In this chapter, we will continue to look at flow control. In Chapter 28, we constructed some simple menus and built the logic used to act on a user’s selection. To do this, we used a series of if commands to identify which of the possible choices has been selected. This type of construct appears frequently in programs, so much so that many programming languages (including the shell) provide a flow control mechanism for multiple-choice decisions.
在這一章中，我們將繼續看一下程式的流程控制。在第28章中，我們建構了一些簡單的選單並建立了用來 應對各種使用者選擇的程式邏輯。為此，我們使用了一系列的 if 命令來識別哪一個可能的選項已經被選中。 這種型別的構造經常出現在程式中，出現頻率如此之多，以至於許多程式語言（包括 shell） 專門為多選決策提供了一種流程控制機制。
The bash multiple-choice compound command is called case. It has the following syntax:
Bash 的多選複合命令稱為 case。它的語法規則如下所示：
case word in
[pattern [| pattern]...) commands ;;]...
esac
If we look at the read-menu program from Chapter 28, we see the logic used to act on a user’s selection:
如果我們看一下第28章中的讀選單程式，我們就知道了用來應對一個使用者選項的邏輯流程：
#!/bin/bash
# read-menu: a menu driven system information program
clear
echo "
Please Select:
1. Display System Information
2. Display Disk Space
3. Display Home Space Utilization
0. Quit
"
read -p "Enter selection [0-3] > "
if [[ $REPLY =~ ^[0-3]$ ]]; then
if [[ $REPLY == 0 ]]; then
echo "Program terminated."
exit
fi
if [[ $REPLY == 1 ]]; then
echo "Hostname: $HOSTNAME"
uptime
exit
fi
if [[ $REPLY == 2 ]]; then
df -h
exit
fi
if [[ $REPLY == 3 ]]; then
if [[ $(id -u) -eq 0 ]]; then
echo "Home Space Utilization (All Users)"
du -sh /home/*
else
echo "Home Space Utilization ($USER)"
du -sh $HOME
fi
exit
fi
else
echo "Invalid entry." >&2
exit 1
fi
Using case, we can replace this logic with something simpler:
使用 case 語句，我們可以用更簡單的程式碼替換這種邏輯：
#!/bin/bash
# case-menu: a menu driven system information program
clear
echo "
Please Select:
1. Display System Information
2. Display Disk Space
3. Display Home Space Utilization
0. Quit
"
read -p "Enter selection [0-3] > "
case $REPLY in
0) echo "Program terminated."
exit
;;
1) echo "Hostname: $HOSTNAME"
uptime
;;
2) df -h
;;
3) if [[ $(id -u) -eq 0 ]]; then
echo "Home Space Utilization (All Users)"
du -sh /home/*
else
echo "Home Space Utilization ($USER)"
du -sh $HOME
fi
;;
*) echo "Invalid entry" >&2
exit 1
;;
esac
The case command looks at the value of word, in our example, the value of the REPLY variable, and then attempts to match it against one of the specified patterns. When a match is found, the commands associated with the specified pattern are executed. After a match is found, no further matches are attempted.
case 命令檢查一個變數值，在我們這個例子中，就是 REPLY 變數的變數值，然後試圖去匹配其中一個具體的模式。 當與之相匹配的模式找到之後，就會執行與該模式相關聯的命令。若找到一個模式之後，就不會再繼續尋找。
The patterns used by case are the same as those used by pathname expansion. Patterns are terminated with a “)” character. Here are some valid patterns:
這裡 case 語句使用的模式和路徑展開中使用的那些是一樣的。模式以一個「)」為終止符。這裡是一些有效的模式。
| Pattern | Description |
|---|---|
| a) | Matches if word equals "a". |
| [[:alpha:]]) | Matches if word is a single alphabetic character. |
| ???) | Matches if word is exactly three characters long. |
| *.txt) | Matches if word ends with the characters “.txt”. |
| *) | Matches any value of word. It is good practice to include this as the last pattern in a case command, to catch any values of word that did not match a previous pattern; that is, to catch any possible invalid values. |
| 模式 | 描述 |
|---|---|
| a) | 若單詞為 “a”，則匹配 |
| [[:alpha:]]) | 若單詞是一個字母字元，則匹配 |
| ???) | 若單詞只有3個字元，則匹配 |
| *.txt) | 若單詞以 “.txt” 字元結尾，則匹配 |
| *) | 匹配任意單詞。把這個模式做為 case 命令的最後一個模式，是一個很好的做法， 可以捕捉到任意一個與先前模式不匹配的數值；也就是說，捕捉到任何可能的無效值。 |
Here is an example of patterns at work:
這裡是一個模式使用範例：
#!/bin/bash
read -p "enter word > "
case $REPLY in
[[:alpha:]]) echo "is a single alphabetic character." ;;
[ABC][0-9]) echo "is A, B, or C followed by a digit." ;;
???) echo "is three characters long." ;;
*.txt) echo "is a word ending in '.txt'" ;;
*) echo "is something else." ;;
esac
It is also possible to combine multiple patterns using the vertical bar character as a separator. This creates an “or” conditional pattern. This is useful for such things as handling both upper- and lowercase characters. For example:
還可以使用豎線字元作為分隔符，把多個模式結合起來。這就建立了一個「或」條件模式。這對於處理諸如大小寫字元很有用處。例如：
#!/bin/bash
# case-menu: a menu driven system information program
clear
echo "
Please Select:
A. Display System Information
B. Display Disk Space
C. Display Home Space Utilization
Q. Quit
"
read -p "Enter selection [A, B, C or Q] > "
case $REPLY in
q|Q) echo "Program terminated."
exit
;;
a|A) echo "Hostname: $HOSTNAME"
uptime
;;
b|B) df -h
;;
c|C) if [[ $(id -u) -eq 0 ]]; then
echo "Home Space Utilization (All Users)"
du -sh /home/*
else
echo "Home Space Utilization ($USER)"
du -sh $HOME
fi
;;
*) echo "Invalid entry" >&2
exit 1
;;
esac
Here, we modify the case-menu program to use letters instead of digits for menu selection. Notice how the new patterns allow for entry of both upper- and lowercase letters.
這裡，我們更改了 case-menu 程式的程式碼，用字母來代替數字做為選單選項。注意新模式如何使得大小寫字母都是有效的輸入選項。
In versions of bash prior to 4.0, case allowed only one action to be performed on a successful match. After a successful match, the command would terminate. Here we see a script that tests a character:
早於版本號4.0的 bash，case 語法只允許執行與一個成功匹配的模式相關聯的動作。 匹配成功之後，命令將會終止。這裡我們看一個測試一個字元的指令碼：
#!/bin/bash
# case4-1: test a character
read -n 1 -p "Type a character > "
echo
case $REPLY in
[[:upper:]]) echo "'$REPLY' is upper case." ;;
[[:lower:]]) echo "'$REPLY' is lower case." ;;
[[:alpha:]]) echo "'$REPLY' is alphabetic." ;;
[[:digit:]]) echo "'$REPLY' is a digit." ;;
[[:graph:]]) echo "'$REPLY' is a visible character." ;;
[[:punct:]]) echo "'$REPLY' is a punctuation symbol." ;;
[[:space:]]) echo "'$REPLY' is a whitespace character." ;;
[[:xdigit:]]) echo "'$REPLY' is a hexadecimal digit." ;;
esac
Running this script produces this:
執行這個指令碼，輸出這些內容：
[me@linuxbox ~]$ case4-1
Type a character > a
'a' is lower case.
The script works for the most part, but fails if a character matches more than one of the POSIX characters classes. For example, the character “a” is both lower case and alphabetic, as well as a hexadecimal digit. In bash prior to version 4.0 there was no way for case to match more than one test. Modern versions of bash, add the “;;&” notation to terminate each action, so now we can do this:
大多數情況下這個指令碼工作是正常的，但若輸入的字元不止與一個 POSIX 字符集匹配的話，這時指令碼就會出錯。 例如，字元 “a” 既是小寫字母，也是一個十六進位制的數字。早於4.0的 bash，對於 case 語法絕不能匹配 多個測試條件。現在的 bash 版本，新增 “;;&” 表示式來終止每個行動，所以現在我們可以做到這一點：
#!/bin/bash
# case4-2: test a character
read -n 1 -p "Type a character > "
echo
case $REPLY in
[[:upper:]]) echo "'$REPLY' is upper case." ;;&
[[:lower:]]) echo "'$REPLY' is lower case." ;;&
[[:alpha:]]) echo "'$REPLY' is alphabetic." ;;&
[[:digit:]]) echo "'$REPLY' is a digit." ;;&
[[:graph:]]) echo "'$REPLY' is a visible character." ;;&
[[:punct:]]) echo "'$REPLY' is a punctuation symbol." ;;&
[[:space:]]) echo "'$REPLY' is a whitespace character." ;;&
[[:xdigit:]]) echo "'$REPLY' is a hexadecimal digit." ;;&
esac
When we run this script, we get this:
當我們執行這個指令碼的時候，我們得到這些：
[me@linuxbox ~]$ case4-2
Type a character > a
'a' is lower case.
'a' is alphabetic.
'a' is a visible character.
'a' is a hexadecimal digit.
The addition of the “;;&” syntax allows case to continue on to the next test rather than simply terminating.
新增的 “;;&” 的語法允許 case 語句繼續執行下一條測試，而不是簡單地終止執行。
The case command is a handy addition to our bag of programming tricks. As we will see in the next chapter, it’s the perfect tool for handling certain types of problems.
case 命令是我們程式設計技巧口袋中的一個便捷工具。在下一章中我們將看到， 對於處理某些型別的問題來說，case 命令是一個完美的工具。
The Bash Reference Manual section on Conditional Constructs describes the case command in detail:
Bash 參考手冊的條件構造一節詳盡的介紹了 case 命令：
The Advanced Bash-Scripting Guide provides further examples of case applications:
高階 Bash 指令碼指南提供了更深一層的 case 應用範例：