Basic script constructs

2.6 Summarize scripting basics for server administration.

📘CompTIA Server+ (SK0-005) 

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Scripts are a series of commands that a computer executes automatically. To make them powerful, scripts use constructs—basic building blocks that control how the script behaves. The main constructs are variables, loops, conditionals, and comparators.

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1. Variables

Definition:
A variable is a storage location in a script that holds data. Think of it as a named “container” for information that can change.

Why it’s used in IT:

• Store server names, IP addresses, or configuration settings.
• Store user input or results from a command to use later in the script.

Examples:

• In a PowerShell script, you can store a server name:

$ServerName = "Server01"

• In a Bash script, you can store an IP address:

IP="192.168.1.10"

Notes for the exam:

• Variables make scripts flexible. You don’t have to change the script everywhere; just update the variable.
• Variable names usually start with a letter and contain letters, numbers, or underscores.

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2. Loops

Definition:
Loops let scripts repeat actions automatically, saving time and reducing manual effort.

Types of loops commonly used:

• For loop: Repeats a task a specific number of times.
• While loop: Repeats a task as long as a condition is true.
• ForEach loop: Repeats a task for each item in a list.

IT examples:

1. Deploying updates to multiple servers:

$Servers = @("Server01","Server02","Server03")
foreach ($Server in $Servers) {
    Restart-Computer -ComputerName $Server
}

This script restarts three servers automatically without doing each one manually.

2. Checking log files continuously:

while [ -f /var/log/syslog ]; do
    tail -n 10 /var/log/syslog
    sleep 60
done

This monitors the last 10 lines of a log file every 60 seconds.

Notes for the exam:

• Loops reduce repetitive work.
• Infinite loops can happen if the exit condition is never met—scripts must have a stop condition.

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3. Conditionals (If statements)

Definition:
Conditionals let the script make decisions based on certain conditions. They are “if this happens, do that; otherwise, do something else.”

IT examples:

1. Check if a service is running:

if ((Get-Service "Spooler").Status -eq "Running") {
    Write-Output "Print service is running"
} else {
    Start-Service "Spooler"
    Write-Output "Print service started"
}

• Checks if the Print Spooler service is running. If not, it starts it.

2. Check free disk space on Linux:

if [ $(df / | tail -1 | awk '{print $5}' | sed 's/%//') -gt 80 ]; then
    echo "Disk usage over 80%, cleanup required"
fi

Notes for the exam:

• Conditional statements are often paired with comparators.
• Common keywords: if, else, elif (in Bash), elseif (in PowerShell).

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4. Comparators

Definition:
Comparators are symbols or words that compare values. Scripts use them in conditionals and loops to make decisions.

Common Comparators:

Type	Bash	PowerShell	Meaning
Equal to	-eq	-eq	Checks if values are equal
Not equal	-ne	-ne	Checks if values are not equal
Greater than	-gt	-gt	Checks if left value is greater
Less than	-lt	-lt	Checks if left value is smaller
Greater or equal	-ge	-ge	Checks if left value is greater or equal
Less or equal	-le	-le	Checks if left value is smaller or equal

Example in IT:

• Check CPU load:

$CPUUsage = 75
if ($CPUUsage -gt 70) {
    Write-Output "CPU is high, consider load balancing"
}

• Check if a server is online:

ping -c 1 192.168.1.10
if [ $? -eq 0 ]; then
    echo "Server is online"
else
    echo "Server is offline"
fi

Notes for the exam:

• Comparators are always used in conditionals or loops.
• $? in Bash is a special variable that stores the exit status of the last command (0 = success).

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✅ Key Takeaways for the Exam

1. Variables store data for reuse and flexibility.
2. Loops automate repetitive tasks (For, While, ForEach).
3. Conditionals make decisions based on conditions (if, else).
4. Comparators allow scripts to evaluate data (-eq, -ne, -gt, -lt).
5. These constructs often work together. For example, a loop may use a conditional to act only on certain items.

Tip: Most exam questions may ask you to identify what a script does, choose the right construct, or predict the output of a script snippet.