4.1. 变量替换
变量的名称是其值（它所保存的数据）的占位符。引用（检索）其值称为变量替换。
- $
让我们仔细区分变量的名称和它的值。如果variable1是变量的名称，那么$variable1是对它的值（它包含的数据项）的引用。[1]
bash$ variable1=23 bash$ echo variable1 variable1 bash$ echo $variable1 23
变量仅在“裸露”出现时——没有$前缀——才是在声明或赋值时，当取消设置时，当导出时，在双括号 (( ... ))内的算术表达式中，或者在代表信号的变量的特殊情况下（参见示例 32-5）。赋值可以使用=（如var1=27中所示），在 read 语句中，以及在循环的开头（for var2 in 1 2 3).
将引用的值用双引号 (" ... ") 括起来不会干扰变量替换。这被称为部分引用，有时被称为“弱引用”。使用单引号 (' ... ') 会导致变量名按字面意义使用，并且不会发生替换。这是完全引用，有时被称为“强引用”。有关详细讨论，请参见第 5 章。
请注意$variable实际上是${variable}的简化形式。在$variable语法导致错误的情况下，较长的形式可能会起作用（请参见下面的10.2 节）。
示例 4-1. 变量赋值和替换
#!/bin/bash # ex9.sh # Variables: assignment and substitution a=375 hello=$a # ^ ^ #------------------------------------------------------------------------- # No space permitted on either side of = sign when initializing variables. # What happens if there is a space? # "VARIABLE =value" # ^ #% Script tries to run "VARIABLE" command with one argument, "=value". # "VARIABLE= value" # ^ #% Script tries to run "value" command with #+ the environmental variable "VARIABLE" set to "". #------------------------------------------------------------------------- echo hello # hello # Not a variable reference, just the string "hello" ... echo $hello # 375 # ^ This *is* a variable reference. echo ${hello} # 375 # Likewise a variable reference, as above. # Quoting . . . echo "$hello" # 375 echo "${hello}" # 375 echo hello="A B C D" echo $hello # A B C D echo "$hello" # A B C D # As we see, echo $hello and echo "$hello" give different results. # ======================================= # Quoting a variable preserves whitespace. # ======================================= echo echo '$hello' # $hello # ^ ^ # Variable referencing disabled (escaped) by single quotes, #+ which causes the "$" to be interpreted literally. # Notice the effect of different types of quoting. hello= # Setting it to a null value. echo "\$hello (null value) = $hello" # $hello (null value) = # Note that setting a variable to a null value is not the same as #+ unsetting it, although the end result is the same (see below). # -------------------------------------------------------------- # It is permissible to set multiple variables on the same line, #+ if separated by white space. # Caution, this may reduce legibility, and may not be portable. var1=21 var2=22 var3=$V3 echo echo "var1=$var1 var2=$var2 var3=$var3" # May cause problems with legacy versions of "sh" . . . # -------------------------------------------------------------- echo; echo numbers="one two three" # ^ ^ other_numbers="1 2 3" # ^ ^ # If there is whitespace embedded within a variable, #+ then quotes are necessary. # other_numbers=1 2 3 # Gives an error message. echo "numbers = $numbers" echo "other_numbers = $other_numbers" # other_numbers = 1 2 3 # Escaping the whitespace also works. mixed_bag=2\ ---\ Whatever # ^ ^ Space after escape (\). echo "$mixed_bag" # 2 --- Whatever echo; echo echo "uninitialized_variable = $uninitialized_variable" # Uninitialized variable has null value (no value at all!). uninitialized_variable= # Declaring, but not initializing it -- #+ same as setting it to a null value, as above. echo "uninitialized_variable = $uninitialized_variable" # It still has a null value. uninitialized_variable=23 # Set it. unset uninitialized_variable # Unset it. echo "uninitialized_variable = $uninitialized_variable" # uninitialized_variable = # It still has a null value. echo exit 0
未初始化的变量具有“空”值——根本没有赋值（不是零！）。
if [ -z "$unassigned" ] then echo "\$unassigned is NULL." fi # $unassigned is NULL.
在为变量赋值之前使用变量可能会导致问题。然而，对未初始化的变量执行算术运算是可能的。
另请参见示例 15-23。echo "$uninitialized" # (blank line) let "uninitialized += 5" # Add 5 to it. echo "$uninitialized" # 5 # Conclusion: # An uninitialized variable has no value, #+ however it evaluates as 0 in an arithmetic operation.