Shell读取*有时*去除尾随定界符 [英] Shell read *sometimes* strips trailing delimiter
问题描述
要解析冒号分隔的字段，可以将read与自定义IFS一起使用:
To parse colon-delimited fields I can use read with a custom IFS:
$ echo 'foo.c:41:switch (color) {' | { IFS=: read file line text && echo "$file | $line | $text"; }
foo.c | 41 | switch (color) {
如果最后一个字段包含冒号，则没问题，冒号将保留.
If the last field contains colons, no problem, the colons are retained.
$ echo 'foo.c:42:case RED: //alert' | { IFS=: read file line text && echo "$file | $line | $text"; }
foo.c | 42 | case RED: //alert
尾部定界符也被保留...
A trailing delimiter is also retained...
$ echo 'foo.c:42:case RED: //alert:' | { IFS=: read file line text && echo "$file | $line | $text"; }
foo.c | 42 | case RED: //alert:
...除非它是 only 额外的定界符.然后将其剥离. 等等，
...Unless it's the only extra delimiter. Then it's stripped. Wait, what?
$ echo 'foo.c:42:case RED:' | { IFS=: read file line text && echo "$file | $line | $text"; }
foo.c | 42 | case RED
Bash，ksh93和破折号都可以做到这一点，所以我猜这是POSIX标准行为.
Bash, ksh93, and dash all do this, so I'm guessing it is POSIX standard behavior.
- 为什么会发生?
- 什么是最好的选择?
我想将上面的字符串解析为三个变量，并且我不想破坏第三字段中的任何文本.我以为read是应该走的路，但是现在我正在重新考虑.
I want to parse the strings above into three variables and I don't want to mangle any text in the third field. I had thought read was the way to go but now I'm reconsidering.
推荐答案
是的，这是标准行为(请参见 read规范和字段拆分).少数外壳程序(至少基于ash，包括dash，基于pdksh，zsh，yash)用于执行此操作，但用于zsh除外(当未处于POSIX模式时)， busybox sh，其中大多数已更新为符合POSIX.
Yes, that's standard behaviour (see the read specification and Field Splitting). A few shells (ash-based including dash, pdksh-based, zsh, yash at least) used not to do it, but except for zsh (when not in POSIX mode), busybox sh, most of them have been updated for POSIX compliance.
以下内容相同:
$ var='a:b:c:' IFS=:
$ set -- $var; echo "$#"
3
(请参阅read的POSIX规范实际上是如何遵循 Field Splitting 机制的，其中a:b:c:被分为3个字段，因此对于IFS=: read -r a b c，存在的字段数与变量).
(see how the POSIX specification for read actually defers to the Field Splitting mechanism where a:b:c: is split into 3 fields, and so with IFS=: read -r a b c, there are as many fields as variables).
基本原理是，在ksh(POSIX规范所基于的基础)中，$IFS(最初在Bourne Shell中，内部字段分隔符)成为了字段定界符，我认为因此可以表示任何元素列表(不包含定界符).
The rationale is that in ksh (on which the POSIX spec is based) $IFS (initially in the Bourne shell the internal field separator) became a field delimiter, I think so any list of elements (not containing the delimiter) could be represented.
当$IFS是分隔符时，不能代表一个空元素的列表(""分为0个元素的列表，":"分为两个元素的列表空元素¹).当它是定界符时，可以用""表示零元素的列表，或者用":"表示一个空元素的列表，或者用"::"表示两个空元素的列表.
When $IFS is a separator, one can't represent a list of one empty element ("" is split into a list of 0 element, ":" into a list of two empty elements¹). When it's a delimiter, you can express a list of zero element with "", or one empty element with ":", or two empty elements with "::".
不幸的是，$IFS的最常见用法之一是拆分$PATH.像/bin:/usr/bin:这样的$PATH应该被拆分为"/bin"，"/usr/bin"，""，而不仅仅是"/bin"和"/usr/bin".
It's a bit unfortunate as one of the most common usages of $IFS is to split $PATH. And a $PATH like /bin:/usr/bin: is meant to be split into "/bin", "/usr/bin", "", not just "/bin" and "/usr/bin".
现在，使用POSIX外壳程序(但不是所有外壳程序都兼容)，可以在参数扩展时进行单词拆分，可以解决以下问题:
Now, with POSIX shells (but not all shells are compliant in that regard), for word splitting upon parameter expansion, that can be worked around with:
IFS=:; set -o noglob
for dir in $PATH""; do
something with "${dir:-.}"
done
结尾的""确保如果$PATH以结尾的:结尾，则会添加一个额外的空元素.而且，空的$PATH应当被视为一个空元素.
That trailing "" makes sure that if $PATH ends in a trailing :, an extra empty element is added. And also that an empty $PATH is treated as one empty element as it should be.
但是，该方法不能用于read.
That approach can't be used for read though.
切换到zsh的快捷方式，除了插入一个额外的:并随后将其删除外，没有其他容易的解决方法，例如:
Short of switching to zsh, there's no easy work around other than inserting an extra : and remove it afterwards like:
echo a:b:c: | sed 's/:/::/2' | { IFS=: read -r x y z; z=${z#:}; echo "$z"; }
或(便携式程度较低):
Or (less portable):
echo a:b:c: | paste -d: - /dev/null | { IFS=: read -r x y z; z=${z%:}; echo "$z"; }
我还添加了使用read时通常需要的-r.
I've also added the -r which you generally want when using read.
最有可能在这里您想使用诸如sed/awk/perl之类的适当文本处理实用程序来代替编写在read 周围复杂且可能效率低下的代码，而并非为此设计的.
Most likely here you'd want to use a proper text processing utility like sed/awk/perl instead of writing convoluted and probably inefficient code around read which has not been designed for that.
¹尽管在Bourne外壳程序中，由于IFS空格字符和IFS非非空格字符之间没有区别，因此仍被分成零个元素，ksh也添加了一些内容
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