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This article mainly explains the "shell variable declaration judgment and string operation detailed explanation", the article explains the content is simple and clear, easy to learn and understand, the following please follow the editor's ideas slowly in-depth, together to study and learn "shell variable declaration judgment and string operation detailed explanation"!

1. Judgment of shell variable declaration

The expression means the value of the ${var} variable var, which is the same as $var

${var-DEFAULT} if var is not declared, then take $DEFAULT as its value * ${var:-DEFAULT} if var is not declared or its value is empty, then take $DEFAULT as its value *

${var=DEFAULT} if var is not declared, then take $DEFAULT as its value * ${var:=DEFAULT} if var is not declared or its value is empty, then take $DEFAULT as its value *

${var+OTHER} if var is declared, its value is $OTHER, otherwise it is null string ${var:+OTHER} if var is set, its value is $OTHER, otherwise it is null string

${var?ERR_MSG} if var is not declared, print $ERR_MSG * ${var:?ERR_MSG} if var is not set, print $ERR_MSG *

${! varprefix*} matches all variables previously declared with varprefix ${! varprefix@} matches all variables previously declared with varprefix

* it's very simple. Just try and understand. Note that there can be no spaces between the dollar sign and the left curly braces, between the left curly braces and the variable name, and between the variable name and the judgment.

2. String operations (length acquisition, reading, matching deletion, replacement)

The expression means the length of ${# string} $string

${string:position} in $string, extract substrings from location $position ${string:position:length} in $string, extract substrings of length $length from location $position

${string#substring} from the beginning of the variable $string, delete the shortest matching $substring substring ${string##substring} from the beginning of the variable $string, delete the longest matching $substring substring ${string%substring} from the end of the variable $string, delete the shortest matching $substring substring ${string%%substring} from the end of the variable $string, delete the longest matching $substring substring

${string/substring/replacement} use $replacement instead of the first matching $substring$ {string//substring/replacement} use $replacement instead of all matching $substring$ {string/#substring/replacement} if the prefix of $string matches $substring, then use $replacement instead of matching $substring$ {string/%substring/replacement} if the suffix of $string matches $substring, use $replacement instead of matching $substring

* it should be noted that substring can be a regular expression.

Alternative commands: cut sed awk, these three are more powerful string processing commands that can do a lot of things about strings. There are many examples here.

3. Performance comparison

Time for i in $(seq 10000); do asides ${# test}; done

Time for i in $(seq 10000); do asides $(expr length $test); done

The code is as follows:

Real 0m0.181s

User 0m0.170s

Sys 0m0.000s

Real 0m8.580s

User 0m2.497s

Sys 0m6.075s

This is equivalent to looping through external commands such as awk, sed, cut, length, and so on, to process strings, which takes time for granted. There was a previous article on shell optimization, but I won't repeat it here. The shell loop is much slower than awk. The performance test here is actually not very meaningful. The specific reasons are combined with the shell optimization article.

4. String regular judgment

You can use commands similar to the following:

The code is as follows:

If [["${lastday?ERR_MSG}" = ~ "^ [0-9] {4}-[0-9] {2}-[0-9] {2} $"]]

Then

...

Fi

It is said that bash version 3.0 or above is needed to use it. The way to check bash version is: bash-version, view the current interpreter type command: echo $0

Thank you for your reading, the above is the content of "the judgment of shell variable declaration and the detailed explanation of string operation". After the study of this article, I believe you have a deeper understanding of the judgment of shell variable declaration and the detailed interpretation of string operation, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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