Linux shell String Operations (length, find, replace) detailed

First, Judge read the string value

An expression	meaning
${var}	Variable var with the same value as $var
${var-default}	If Var is not declared, then use $default as its value *
${var:-default}	If Var is not declared, or its value is empty, then $default is used as its value *
${var=default}	If Var is not declared, then use $default as its value *
${var:=default}	If Var is not declared, or its value is empty, then $default is used as its value *
${var+other}	If Var is declared, then its value is $other, otherwise it is a null string
${var:+other}	If Var is set, then its value is $other, otherwise it will be a null string
${var? ERR_MSG}	If Var is not declared, print $err_msg *
${var:? ERR_MSG}	If Var is not set, then print $err_msg *
${!varprefix*}	Matches all previously declared variables beginning with Varprefix
${[email protected]}	Matches all previously declared variables beginning with Varprefix

Adding "*" does not mean: Of course, if the variable var is already set, then its value is $var.

[[email protected] ~]$ echo ${abc-' OK '}
Ok
[Email protected] ~]$ echo $ABC

[[email protected] ~]$ echo ${abc= ' OK '}
Ok
[Email protected] ~]$ echo $ABC
Ok

If ABC does not declare "=" It will also assign a value to ABC.

[Email protected] ~]$ var1=11;var2=12;var3=
[[email protected] ~]$ echo ${[email protected]}
Var1 var2 Var3
[[email protected] ~]$ echo ${!v*}
Var1 var2 Var3

${!varprefix*} similar to ${[email protected]}, a variable name prefix character can be used to search for a variable that has already been defined, regardless of whether it is a null value.

Second, string operation (length, read, replace)

An expression	meaning
${#string}	Length of $string
${string:position}	In $string, start extracting substrings from position $position
${string:position:length}	In $string, a substring of length $length is extracted starting from position $position
${string#substring}	Delete the substring of the shortest match $substring from the beginning of the variable $string
${string# #substring}	Delete the substring of the longest matching $substring from the beginning of the variable $string
${string%substring}	Delete the substring of the shortest match $substring from the end of the variable $string
${string%%substring}	Delete the substring of the longest matching $substring from the end of the variable $string
${string/substring/replacement}	Use $replacement to replace the first matching $substring
${string//substring/replacement}	Use $replacement instead of all matching $substring
${string/#substring/replacement}	If the $string prefix matches the $substring, then the $replacement is used instead of the matching $substring
${string/%substring/replacement}	If the $string suffix matches the $substring, then the $replacement is used instead of the matching $substring

Description: "* $substring" can be a regular expression .

1. Length

[Email protected] ~]$ test= ' I love China '
[Email protected] ~]$ echo ${#test}
12

${#变量名} to get the string length

2. Intercept strings

[Email protected] ~]$ test= ' I love China '
[[email protected] ~]$ echo ${test:5}
E China
[[email protected] ~]$ echo ${test:5:10}
E China

${variable Name: start: length} get substring

3. String deletion

[Email protected] ~]$ test= ' C:/windows/boot.ini '
[[email protected] ~]$ echo ${test#/}
C:/windows/boot.ini
[[email protected] ~]$ echo ${test#*/}
Windows/boot.ini
[[email protected] ~]$ echo ${test##*/}
Boot. ini

[[email protected] ~]$ echo ${test%/*}
C:/windows
[[email protected] ~]$ echo ${test%%/*}

${variable name #substring regular expression} starts with substring from the beginning of the string and deletes the expression on the match.

${variable name%substring regular expression} starts with substring from the end of the string and deletes the expression on the match.

Note: ${test##*/},${test%/*} is the simplest way to get the file name, or directory address, respectively.

4. String substitution

[Email protected] ~]$ test= ' C:/windows/boot.ini '
[[email protected] ~]$ echo ${test/\//\\}
C:\windows/boot.ini
[[email protected] ~]$ echo ${test//\//\\}
C:\windows\boot.ini

${variable/find/Replace value} A "/" means replacing the first, "//" means replacing all, when the lookup appears: "/" Please add escape character "/" to indicate.

Third, performance comparison

In the shell, through awk,sed,expr and so on can be implemented, the string above operation. Below we perform a performance comparison.

[Email protected] ~]$ test= ' C:/windows/boot.ini '
[[Email protected] ~]$ time for I in $ (seq 10000);d o a=${#test};d one;

Real 0m0.173s
User 0m0.139s
SYS 0m0.004s

[[Email protected] ~]$ time for I in $ (seq 10000);d o a=$ (length of expr $test);d one;

Real 0m9.734s
User 0m1.628s

The speed difference is hundreds of times, call external command processing, and built-in operator performance is very different. In shell programming, try to complete with built-in operators or functions. Similar results can occur with awk,sed.