字符串的操作

1 字符串的截取
${字符串变量名:截取的开始下标} ,直到字符串最后。下标从1开始，空格也是一个字符串。原字符串不变

shiyanlou:~/ $ st="hello world shellgood"                                                                                          [22:22:59]
shiyanlou:~/ $ echo ${st:5}                                                                                                        [22:28:06]
 world shellgood
shiyanlou:~/ $ echo ${st:6}                                                                                                        [22:28:25]
world shellgood
shiyanlou:~/ $ echo ${st:7}                                                                                                        [22:28:35]
orld shellgood
shiyanlou:~/ $ echo $st                                                                                                            [22:29:16]
hello world shellgood
shiyanlou:~/ $ 

${字符串变量名:开始的下标，需要截取的字符个数}

hiyanlou:~/ $ st2=${st:6:5}                                                                                                       [22:33:07]
shiyanlou:~/ $ echo $st2                                                                                                           [22:33:19]
world
shiyanlou:~/ $ echo $st                                                                                                            [22:33:22]
hello world shellgood

2 获取字符串的长度
${#字符串变量名｝

shiyanlou:~/ $ echo ${#st}                                                                                                         [22:33:38]
21
shiyanlou:~/ $  

2 从头部开始去除
${字符串变量名#从头部开始需要去除的子串｝

shiyanlou:~/ $ echo ${st#hel} 去除开始的子串hel                                                                                                 [22:34:52]
lo world shellgood
shiyanlou:~/ $ echo ${st#*o}  去除从头开始到出现o的子串                                                                                              [22:36:57]
 world shellgood
shiyanlou:~/ $ echo ${st#*o }  去除从头开始到出现o空格的子串                                                                                                       [22:41:16]
world shellgood
shiyanlou:~/ $ echo ${st#*d}    去除从头开始到出现d的子串                                                                                                     [22:41:24]
 shellgood
shiyanlou:~/ $ echo $st     st内容不变                                                                                                       [22:41:59]
hello world shellgood
shiyanlou:~/ $ echo ${st#llo}  llo子串不和头部匹配，不进行去除                                                                                                    [22:45:21]
hello world shellgood
shiyanlou:~/ $ echo ${st#world}  world子串不和头部匹配，不进行去除                                                                                                  [22:45:30]
hello world shellgood

2.1 贪婪匹配
${字符串变量名##子串}，如果子串匹配字符串的头部，一直要删除到最后一次

hello world shellgood
shiyanlou:~/ $ echo "${st##*o}"  去除最后一次出现o的位置                                                                                                    [22:45:53]
d
shiyanlou:~/ $  

注意：使用括号括起来，表示一个整体，不容易出现误会和不可知的错误
3 从尾巴开始去除，%表示去尾巴，用法和去头相同

hiyanlou:~/ $ echo $st                                                                                                            [22:56:15]
hello world shellgood
shiyanlou:~/ $ echo "${st%l*}"    从尾巴开始去除第一次出现l的位置                                                                                                  [23:03:29]
hello world shel
shiyanlou:~/ $ echo "${st%%l*}"  从尾巴开始去除最后一次出现l的位置                                                                                                  [23:03:54]
he
shiyanlou:~/ $ echo "${st%good}"  从尾巴开始匹配到good,进行去除                                                                                                 [23:04:03]
hello world shell
shiyanlou:~/ $ echo $st                                                                                                            [23:04:26]
hello world shellgood
shiyanlou:~/ $   

4 字符串的替换,原字符串内容不变
${字符串变量名/old子串/new子串｝，用new子串替换old子串

shiyanlou:~/ $ echo $st                                                                                                            [23:04:26]
hello world shellgood
shiyanlou:~/ $ echo "${st/good/}"                                                                                                  [23:04:32]
hello world shell
shiyanlou:~/ $ echo "${st/shell/}"                                                                                                 [23:09:24]
hello world good
shiyanlou:~/ $ echo "${st/shell/java}"                                                                                             [23:09:39]
hello world javagood
shiyanlou:~/ $ echo $st                                                                                                            [23:09:54]
hello world shellgood