Bash Conditional Expressions and Operator Patterns
Extended Pattern Matching
Using double brackets to validate input falls outside a specific range:
[[ ! $score =~ ^[1-3]$ ]] && {
echo "Value must be between 1 and 3"
exit 1
}
Example 1: Single Character Validation
Accept a single digit input and output the corresponding value, displaying an error for invalid entries.
Script: /opt/tools/checker.sh
#!/bin/bash
echo -n "Enter a digit: "
read input
[ "$input" = "7" ] && {
echo "Seven"
exit 0
}
[ "$input" -eq 9 ] && {
echo "Nine"
exit 0
}
[ "$input" != "7" -a "$input" -ne 9 ] && {
echo "Invalid input"
exit 0
}
Execution results:
$ bash /opt/tools/checker.sh
Enter a digit: 7
Seven
$ bash /opt/tools/checker.sh
Enter a digit: 9
Nine
$ bash /opt/tools/checker.sh
Enter a digit: 4
Invalid input
Example 2: Comparing Two Integer Values
Read two integers from the user and display their relationship without using if statements. The script validates numeric input and checks parameter count.
Script: /opt/tools/compare.sh
#!/bin/bash
read -t 15 -p "Enter two integers: " x y
[ -z "$x" ] || [ -z "$y" ] && {
echo "Error: provide both numbers"
exit 1
}
[[ ! $x =~ ^[0-9]+$ ]] && {
echo "Error: first value not numeric"
exit 2
}
[[ ! $y =~ ^[0-9]+$ ]] && {
echo "Error: second value not numeric"
exit 3
}
[ $x -eq $y ] && {
echo "x equals y"
exit 0
}
[ $x -gt $y ] && echo "x is greater than y" || echo "x is less than y"
Example 3: Web Stack Installation Menu
Display a selection menu for various web service installations and execute the corresponding setup script.
Script: /opt/tools/install.sh
#!/bin/bash
base=/opt/tools
[ ! -d "$base" ] && mkdir -p "$base"
cat <<MENU
1. Deploy Apache + MySQL + PHP
2. Deploy Nginx + MySQL + PHP
3. Quit
Select an option:
MENU
read choice
expr "$choice" + 0 >/dev/null 2>&1
[ $? -ne 0 ] && {
echo "Invalid selection: $choice"
exit 1
}
case $choice in
1)
echo "Installing LAMP stack..."
sleep 2
[ -f "$base/lamp_install.sh" ] && {
[ -x "$base/lamp_install.sh" ] || {
echo "LAMP script lacks execute permission"
exit 2
}
"$base/lamp_install.sh"
}
;;
2)
echo "Installing LNMP stack..."
sleep 2
[ -f "$base/lnmp_install.sh" ] && source "$base/lnmp_install.sh"
;;
3)
echo "Exiting installation"
exit 0
;;
*)
echo "Valid options: 1, 2, or 3"
exit 5
;;
esac
The script creates the target directory if absant, presents a menu interface, validates numeric input, and handles three scenarios: Apache stack enstallation, Nginx stack installation via sourcing, and graceful exit.
