I have posted related articles before. Here, we recommend that you use some built-in functions.
When writing shell programs, string-related operations are often involved. There are many command statements, such as awk and sed, which can perform various string operations. In fact, the shell has a series of operating symbols built-in to achieve similar effects. Using internal operators will omit the time to start external programs, so the speed will be very fast. If built-in operators can do it, use built-in first.
1 Read the string value
| expression | meaning |
| ${var} | The original value of the variable var |
| ${var-default} | var does not declare to return default, but does not change the value of var |
| ${var:-default} | var is not declared or its value is empty and returns default, but does not change the value of var |
| ${var=default} | var does not declare that it returns default, and sets the value of var to default |
| ${var:=default} | var is not declared or its value is empty and returns default, and sets the value of var to default |
| ${var+other} | var is declared to return other, but does not change the value of var |
| ${var:+other} | var is declared and does not return other as empty, but does not change the value of var |
| ${var?err_msg} | var is not declared, send the message err_msg to the standard error output |
| ${var:?err_msg} | var is not declared or empty, send the message err_msg to the standard error output |
| ${!varprefix*} | All variables declared before matching begin with varprefix |
| ${!varprefix@} | All variables declared before matching begin with varprefix |
2 string operations
| expression | meaning |
| ${#string} | length of string |
| ${string:position} | In string, extract substrings from position position |
| ${string:position:length} | In string, extract the substring of length $length starting from position position |
| ${string#substring} | From the beginning of the variable string, delete the substring that matches the shortest substring |
| ${string##substring} | From the beginning of the variable string, delete the substring that matches the longest substring |
| ${string%substring} | From the end of the variable string, delete the substring that matches the shortest substring |
| ${string%%substring} | From the end of the variable string, delete the substring that matches the longest substring |
| ${string/substring/replacement} | Use replacement, to replace the first matching substring |
| ${string//substring/replacement} | Use replacement, replace all matching substrings |
| ${string/#substring/replacement} | Assuming that the prefix of string matches substring, then replace the matching substring |
| ${string/%substring/replacement} | Assuming that the suffix of string matches substring, then replace the matching substring |
| Description: "substring" can be a regular expression | |
Author: Heaven and Soul and Earth Evil