Writing room BASH Script

bash shell The first line of the script begins with ： #!/bin/bash

Created shell Script , Need to use chmod Command to add executable permissions , There are also willing to be with chown Command combination changes the file ownership of the script . Grant execute permission only to the target user of the script .

Quote special characters , Three tools to cancel or escape special meanings ： The backslash （\）、 Single quotation marks （''）、 Double quotes （""）.

The backslash escape character removes the special meaning of the single character immediately following it . Example ：

When you need to escape multiple characters in a text string , You can use single quotation marks （''）. A single quotation mark retains the literal meaning of all characters it contains .：

Using double quotation marks can prevent generic matching and shell Expand , But it still allows command and variable substitution . Variable substitution is conceptually the same as command substitution

echo Command in shell Scripts are widely used to display information and error messages .

shell The cycle in

bash Of for Loop structure Use the grammar .

for VARIABLE in LTST; do

COMMAND VARIABLE

done

The lower case part is fixed .VARIABLE Is a variable name. ,COMMAND VARIABLE A command block that represents a reference variable .

Use exit code in scripts

Use exit command . When the script encounters exit On command , The script exits immediately and does not process the rest of the script .

It can be an integer parameter （ Optional ,0-255 Between , Indicates the exit code ） To execute exit command . The exit code is returned to the parent process , Store it in ？ variable , Can pass $? Visit .

Conditional structure

if/then The structural grammar is as follows ：

also if/then/else structure ：

also if/then/elif/then/else structure

Use regular expressions to match text in command output

vim、grep and less All commands can use regular expressions .

The simplest perfect match , That is, the characters in the regular expression match the type and order of the data being searched , It's a perfect match .

Match the beginning and end of a line , Search at the beginning of the line , Use the caret （^）. Search at the end of the line , Use the dollar sign （$）.

Regular expressions use periods （.） Or point （.） To match any single character other than a newline character .c.t Include the search for c Add any single character and t String .

c[aou]t Will match ： With c start , Follow behind a or o or u, And then there was t, namely cat、cot、cut.

A mechanism that is often used with wildcards . The multiple is applied to the previous character in the expression , One of the more common multiples is the asterisk （*）.

c*t, Match anything with c start , Followed by zero or more characters , Finally t Data at the end .

Another type of multiple will indicate the expected number of preceding characters in the pattern .‘c.\{2\}t’ Is an example of using display multiples , It matches with c start , Followed by any string , Finally t Any words at the end .

Regular expressions

Options	describe
.	Period （.） Match any single character .
？	The front item is optional , And match at most once .
*	The previous items will match zero or more times .
+	The previous items will match one or more times .
{n}	The front item just matches n Time .
{n,}	The front item matches n Times or more .
{,m}	The front items match at most m Time .
{n,m}	The front items at least match n Time , But not more than m Time .
[:alnum:]	Alphanumeric character ：‘[:alpha:]’ and ‘[:digit:]’; stay 'C' Language environment and ASCII In character encoding , It is equivalent to ‘[0-9A-Za-z]’
[:alpha:]	Alphabetic character ：‘[:lower:]’ and '[:upper:]'; stay 'C' Language environment and ASCII In character encoding , It is equivalent to ‘[A-Za-z]’.
[:blank:]	Blank character ： Spaces and tabs .
[:cntrl:]	Control characters . stay ASCII in , These characters correspond to octal code 000 To 037 and 177(DEL). In other character sets , They are equivalent characters .
[;digit;]	Numbers ：0 1 2 3 4 5 6 7 8 9 .
[;graph;]	Image characters ：‘[:alnum:]’ and '[:punct:]'.
[:lower:]	Lowercase letters ; stay ”C“ Language environment and ASCII In character encoding , It corresponds to a b c d e f g h i j k l m n o p q r s t u v w x y z.
[:print:]	Printable characters ：‘[:alnum:]’、'[:punct:]' And Spaces .
[:punct:]	Punctuation ; stay ”C“ Language environment and ASCII In character encoding , It corresponds to ！” # $ %&'()*+,-./:;<=>?@[\]^'{|}~.
[:space:]	Space character ;
[:upper:]	Capital
[:xdigit:]	16 Hexadecimal Numbers ： 0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f .
\b	Matches the empty string on both sides of the word .
\B	Matches an empty string in the middle of a word .
\<	Matches the empty string at the beginning of the word .
\>	Matches the empty string at the end of the word .
\w	Match word components
\W	Match non word components
\s	Match spaces
\S	Match non spaces .

Use grep The command matches the regular expression

The vertical line operator （|）,grep Commands can be used with other commands . Example ：

common grep List of options

Options	function
-i	Use the provided regular expression , But it doesn't force case sensitivity .
-v	Show only lines that do not contain regular expression matches .
-r	Apply a data search that recursively matches a regular expression to a set of files or directories .
-A number	Shows the number of lines after a regular expression match .
-B number	Shows the number of lines before a regular expression match .
-e	You can provide multiple regular expressions , And will be associated with logic OR Use it together .