shell 变量替换

转自：http://blog.csdn.net/xuhongning/article/details/6191515

1，参数替换:

                   不含有“:”的，只要定义了，就生效，不管是否为空

                   含有“:”的，即使定义了，但是为空就不生效

                   用来替换的内容可以是字符串、一个变量、命令的输出

                   被替换的内容是变量，如$a、$1等

         1)默认值替换，如果变量$var没有定义，则整体值为default字符串，但var变量本身并没有被设置：

                   ${var-default}                    如果var定义了，但为空，则还使用$var，即为空

                   ${var:-default}          如果var定义了，但为空，则使用default字符串

                   例如（以下例子为顺序执行）：

                   [root@fx_local2 ~]# unset a

                   [root@fx_local2 ~]# unset b

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# a=                      # 定义了$a，但为空

                   [root@fx_local2 ~]# echo ${a-CBA}          # 用字符串CBA替换$a

                                                                 # 没有被替换，输出空，因为$a为空

                   [root@fx_local2 ~]# echo ${a:-CBA}

                   CBA                                       # 已经被替换，没有输出空，使用了CBA来替换

                   [root@fx_local2 ~]# echo $a            # 看一下$a的值是否被设置

                                                                 # 还是空，因为$a本身并没有被设置

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# unset a             # 删除$a

                   [root@fx_local2 ~]# echo ${a:-CBA}

                   CBA

                   [root@fx_local2 ~]# echo ${a-CBA}

                   CBA

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# b=CCC

                   [root@fx_local2 ~]# echo ${a-$b} # 用变量$b替换$a

                   CCC

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${a-$(pwd)}     # 用命令pwd替换$a

                   /root

                   #--------------------------------------------------------------------------------#

                   或者在脚本中使用：${1-deault}等

         2)默认值设置，包含默认值替换的所有功能（标准输出一样），不同在于，如果变量$var没有定义，则在输出default字符串的同时，$var也被设置成default字符串

                   ${var=default}           如果var定义了，但为空，则还使用$var，即为空

                   ${var:=default}                   如果var定义了，但为空，则将$var设置成default字符串

                   例如：

                   [root@fx_local2 ~]# unset a

                   [root@fx_local2 ~]# unset b

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# a=ABC

                   [root@fx_local2 ~]# echo ${a=CBA}

                   ABC

                   [root@fx_local2 ~]# echo $a

                   ABC

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${b=CBA}       # $b没有值

                   CBA                                       # 则输出CBA，即被替换的值

                   [root@fx_local2 ~]# echo $b

                   CBA                                       # 并且$b的值也被设置成了CBA

         3)已有变量替换（并非设置），和默认值替换相反

                   ${var+new_var}

                   ${var:+new_var}

                   例如：

                   [root@fx_local2 ~]# unset a

                   [root@fx_local2 ~]# unset b

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# a=ABC             # 定义了$a

                   [root@fx_local2 ~]# echo ${a+CBA}

                   CBA                                       # 整体输出了CBA，被替换的值

                   [root@fx_local2 ~]# echo $a

                   ABC                                       # $a本身并没有被设置

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${b+CBA}         # $b是一个不存在的变量

                                                                 # 输出空

                   [root@fx_local2 ~]# echo $b

                                                                 # 输出空，证明$b本身没有被设置

         4)标准错误替换，当变量没有定义的时候，输出标准错误

                   ${a?}

                   ${a?:}

                   ${a?AAA} AAA为作为标准错误输出的内容

                   ${a?:AAA}

                   例如：

                   [root@fx_local2 ~]# unset a

                   [root@fx_local2 ~]# unset b

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# a=pwd

                   [root@fx_local2 ~]# echo ${a?}

                   pwd

                   [root@fx_local2 ~]# ${a}

                   /root

                   [root@fx_local2 ~]# ${a?}

                   /root

                   [root@fx_local2 ~]# : ${a?}               # 空命令后面跟${a?}，则无标准输出

                   [root@fx_local2 ~]# echo $?

                   0                                            # 此时状态值为0，表示$a有值

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# ${b?}

                   -bash: b: parameter null or not set

                   [root@fx_local2 ~]# : ${b?}

                   -bash: b: parameter null or not set

                   [root@fx_local2 ~]# echo ${b?}

                   -bash: b: parameter null or not set

                   [root@fx_local2 ~]# (${b?}) >/dev/null 2>&1

                   [root@fx_local2 ~]# echo $?

                   1                                            # 以上这些的状态值都是1

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# ${b?AAA}

                   -bash: b: AAA                               # 错误信息变成了AAA

                   #--------------------------------------------------------------------------------#

                   或者一种灵活的用法：

                   ${1?"Usage: $0 ARGUMENT"}

         2，字符串匹配处理：

         1)字符串长度：

                   ${#var}     表示$var的长度

                   ${#2}         表示脚本的第二个参数的长度

                   ${#@}       表示脚本的所有参数的个数，等效于$#

                   另外，数组中某个元素的长度 ${#array[n]}

         2)字符串的前后截取，pattern一般是个命令行glob，匹配的即为被去掉的字符：

                   ${var#pattern}                   最短头匹配截取

                   ${var##Pattern}                 最大头匹配截取

                   ${var%Pattern}                  最短尾匹配截取

                   ${var%%Pattern}               最大尾匹配截取

                   例如：

                   [root@fx_local2 ABS]# unset a

                   [root@fx_local2 ABS]# a=AAABBBCCC

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ABS]# echo $a

                   AAABBBCCC

                   [root@fx_local2 ABS]# echo ${a#A}         # 并没有用glob，从头去掉一个“A”

                   AABBBCCC

                   [root@fx_local2 ABS]# echo ${a#AA}

                   ABBBCCC

                   [root@fx_local2 ABS]# echo ${a#AAA}

                   BBBCCC

                   [root@fx_local2 ABS]# echo ${a##AAA}

                   BBBCCC                                        # 以上在没有glob的时候，#与##是相同的

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ABS]# echo ${a%CC}     # 从尾去掉两个“C”（或者说一个“CC”）

                   AAABBBC

                   [root@fx_local2 ABS]# echo ${a%%BBCCC}

                   AAAB                                             # %与%%的原理同上一样

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ABS]# echo ${a#BB}

                   AAABBBCCC                                # 本是从头，但字符在中间就不行了

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ABS]# echo ${a##*A}    # 这里用到了glob

                   BBBCCC                                        # 因为是最大头匹配，所以其中*A等效于AAA

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ABS]# a=/home/wwy/bin/ABS/zero.sh                  

                   [root@fx_local2 ABS]# echo ${a%.*}       # 匹配右边数第一个“.”右边的字符

                   /home/wwy/bin/ABS/zero

                   [root@fx_local2 ABS]# echo ${a%%.*}

                   /home/wwy/bin/ABS/zero                          # 因为只有一个“.”，所以%与%%是一样的

                   [root@fx_local2 ABS]# echo ${a#*.}        # 匹配左边数第一个“.”左边的字符

                   sh

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ABS]# echo ${a%/*}     # 最短尾匹配，从右往左看

                   /home/wwy/bin/ABS                          # /* 即为从右往左看的第一个/的右边

                   [root@fx_local2 ABS]# echo ${a##*/}     # 最长头匹配，从左往右看

                   zero.sh                                          # */ 即为从左往右看的最后一个/的左边

         3)字符串位置，postion为第几个位置，此位置前面的字符为被匹配的（被删除的），length表示剩下的字符的长度：

                   ${var:postion}  # 从左往右看，匹配postion位置之左的，留下之后的

                   ${var:(-postion)} # 从右往左看，匹配postion位置之左的，()是为了避免和${var-default}冲突

                   ${var:postion:length} # 匹配postion位置之前的同时，显示之后的长度为length

                   例如：

                   [root@fx_local2 ~]# unset a

                   [root@fx_local2 ~]# a=123456789

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${a:5}

                   6789                                               # 第五个字符之前的被删除了（被匹配）

                   [root@fx_local2 ~]# echo ${a:(-2)}

                   89                                          # 倒数第二个字符之前的被删除了（被匹配）

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${a:5:2} 

                   67                                          # 删除之后，只输出了剩下的前两位

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${a:5:0}

                                                                 # 这样就什么都不剩了

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${a:5:1}

                   6                                            # 只剩下第一个，此方法即，显示第五个字符

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# b=3

                   [root@fx_local2 ~]# echo ${a:`echo 5`:$b}      # 也可以这样     

                   678

         4)字符串替换，分第一个替换和全局替换，前缀替换后缀替换，同样支持glob：

                   ${var/Pattern/Replacement}            # 第一次匹配的被替换，类似：sed ‘s/Pattern/Replacement/‘

                   ${var//Pattern/Replacement}          # 全局的匹配被替换，类似：sed ‘s/Pattern/Replacement/g‘

                   ${var#/prefix/Replacement}             # 前缀替换

                   ${var%/suffix/Replacement}             # 后缀替换

                   例如：

                   [root@fx_local2 ~]# unset a

                   [root@fx_local2 ~]# a=ABCxxABC

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${a/x/X}  # 等效于：sed ‘s/x/X/‘  <<<$a

                   ABCXxABC

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${a//x/X}          # 等效于：sed ‘s/x/X/g‘ <<<$a

                   ABCXXABC

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${a/#ABC/ZZZ}

                   ZZZxxABC                                     # 前面的ABC被替换

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${a/%ABC/ZZZ}

                   ABCxxZZZ                                     # 后面的ABC被替换

         5)得到变量名，输出所有前缀一样的变量名，不支持glob：

                  ${!var_prefix*} 或 ${!var_prefix@}

                   例如：

                   [root@fx_local2 ~]# xyz111=something # 第一两个变量，值随意

                   [root@fx_local2 ~]# xyz222=something

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# echo ${!xyz*}  # 输出所有xyz为前缀的变量名

                   xyz111 xyz222

                   [root@fx_local2 ~]# echo ${!xyz@}         # 同样的效果

                   xyz111 xyz222

                   #--------------------------------------------------------------------------------#

                   [root@fx_local2 ~]# Axyz333=somthing

                   [root@fx_local2 ~]# echo ${!xyz@}  

                   xyz111 xyz222                             # Axyz并没有输出

                   [root@fx_local2 ~]# echo ${!Axyz@}       # 应该这样

                   Axyz333