什么是IFS =做这个庆典循环：`猫文件|而IFS =读-r线; DO ... done` [英] What does IFS= do in this bash loop: `cat file | while IFS= read -r line; do ... done`

问题描述

我学习bash和我看到这种结构：

I'm learning bash and I saw this construction:

cat file | while IFS= read -r line;
do
    ...
done

谁能解释一下 IFS = 吗？我知道这是输入字段sepereter，但它被设置成什么？

Can anyone explain what IFS= do? I know it's input field sepereter, but it's being set to nothing?

推荐答案

IFS 做很多事情，但你问的那个特定的循环。

IFS does many things but you are asking about that particular loop.

在该循环的作用是的 preserve开头和结尾的空格的在行。为了说明这一点，先用IFS设置为nothing观察：

The effect in that loop is to preserve leading and trailing white space in line. To illustrate, first observe with IFS set to nothing:

$ echo " this   is a test " | while IFS= read -r line; do echo "=$line=" ; done
= this   is a test =

的行变量包含了所有它在它的标准输入收到的空白。现在，考虑使用默认IFS相同的语句：

The line variable contains all the white space it received on its stdin. Now, consider the same statement with the default IFS:

$ echo " this   is a test " | while read -r line; do echo "=$line=" ; done
=this   is a test=

在这个版本中，空白内部的线路仍然是preserved。但是，领先的和尾随空白已被删除。

In this version, the white space internal to the line is still preserved. But, the leading and trailing white space have been removed.

的 -r 选项prevents 读从治疗反斜杠作为特殊字符。

The -r option prevents read from treating backslash as a special character.

要说明这一点，我们使用提供两行到，而回路内回声命令。观察与 -r 会发生什么：

To illustrate, we use two echo commands that supply two lines to the while loop. Observe what happens with -r:

$ { echo 'this \\ line is \' ; echo 'continued'; } | while IFS= read -r line; do echo "=$line=" ; done
=this \\ line is \=
=continued=

现在，没有观察到 -r 会发生什么：

Now, observe what happens without -r:

$ { echo 'this \\ line is \' ; echo 'continued'; } | while IFS= read line; do echo "=$line=" ; done
=this \ line is continued=

没有 -r ，两个变化发生了。首先，双反斜线被转换为一个反斜杠。其次，在第一行的末尾反斜杠是PTED作为续行字符间$ P $和两行合并成一个。

Without -r, two changes happened. First, the double-backslash was converted to a single backslash. Second, the backslash on the end of the first line was interpreted as a line-continuation character and the two lines were merged into one.

总之，如果你想在输入反斜杠有特殊的意义，不使用 -r 。如果你想在输入反斜杠被当作普通的字符，然后用 -r 。

In sum, if you want backslashes in the input to have special meaning, don't use -r. If you want backslashes in the input to be taken as plain characters, then use -r.

由于读需要输入一行的时间，IFS行为影响以相同的方式，它影响单行输入多行输入的每一行。 -r 行为异常与同样，如果没有 -r ，多条线路可以使用尾随合并成一条线如上图所示反斜杠。

Since read takes input one line at a time, IFS behaves affects each line of multiple line input in the same way that it affects single line input. -r behaves similarly with the exception that, without -r, multiple lines can be combined into one line using the trailing backslash as shown above.

与多个线路输入的行为，但是，可以大大利用读的 -d 标志改变。 -d 更改分隔符的读用来标记输入行的末尾。例如，我们可以终止与制表符行：

The behavior with multiple line input, however, can be changed drastically using read's -d flag. -d changes the delimiter character that read uses to mark the end of an input line. For example, we can terminate lines with a tab character:

$ echo $'line one \n line\t two \n line three\t ends here'
line one 
 line    two 
 line three      ends here
$ echo $'line one \n line\t two \n line three\t ends here' | while IFS= read -r -d$'\t' line; do echo "=$line=" ; done
=line one 
 line=
= two 
 line three=

在这里， $...构造用来输入特殊字符，如换行， \\ n 和标签， \\ t 。观察与 -d $'\\ t'，读把它投入的基础上制表符的线。最后一个选项卡后，都被忽略。

Here, the $'...' construct was used to enter special characters like newline, \n and tab, \t. Observe that with -d$'\t', read divides its input into "lines" based on tab characters. Anything after the final tab is ignored.

的上述特征的最重要的用途是处理困难的文件名。因为这不能出现在路径/文件名的一个字符是一个空字符，空字符可用于分隔的文件名的列表。作为一个例子：

The most important use of the features described above is to process difficult file names. Since the one character that cannot appear in path/filenames is the null character, the null character can be used to separate a list of file names. As an example:

while IFS= read -r -d $'\0' file
do
    # do something to each file
done < <(find ~/music -type f -print0)

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