Unix / Linux - Shell Decision Making

안녕하세요, 열망하는 프로그래머 여러분! 오늘 우리는 Unix와 Linux 셸 스크립팅에서 결정을 내리는 흥미로운 세상으로 뛰어들어 보겠습니다. 여러분의 친절한 이웃 컴퓨터 선생님으로서 저는 이 여정을 단계별로 안내해 드리겠습니다. 그러니 마음에 드는 음료를 한 잔 챙겨, 편안하게 자리 잡고 함께 이 모험을 떠나보겠습니다!

if...else 문

imaginе you're at a crossroads in your code, and you need to make a decision based on certain conditions. That's where the if...else statements come in handy. They're like the traffic lights of programming, guiding your code down different paths based on whether conditions are true or false.

기본 if 문

let's start with the simplest form of the if statement:

if [ condition ]
then
command1
command2
...
commandN
fi

Here's a real-world example:

#!/bin/bash
age=18

if [ $age -ge 18 ]
then
echo "You are old enough to vote!"
fi

In this example, we're checking if the age variable is greater than or equal to 18. If it is, the message "You are old enough to vote!" will be displayed.

if...else 문

Now, what if we want to do something when the condition is false? That's where the else clause comes in:

if [ condition ]
then
command1
command2
...
commandN
else
alternative_command1
alternative_command2
...
alternative_commandN
fi

Let's modify our voting example:

#!/bin/bash
age=16

if [ $age -ge 18 ]
then
echo "You are old enough to vote!"
else
echo "Sorry, you're too young to vote."
fi

Now, if the age is less than 18, it will display the message "Sorry, you're too young to vote."

if...elif...else 문

Sometimes, life (and code) isn't just black and white. We often need to check multiple conditions. That's where the elif (else if) clause comes in:

if [ condition1 ]
then
command1
elif [ condition2 ]
then
command2
else
alternative_command
fi

Let's create a more complex example:

#!/bin/bash
grade=75

if [ $grade -ge 90 ]
then
echo "A - Excellent!"
elif [ $grade -ge 80 ]
then
echo "B - Good job!"
elif [ $grade -ge 70 ]
then
echo "C - You passed."
else
echo "F - You need to study more."
fi

This script evaluates a student's grade and provides appropriate feedback based on different grade ranges.

The case...esac Statement

Now, imagine you're at a buffet with multiple food options. That's what the case...esac statement is like in programming. It allows you to match a value against a list of patterns and execute commands based on the match.

The basic syntax looks like this:

case expression in
pattern1)
command1
command2
...
commandN
;;
pattern2)
command1
command2
...
commandN
;;
*)
default_command
;;
esac

Let's look at a practical example:

#!/bin/bash
fruit="apple"

case $fruit in
"apple")
echo "It's a delicious red fruit!"
;;
"banana")
echo "It's a yellow curved fruit!"
;;
"orange")
echo "It's a juicy citrus fruit!"
;;
*)
echo "I don't know that fruit."
;;
esac

In this example, we're matching the value of fruit against different patterns. If it matches "apple", it will echo the corresponding message. If it doesn't match any of the specified patterns, it will execute the default case (*).

Multiple patterns

You can also match multiple patterns for the same set of commands:

#!/bin/bash
day=$(date +%A)

case $day in
Monday|Tuesday|Wednesday|Thursday|Friday)
echo "It's a weekday. Time to work!"
;;
Saturday|Sunday)
echo "It's the weekend. Time to relax!"
;;
*)
echo "What planet are you on?"
;;
esac

This script checks the current day of the week and outputs whether it's a weekday or weekend.

Comparison of if...else and case...esac

Now, let's compare these two decision-making structures:

Feature	if...else	case...esac
Complexity	Can handle complex conditions	Best for simple value matching
Readability	Can become hard to read with many conditions	More readable with multiple conditions
Performance	Slightly slower for many conditions	Faster for many conditions
Flexibility	Can use any type of condition	Limited to pattern matching

In my years of teaching, I've found that students often struggle with choosing between if...else and case...esac. Here's a simple rule of thumb: if you're checking a single variable against multiple possible values, case...esac is your friend. For everything else, stick with if...else.

Remember, programming is like cooking. These decision-making structures are your ingredients, and with practice, you'll learn when to use each one to create the perfect dish... I mean, script!

In conclusion, decision making is a crucial aspect of shell scripting. It allows your scripts to adapt and respond to different situations, making them more powerful and flexible. Whether you're using if...else or case...esac, you're now equipped with the tools to make your scripts smarter. Keep practicing, and soon you'll be writing scripts that can make decisions faster than you can say "sudo"!

Credits: Image by storyset