Bash 脚本高级指南
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5.1. 变量引用

当引用变量时，通常建议将其名称用双引号括起来。这可以防止对引号字符串中所有特殊字符的重新解释 -- 除了 $、` (反引号) 和 \ (转义符)。 [1] 在双引号内保留 $ 作为特殊字符允许引用带引号的变量 ("$variable")，也就是说，将变量替换为其值 (参见上面的示例 4-1)。

使用双引号来防止单词分割。[2] 即使参数包含空白分隔符，用双引号括起来的参数也会被视为一个单词。

List="one two three"

for a in $List     # Splits the variable in parts at whitespace.
do
  echo "$a"
done
# one
# two
# three

echo "---"

for a in "$List"   # Preserves whitespace in a single variable.
do #     ^     ^
  echo "$a"
done
# one two three

一个更详细的例子

variable1="a variable containing five words"
COMMAND This is $variable1    # Executes COMMAND with 7 arguments:
# "This" "is" "a" "variable" "containing" "five" "words"

COMMAND "This is $variable1"  # Executes COMMAND with 1 argument:
# "This is a variable containing five words"


variable2=""    # Empty.

COMMAND $variable2 $variable2 $variable2
                # Executes COMMAND with no arguments. 
COMMAND "$variable2" "$variable2" "$variable2"
                # Executes COMMAND with 3 empty arguments. 
COMMAND "$variable2 $variable2 $variable2"
                # Executes COMMAND with 1 argument (2 spaces). 

# Thanks, St�phane Chazelas.

只有当单词分割或保留空白成为问题时，才需要将 echo 语句的参数用双引号括起来。

示例 5-1. 回显奇怪的变量

#!/bin/bash
# weirdvars.sh: Echoing weird variables.

echo

var="'(]\\{}\$\""
echo $var        # '(]\{}$"
echo "$var"      # '(]\{}$"     Doesn't make a difference.

echo

IFS='\'
echo $var        # '(] {}$"     \ converted to space. Why?
echo "$var"      # '(]\{}$"

# Examples above supplied by Stephane Chazelas.

echo

var2="\\\\\""
echo $var2       #   "
echo "$var2"     # \\"
echo
# But ... var2="\\\\"" is illegal. Why?
var3='\\\\'
echo "$var3"     # \\\\
# Strong quoting works, though.


# ************************************************************ #
# As the first example above shows, nesting quotes is permitted.

echo "$(echo '"')"           # "
#    ^           ^


# At times this comes in useful.

var1="Two bits"
echo "\$var1 = "$var1""      # $var1 = Two bits
#    ^                ^

# Or, as Chris Hiestand points out ...

if [[ "$(du "$My_File1")" -gt "$(du "$My_File2")" ]]
#     ^     ^         ^ ^     ^     ^         ^ ^
then
  ...
fi
# ************************************************************ #

单引号 (' ') 的操作类似于双引号，但不允许引用变量，因为 $ 的特殊含义被关闭了。在单引号内，除了 ' 之外的每个特殊字符都会被字面解释。可以将单引号 ("完全引用") 视为比双引号 ("部分引用") 更严格的引用方法。

由于即使是转义字符 (\) 在单引号内也会被字面解释，因此尝试在单引号内包含单引号将不会产生预期的结果。

echo "Why can't I write 's between single quotes"

echo

# The roundabout method.
echo 'Why can'\''t I write '"'"'s between single quotes'
#    |-------|  |----------|   |-----------------------|
# Three single-quoted strings, with escaped and quoted single quotes between.

# This example courtesy of St�phane Chazelas.

5.1. 变量引用

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