bash字符串前导美元符号的作用

problem

bash内置变量IFS作为内部单词分隔符，其默认值为<space><tab><newline>, 我想设置它仅为\n,于是：

OLD_IFS=$IFS
IFS='\n'
# do some work here
IFS=$OLD_IFS

但结果为：IFS把单独的字符当作了分隔符，即分隔符被设置成下划线和字母n 。

Why ?

Solution

通过google搜索，得知需要把\n转化成ANSI-C Quoting,

方法是把字符串放入$'string'中，即应该设置成:

IFS=$'\n'

顺便搜了下$字符的用途，在Unix & Linux,

中解释了字符串前面加$字符的两种形式，一种是单引号，一种是双引号，即

There are two different things going on here, both documented in the bash manual

$'

Dollar-sign single quote is a special form of quoting:

ANSI C Quoting

Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard.

$"

Dollar-sign double-quote is for localization:

Locale translation

A double-quoted string preceded by a dollar sign (‘$’) will cause the string to be translated according to the current locale. If the current locale is C or POSIX, the dollar sign is ignored.

If the string is translated and replaced, the replacement is double-quoted.

因此单引号表示转化成ANSI-C字符，双引号则表示将字符串本地化。

以下是一个实例，ping /etc/hosts的主机名为video-开头的主机名，检查网络状况!

#!/bin/bash
trap "echo 'interrupted!';exit 1" SIGHUP SIGINT SIGTERM
OLD_IFS=$IFS
IFS=$'\n'
for i in `awk '$0!~/^$/ && $0!~/^#/ && $2~/^video/ {print $1,$2}' /etc/hosts`
do
	ADDR=$(echo $i | cut -d' ' -f 1)
	DOMAIN=$(echo $i | cut -d' ' -f 2)
	if ping -c 2 $ADDR &>/dev/null
	then
		echo $DOMAIN ok!
	else
		echo $DOMIN dead!
	fi
done
IFS=$OLD_IFS