Home/ COMP2041/ Week 1/ Lab 03: Shell ScriptingLab 03: Shell脚本编程

Lab 03: Shell ScriptingLab 03: Shell脚本编程

POSIX Shell Scripts: Variables, Conditions, Loops, and APIsPOSIX Shell脚本：变量、条件、循环与API

Exercise 1: Balancing Numbers with tr练习1：使用tr平衡数字

Task任务

Convert digits using tr: 0-4 → <, 6-9 → >, 5 unchanged.

使用 tr 转换数字：0-4 → <, 6-9 → >, 5 保持不变。

The Solution解决方案

$ cat balancing_numbers.sh
#!/bin/dash
tr '01234' '<' | tr '6789' '>'

How it Works工作原理

Part部分 Meaning含义
tr '01234' '<' Map characters 0,1,2,3,4 to <将字符 0,1,2,3,4 映射为 <
| Pipe output to next command将输出通过管道传给下一个命令
tr '6789' '>' Map characters 6,7,8,9 to >将字符 6,7,8,9 映射为 >

Example示例

$ echo "1 234 5 678 9" | ./balancing_numbers.sh
< <<< 5 >>> >

💡 Key Insight: tr automatically repeats the last character in SET2 to match the length of SET1. So tr '01234' '<' works the same as tr '01234' '<<<<<'.

💡 关键洞察： tr 会自动重复 SET2 的最后一个字符来匹配 SET1 的长度。所以 tr '01234' '<' 等同于 tr '01234' '<<<<<'。

Exercise 2: Echo with Conditions (if/while)练习2：带条件的Echo（if/while）

Task任务

Write echon.sh that prints a string N times with proper error handling.

编写 echon.sh，打印字符串 N 次，并正确处理错误。

Key Concepts核心概念

1. Shebang (#!/bin/dash)1. Shebang (#!/bin/dash)

#!/bin/dash

Tells the system to use dash shell to execute this script. Must be the first line, no spaces before #.

告诉系统使用 dash shell 执行此脚本。必须是第一行，# 前不能有空格。

2. Special Variables2. 特殊变量

Variable变量 Meaning含义
$# Number of arguments参数个数
$1, $2, ... 1st, 2nd, ... argument第1个、第2个...参数
$@ All arguments所有参数
$? Exit status of last command上一个命令的退出状态

3. if Statement Syntax3. if 语句语法

$ # Basic if
if [ condition ]; then
# commands
fi
$ # if-elif-else
if [ condition ]; then
# commands
elif [ condition ]; then
# commands
else
# commands
fi

4. Test Operators ([ ])4. 测试运算符 ([ ])

Operator运算符 Meaning含义 Example示例
-eq Equal (numbers)等于（数字） [ $a -eq 5 ]
-ne Not equal (numbers)不等于（数字） [ $# -ne 2 ]
-lt Less than小于 [ $i -lt 10 ]
-gt Greater than大于 [ $1 -gt 0 ]
-f Is a regular file是普通文件 [ -f "$file" ]
-d Is a directory是目录 [ -d "$dir" ]

5. while Loop5. while 循环

i=0
while [ $i -lt $1 ]; do
echo "$2"
i=$((i + 1)) # Arithmetic: i = i + 1
done

6. case Statement6. case 语句

case "$1" in
*[!0-9]*)
echo "Not a number"
exit 1
;;
esac

*[!0-9]* matches if the string contains any non-digit character.

*[!0-9]* 匹配字符串中包含任何非数字字符的情况。

Complete Solution完整解决方案

$ cat echon.sh
#!/bin/dash
# Check argument count
if [ $# -ne 2 ]; then
echo "Usage: ./echon.sh "
exit 1
fi
# Check if first argument is a non-negative integer
case "$1" in
*[!0-9]*)
echo "./echon.sh: argument 1 must be a non-negative integer"
exit 1
;;
esac
# Loop and print
i=0
while [ $i -lt $1 ]; do
echo "$2"
i=$((i + 1))
done

⚠️ Common Mistakes:

⚠️ 常见错误：

  • [ ] must have spaces inside: [ $i -lt 10 ] not [$i -lt 10][ ] 内部必须有空格：[ $i -lt 10 ] 而不是 [$i -lt 10]
  • Variable assignment: i=0 not i = 0 (no spaces!)变量赋值：i=0 而不是 i = 0（不能有空格！）
  • Every if needs a fi, every case needs an esac每个 if 需要一个 fi，每个 case 需要一个 esac

Exercise 3: File Sizes (for/wc)练习3：文件大小（for/wc）

Task任务

Categorize files by line count: small (<10), medium (10-99), large (≥100).

按行数对文件分类：小文件（<10行）、中文件（10-99行）、大文件（≥100行）。

Key Concepts核心概念

1. for Loop1. for 循环

for file in *; do
echo "$file"
done

* is a wildcard that matches all files in the current directory.

* 是通配符，匹配当前目录下的所有文件。

2. wc -l (Count Lines)2. wc -l（计算行数）

$ wc -l < filename
5

< redirects file to stdin, so wc only outputs the count.

< 将文件重定向到标准输入，所以 wc 只输出计数。

3. Command Substitution $()3. 命令替换 $()

lines=$(wc -l < "$file")

Captures command output into a variable.

将命令输出捕获到变量中。

4. String Concatenation4. 字符串拼接

small="$small $file"

Append $file to the small variable.

将 $file 追加到 small 变量。

Complete Solution完整解决方案

$ cat file_sizes.sh
#!/bin/dash
small=""
medium=""
large=""
for file in *; do
if [ -f "$file" ]; then
lines=$(wc -l < "$file")
if [ $lines -lt 10 ]; then
small="$small $file"
elif [ $lines -lt 100 ]; then
medium="$medium $file"
else
large="$large $file"
fi
fi
done
echo "Small files:$small"
echo "Medium-sized files:$medium"
echo "Large files:$large"

💡 Why [ -f "$file" ]? The * wildcard matches both files AND directories. We use -f to check if it's a regular file before processing.

💡 为什么用 [ -f "$file" ]？ * 通配符匹配文件和目录。我们用 -f 在处理前检查它是否是普通文件。

Exercise 4: Scraping JSON APIs (curl/jq)练习4：抓取JSON API（curl/jq）

Task任务

Fetch course data from UNSW Handbook API and filter by prefix.

从 UNSW Handbook API 获取课程数据并按前缀过滤。

Key Concepts核心概念

1. curl - Fetch Data1. curl - 获取数据

$ curl -sL "https://api.example.com/data"
Option选项 Meaning含义
-s Silent mode (no progress bar)静默模式（无进度条）
-L Follow redirects跟随重定向

2. jq - Parse JSON2. jq - 解析JSON

$ # Basic structure
jq -r '.data.results[] | select(...) | "..."

3. jq Operators3. jq 运算符

Operator运算符 Meaning含义 Example示例
. Current object当前对象 .data
[] Iterate array遍历数组 .results[]
| Pipe (jq internal)管道（jq内部） .data | .results
select() Filter过滤 select(.type == "Course")
startswith() String starts with字符串以...开头 startswith("COMP")
map() Transform array elements转换数组元素 map(ascii_downcase)
any() Check if any matches检查是否有任何匹配 any(. == "x")

4. Injecting Shell Variables into jq4. 将Shell变量注入jq

# Use '"'"$var"'"' pattern inside jq string
jq -r '... | startswith("'"$2"'") | ...'

Complete Solution完整解决方案

$ cat scraping_courses.sh
#!/bin/dash
if [ $# -ne 2 ]; then
echo "Usage: ./scraping_courses.sh "
exit 1
fi
if ! echo "$1" | grep -qE '^[0-9]+$'; then
echo "./scraping_courses.sh: argument 1 must be an integer between 2019 and 2026"
exit 1
fi
if [ "$1" -lt 2019 ] || [ "$1" -gt 2026 ]; then
echo "./scraping_courses.sh: argument 1 must be an integer between 2019 and 2026"
exit 1
fi
url="https://handbook-proxy.cse.unsw.edu.au/current/api/search-all?searchType=advanced&siteId=unsw-prod-pres&query=&siteYear=$1"
curl -sL "$url" | jq -r '.data.results[] | select(.contentTypeLabel == "Course") | select(.lines | map(ascii_downcase) | any(. == "undergraduate" or . == "postgraduate")) | select(.code | startswith("'"$2"'")) | "\(.code) \(.title)"' | tr -s ' ' | sort | uniq

💡 Debugging Tips:

💡 调试技巧：

  • Run bash -n script.sh to check syntax运行 bash -n script.sh 检查语法
  • Add set -x at the top to trace execution在顶部添加 set -x 跟踪执行
  • Use echo "DEBUG: $var" to see variable values使用 echo "DEBUG: $var" 查看变量值
  • Test API URL in browser first to see JSON structure先在浏览器中测试 API URL 查看 JSON 结构

Summary: Key Patterns总结：关键模式

Pattern模式 When to Use何时使用 Example示例
if [ ] True/false conditions真/假条件 if [ $x -gt 5 ]
case Multiple value matching多值匹配 case "$1" in ... esac
while Loop until condition false循环直到条件为假 while [ $i -lt $n ]
for Iterate over items遍历项目 for file in *
$() Command substitution命令替换 lines=$(wc -l < file)
$((expr)) Arithmetic算术运算 i=$((i + 1))
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