Shell Script Conditional Structures: Using if and case Statements

Shell scripts are an essential skill for backend developers; this tutorial covers their conditional branching structures, focusing on if and case statements.

First, the basic script header is shown:

<code>#!/bin/bash</code>

A simple "hello world" script demonstrates execution:

<code>#!/bin/bash
echo 'hello world'</code>

The if statement in shell supports single‑branch, double‑branch, and multi‑branch forms. Examples of each syntax are provided:

<code># Single branch
if condition ; then
  ...
fi

# Double branch
if condition ; then
  ...
else
  ...
fi

# Multi‑branch
if condition ; then
  ...
elif other_condition ; then
  ...
else
  ...
fi</code>

An example grading script reads a score, validates the input, and uses an if single‑branch to check format, then a multi‑branch to output comments such as "优秀", "良好", "一般", "及格" or "不及格".

<code>read -p "请输入成绩，成绩范围0-100: " score
if [ -z `echo $score | egrep '^[0-9]+$'` ]; then
    echo "输入的成绩格式不正确"
fi

if ((score >= 90)); then
    echo '优秀'
elif ((score >= 80)); then
    echo '良好'
elif ((score >= 70)); then
    echo '一般'
elif ((score >= 60)); then
    echo '及格'
else
    echo '不及格'
fi</code>

The case statement provides another way to branch based on a variable's value. Its general form is shown:

<code>case $variable in
    "value1")
        # commands for value1
        ;;
    "value2")
        # commands for value2
        ;;
    *)
        # default commands
        ;;
esac</code>

A practical case example handles command‑line arguments start , stop , and restart , printing corresponding messages or a usage hint.

<code>#!/bin/bash
case $1 in
    "start")
        echo "this code is start"
        ;;
    "stop")
        echo "this code is stop"
        ;;
    "restart")
        echo "this code is restart"
        ;;
    *)
        echo "Usage ${0} {start|stop|restart}"
        ;;
esac</code>