目錄

流程控制：for 迴圈


In this final chapter on flow control, we will look at another of the shell’s looping constructs. The for loop differs from the while and until loops in that it provides a means of processing sequences during a loop. This turns out to be very useful when programming. Accordingly, the for loop is a very popular construct in bash scripting.

在這關於流程控制的最後一章中，我們將看看另一種 shell 迴圈構造。for 迴圈不同於 while 和 until 迴圈，因為 在迴圈中，它提供了一種處理序列的方式。這在程式設計時非常有用。因此在 bash 指令碼中，for 迴圈是非常流行的構造。

A for loop is implemented, naturally enough, with the for command. In modern versions of bash, for is available in two forms.

實現一個 for 迴圈，很自然的，要用 for 命令。在現代版的 bash 中，有兩種可用的 for 迴圈格式。

for: 傳統 shell 格式

The original for command’s syntax is:

for 命令語法是：

for variable [in words]; do
    commands
done

Where variable is the name of a variable that will increment during the execution of the loop, words is an optional list of items that will be sequentially assigned to variable, and commands are the commands that are to be executed on each iteration of the loop.

這裡的 variable 是一個變數的名字，這個變數在迴圈執行期間會增加，words 是一個可選的條目列表， 其值會按順序賦值給 variable，commands 是在每次迴圈迭代中要執行的命令。

The for command is useful on the command line. We can easily demonstrate how it works:

在命令列中 for 命令是很有用的。我們可以很容易的說明它是如何工作的：

[me@linuxbox ~]$ for i in A B C D; do echo $i; done
A
B
C
D

In this example, for is given a list of four words: “A”, “B”, “C”, and “D”. With a list of four words, the loop is executed four times. Each time the loop is executed, a word is as- signed to the variable i. Inside the loop, we have an echo command that displays the value of i to show the assignment. As with the while and until loops, the done keyword closes the loop.

在這個例子中，for 迴圈有一個四個單詞的列表：“A”、“B”、“C”和 “D”。由於這四個單詞的列表，for 迴圈會執行四次。 每次迴圈執行的時候，就會有一個單詞賦值給變數 i。在迴圈體內，我們有一個 echo 命令會顯示 i 變數的值，來示範賦值結果。 正如 while 和 until 迴圈，done 關鍵字會關閉迴圈。

The really powerful feature of for is the number of interesting ways we can create the list of words. For example, through brace expansion:

for 命令真正強大的功能是我們可以透過許多有趣的方式建立 words 列表。例如，透過花括號展開：

[me@linuxbox ~]$ for i in {A..D}; do echo $i; done
A
B
C
D

or pathname expansion:

或者路徑名展開：

[me@linuxbox ~]$ for i in distros*.txt; do echo $i; done
distros-by-date.txt
distros-dates.txt
distros-key-names.txt
distros-key-vernums.txt
distros-names.txt
distros.txt
distros-vernums.txt
distros-versions.txt

or command substitution:

或者命令替換：

#!/bin/bash
# longest-word : find longest string in a file
while [[ -n $1 ]]; do
    if [[ -r $1 ]]; then
        max_word=
        max_len=0
        for i in $(strings $1); do
            len=$(echo $i | wc -c)
            if (( len > max_len )); then
                max_len=$len
                max_word=$i
            fi
        done
        echo "$1: '$max_word' ($max_len characters)"
    fi
    shift
done

In this example, we look for the longest string found within a file. When given one or more filenames on the command line, this program uses the strings program (which is included in the GNU binutils package) to generate a list of readable text “words” in each file. The for loop processes each word in turn and determines if the current word is the longest found so far. When the loop concludes, the longest word is displayed.

在這個示例中，我們要在一個檔案中查詢最長的字串。當在命令列中給出一個或多個檔名的時候， 該程式會使用 strings 程式（其包含在 GNU binutils 包中），為每一個檔案產生一個可讀的文字格式的 “words” 列表。 然後這個 for 迴圈依次處理每個單詞，判斷當前這個單詞是否為目前為止找到的最長的一個。當迴圈結束的時候，顯示出最長的單詞。

If the optional in words portion of the for command is omitted, for defaults to pro- cessing the positional parameters. We will modify our longest-word script to use this method:

如果省略掉 for 命令的可選項 words 部分，for 命令會預設處理位置引數。 我們將修改 longest-word 指令碼，來使用這種方式：

#!/bin/bash
# longest-word2 : find longest string in a file
for i; do
    if [[ -r $i ]]; then
        max_word=
        max_len=0
        for j in $(strings $i); do
            len=$(echo $j | wc -c)
            if (( len > max_len )); then
                max_len=$len
                max_word=$j
            fi
        done
        echo "$i: '$max_word' ($max_len characters)"
    fi
done

As we can see, we have changed the outermost loop to use for in place of while. By omitting the list of words in the for command, the positional parameters are used instead. Inside the loop, previous instances of the variable i have been changed to the variable j. The use of shift has also been eliminated.

正如我們所看到的，我們已經更改了最外圍的迴圈，用 for 迴圈來代替 while 迴圈。透過省略 for 命令的 words 列表， 用位置引數替而代之。在迴圈體內，之前的變數 i 已經改為變數 j。同時 shift 命令也被淘汰掉了。

Why i?

為什麼是 i？

You may have noticed that the variable i was chosen for each of the for loop examples above. Why? No specific reason actually, besides tradition. The variable used with for can be any valid variable, but i is the most common, followed by j and k.

你可能已經注意到上面所列舉的 for 迴圈的範例都選擇 i 作為變數。為什麼呢？ 實際上沒有具體原因，除了傳統習慣。 for 迴圈使用的變數可以是任意有效的變數，但是 i 是最常用的一個，其次是 j 和 k。

The basis of this tradition comes from the Fortran programming language. In For- tran, undeclared variables starting with the letters I, J, K, L, and M are automati- cally typed as integers, while variables beginning with any other letter are typed as real (numbers with decimal fractions). This behavior led programmers to use the variables I, J, and K for loop variables, since it was less work to use them when a temporary variable (as loop variables often are) was needed. It also led to the following Fortran-based witticism:

“GOD is real, unless declared integer.”

這一傳統的基礎源於 Fortran 程式語言。在 Fortran 語言中，以字母 I、J、K、L 和 M 開頭的未宣告變數的型別 自動設為整形，而以其它字母開頭的變數則為實數型別（帶有小數的數字）。這種行為導致程式設計師使用變數 I、J和 K 作為迴圈變數， 因為當需要一個臨時變數（正如迴圈變數）的時候，使用它們工作量比較少。這也引出瞭如下基於 Fortran 的俏皮話：

“神是實數，除非是宣告的整數。”

for: C 語言格式

Recent versions of bash have added a second form of for command syntax, one that resembles the form found in the C programming language. Many other languages support this form, as well:

最新版本的 bash 已經添加了第二種格式的 for 命令語法，該語法相似於 C 語言中的 for 語法格式。 其它許多程式語言也支援這種格式：

for (( expression1; expression2; expression3 )); do
    commands
done

where expression1, expression2, and expression3 are arithmetic expressions and com- mands are the commands to be performed during each iteration of the loop. In terms of behavior, this form is equivalent to the following construct:

這裡的 expression1、expression2和 expression3 都是算術表示式，commands 是每次迴圈迭代時要執行的命令。 在行為方面，這相當於以下構造形式：

(( expression1 ))
while (( expression2 )); do
    commands
    (( expression3 ))
done

expression1 is used to initialize conditions for the loop, expression2 is used to determine when the loop is finished, and expression3 is carried out at the end of each iteration of the loop.

expression1 用來初始化迴圈條件，expression2 用來決定迴圈結束的時間，還有在每次迴圈迭代的末尾會執行 expression3。

Here is a typical application:

這裡是一個典型應用：

#!/bin/bash
# simple_counter : demo of C style for command
for (( i=0; i<5; i=i+1 )); do
    echo $i
done

When executed, it produces the following output:

指令碼執行之後，產生如下輸出：

[me@linuxbox ~]$ simple_counter
0
1
2
3
4

In this example, expression1 initializes the variable i with the value of zero, expression2 allows the loop to continue as long as the value of i remains less than 5, and expression3 increments the value of i by one each time the loop repeats.

在這個示例中，expression1 初始化變數 i 的值為0，expression2 允許迴圈繼續執行只要變數 i 的值小於5， 還有每次迴圈迭代時，expression3 會把變數 i 的值加1。

The C language form of for is useful anytime a numeric sequence is needed. We will see several applications for this in the next two chapters.

C 語言格式的 for 迴圈對於需要一個數字序列的情況是很有用處的。我們將在接下來的兩章中看到幾個這樣的應用範例。

總結

With our knowledge of the for command, we will now apply the final improvements to our sys_info_page script. Currently, the report_home_space function looks like this:

學習了 for 命令的知識，現在我們將對我們的 sys_info_page 指令碼做最後的改進。 目前，這個 report_home_space 函式看起來像這樣：

report_home_space () {
    if [[ $(id -u) -eq 0 ]]; then
        cat <<- _EOF_
        <H2>Home Space Utilization (All Users)</H2>
        <PRE>$(du -sh /home/*)</PRE>
        _EOF_
    else
        cat <<- _EOF_
        <H2>Home Space Utilization ($USER)</H2>
        <PRE>$(du -sh $HOME)</PRE>
        _EOF_
    fi
    return
}

Next, we will rewrite it to provide more detail for each user’s home directory, and include the total number of files and subdirectories in each:

下一步，我們將重寫它，以便提供每個使用者家目錄的更詳盡資訊，並且包含使用者家目錄中檔案和目錄的總個數：

report_home_space () {
    local format="%8s%10s%10s\n"
    local i dir_list total_files total_dirs total_size user_name
    if [[ $(id -u) -eq 0 ]]; then
        dir_list=/home/*
        user_name="All Users"
    else
        dir_list=$HOME
        user_name=$USER
    fi
    echo "<H2>Home Space Utilization ($user_name)</H2>"
    for i in $dir_list; do
        total_files=$(find $i -type f | wc -l)
        total_dirs=$(find $i -type d | wc -l)
        total_size=$(du -sh $i | cut -f 1)
        echo "<H3>$i</H3>"
        echo "<PRE>"
        printf "$format" "Dirs" "Files" "Size"
        printf "$format" "----" "-----" "----"
        printf "$format" $total_dirs $total_files $total_size
        echo "</PRE>"
    done
    return
}

This rewrite applies much of what we have learned so far. We still test for the superuser, but instead of performing the complete set of actions as part of the if, we set some vari- ables used later in a for loop. We have added several local variables to the function and made use of printf to format some of the output.

這次重寫應用了目前為止我們學過的許多知識。我們仍然測試超級使用者（superuser），但是我們在 if 語句塊內 設定了一些隨後會在 for 迴圈中用到的變數，來取代在 if 語句塊內執行完備的動作集合。我們給 函式添加了幾個本地變數，並且使用 printf 來格式化輸出。

拓展閱讀


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