Linux shell string built-in Common Operations (get length, search, replace), linuxshell

Linux shell string built-in Common Operations (get length, search, replace), linuxshell

String-related operations are often involved when writing shell programs. There are many command statements, suchAwk, sedCan perform various string operations. In fact, shell has a series of built-in operation symbols to achieve similar results. Using Internal operators will omit the time such as starting external programs, so the speed will be very fast. If the built-in operators can do this, the built-in operators are preferred.

1. deduce the string value to be read.

Expression	Description
$ {Var}	Original Value of the variable var
$ {Var-default}	Var does not declare that default is returned, but the value of var is not changed.
$ {Var:-default}	If var is not declared or its value is null, default is returned, but the value of var is not changed.
$ {Var = default}	Var does not declare that default is returned, and the value of var is set to default.
$ {Var: = default}	If var is not declared or its value is null, default is returned, and the value of var is set to default.
$ {Var + other}	Var is declared to return other, but the value of var is not changed
$ {Var: + other}	If var is declared and not empty, other is returned, but the value of var is not changed.
$ {Var? Err_msg}	Var is not declared. The message err_msg is sent 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 *}	Match all variables declared starting with varprefix
$ {! Varprefix @}	Match all variables declared starting with varprefix

2 string operations

Expression	Description
$ {# String}	String Length
$ {String: position}	In string, extract the substring from position
$ {String: position: length}	In string, extract the substring with the length of $ length from the 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 substrings that match the longest substring.
$ {String % substring}	From the end of the string variable, delete the substring that matches the shortest substring.
$ {String % substring}	From the end of the string variable, delete the substring that matches the longest substring.
$ {String/substring/replacement}	Replacement is used to replace the first matched substring.
$ {String // substring/replacement}	Replacement is used to replace all matched substrings.
$ {String/# substring/replacement}	Assume that the string prefix matches the substring, replacement is used to replace the matched substring.
$ {String/% substring/replacement}	Assume that the string suffix matches the substring, replacement is used to replace the matched substring.
Note: "substring" can be a regular expression.