Shell Scripting: Variables, Conditions, Arrays

In our previous blog, we covered the basics of shell scripting. In this post, we will dive into some essential topics: variables, conditions (string, numeric, file, and logical), arrays, and conditional statements.

These concepts are widely used in real-world DevOps tasks. Whenever you write a shell script, conditions and variables are essential for automating decisions and handling dynamic data.

Variables in Shell Scripting

Variables store values that can be reused throughout a script, making your scripts more flexible and easier to maintain.

Global Variables

Global variables are accessible anywhere in the script after declaration.

name="Linux"
echo "Welcome to $name"
echo "Learning $name shell scripting"

Output:

Welcome to Linux
Learning Linux shell scripting

Local Variables

Local variables exist only inside a block, usually a function.

my_function() {
  local city="Hyderabad"
  echo "Inside function: $city"
}
my_function
echo "Outside function: $city"

Output:

Inside function: Hyderabad
# No output for the second echo because the variable is local to the function.

String Conditions

String conditions are used to compare text values.

• = → equals
• != → not equal
• -z → string is empty
• -n → string is not empty

Examples

name="Linux"
if [ "$name" = "Linux" ]; then
  echo "Platform is Linux"
fi
file_name=""
if [ -z "$file_name" ]; then
echo "Filename is empty"
fi

Output:

Platform is Linux
Filename is empty

Numeric Conditions

Numeric conditions compare integer values:

• -eq → equal
• -ne → not equal
• -gt → greater than
• -lt → less than
• -ge → greater than or equal
• -le → less than or equal

Examples

num1=10
num2=20
if [ $num1 -lt $num2 ]; then
echo "$num1 is less than $num2"
fi
if [ $num1 -ne $num2 ]; then
echo "$num1 is not equal to $num2"
fi

Output:

10 is less than 20
10 is not equal to 20

File Conditions

File conditions check file existence, type, and permissions:

• -e → file exists
• -f → regular file
• -d → directory
• -r → readable
• -w → writable
• -x → executable
• -s → not empty
• -L → symbolic link

Examples

file="/etc/passwd"
if [ -e "$file" ]; then
echo "$file exists"
fi
if [ -r "$file" ]; then
echo "$file is readable"
fi
dir="/tmp"
if [ -d "$dir" ]; then
echo "$dir is a directory"
fi

Output:

/etc/passwd exists
/etc/passwd is readable
/tmp is a directory

Logical Conditions

Logical operators help create complex conditions:

• && → AND, executes when all conditions are true
• || → OR, executes when at least one condition is true
• ! → NOT, reverses the condition

Examples

num1=10
num2=20
if [ $num1 -lt $num2 ] && [ $num1 -eq 10 ]; then
echo "Both conditions are true"
fi
if [ $num1 -eq 5 ] || [ $num2 -eq 20 ]; then
echo "At least one condition is true"
fi
if ! [ $num1 -eq 5 ]; then
echo "num1 is not equal to 5"
fi

Output:

Both conditions are true
At least one condition is true
num1 is not equal to 5

Arrays in Shell Scripting

Arrays store multiple values in a single variable. Indexing starts at 0.

• array[@] → prints all elements
• #array[@] → length of array
• array[0] → first element
• array[-1] → last element
• array+=(value) → add element
• unset array[index] → remove specific element
• unset array → remove entire array

Examples

servers=("web01" "db01" "cache01")
echo "All servers: ${servers[@]}"
echo "Total servers: ${#servers[@]}"
echo "First server: ${servers[0]}"
servers+=("proxy01")
unset servers[1]  # removes db01
echo "Updated servers: ${servers[@]}"

Output:

All servers: web01 db01 cache01
Total servers: 3
First server: web01
Updated servers: web01 cache01 proxy01

Conditional Statements

Conditional statements control the flow of execution in scripts based on specific conditions.

if Statement

Commands execute only when the condition is true.

status=$(systemctl is-active nginx)
if [ "$status" = "active" ]; then
  echo "Nginx service is running"
fi

if-else Statement

Executes one block if true, another if false.

if [ -f "/tmp/testfile" ]; then
  echo "File exists"
else
  echo "File does not exist"
fi

if-elif-else Statement

Handles multiple conditions.

cpu_load=$(uptime | awk -F 'load average:' '{ print $2 }' | cut -d, -f1)
if (( $(echo "$cpu_load < 1.0" | bc -l) )); then echo "CPU load is low" elif (( $(echo "$cpu_load >= 1.0 && $cpu_load < 2.0" | bc -l) )); then
  echo "CPU load is medium"
else
  echo "CPU load is high"
fi

case Statement

Matches a variable against multiple patterns. Cleaner for multiple values.

service="nginx"
case $service in
"nginx")
echo "Web server selected";;
"mysql")
echo "Database server selected";;
"redis")
echo "Cache server selected";;
*)
echo "Unknown service";;
esac

Real-World Examples (DevOps)

1. Check if a service is running

services=("nginx" "mysql" "docker")
for svc in "${services[@]}"; do
  if systemctl is-active --quiet $svc; then
    echo "$svc is running"
  else
    echo "$svc is not running"
  fi
done

2. Check disk usage

disk_usage=$(df / | tail -1 | awk '{print $5}' | sed 's/%//')
if [ $disk_usage -gt 80 ]; then
  echo "Warning: Disk usage is above 80%"
else
  echo "Disk usage is normal"
fi

3. Validate file before backup

backup_file="/tmp/data.tar.gz"
if [ -e "$backup_file" ] && [ -s "$backup_file" ]; then
  echo "Backup file exists and is not empty"
else
  echo "Backup file missing or empty"
fi