ansible)

However, knowing Bash gives you a huge advantage, because:

• You understand the foundation

• You can choose the right tool

• You can debug automation at any level

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Final Takeaway

In this lecture, you learned how to:

• Use variables

• Capture command output

• Write conditionals

• Accept parameters

• Read user input

• Use loops effectively

• Control execution flow

These concepts turn Bash scripts from simple command files into real automation tools.