Shell Learning Classic

Seven types of files

D Directory L Symbolic link

s socket file B block device file

C-character device file p named pipe file

-Regular file regular expressions

When you extract or filter text from a file or command output. Regular Expressions (RE) can be used, and regular expressions are a collection of special or not very special string patterns.

Basic meta-Character set:

^ matches only the beginning of the line.

$ matches only the end of the line.

* A single character immediately following *, matching 0 or more characters.

[] matches the characters in [], either as a single character or as a sequence of characters. Can make

Use-to denote a [] inner range, such as [1-5] equivalent to [1,2,3,4,5].

\ masks The special meaning of a meta-character, such as \$ represents the character $, and does not represent a matching row

Tail.

. matches any single character.

Pattern\{n\} matches the number of occurrences of pattern n

Pattern\{n,\}m matches the number of occurrences of pattern, but the minimum number of occurrences is n

Pattern\{n,m\} matches pattern occurrences between N and M (N,m is 0-255)

A few common examples:

Show executable files: ls–l | grep ... x...x. X

Show folders only: Ls–l | grep ^d

Match all blank lines: ^$

Match all the words: [A-Z a-z]*

Match any non-alphabetic character: [^a-z A-z]

Line with eight characters: ^........$ (8). Character class Description

The following is a fairly complete list of the available character classes:

[: Alnum:] alpha-numeric [A-Z 0-9]

[: Alpha:] letter [A-z]

[: Blank:] Space or TAB key

[: Cntrl:] any control character

[:d Igit:] number [0-9]

[: Graph:] Any visible character (no spaces)

[: Lower:] lowercase [A-z]

[:p rint:] Non-control characters

[:p UNCT:] punctuation character

[: Space:] Space

[: Upper:] uppercase [A-z]

[: xdigit:] hex digit [0-9 a-f a-f]

It is advantageous to use character classes as much as possible, as they can be better adapted to non-English locales (including some required accent characters, etc.). Shell's quotation mark type

There are four types of reference in the shell:

"" Double quotation marks

' Single quotation mark

' Anti-quote

\ Backslash

L "" can refer to any character or string other than $, ', \, or, and the variable in "" will display the value of the variable normally.

The difference between "and" "is that the shell ignores any reference value.

Example: Girl= ' GIRL '

echo "The ' $GIRL ' did well '

Then print: The ' girl ' did well

L ' is used to set the output of a system command to a variable, and the shell will use the contents of "as a system command and perform the quality."

For example, echo ' date ' prints the current system time.

L \ used to block special meanings of characters:& * + ^ $ ' "| ?

For example: Expr 12 \* 12 will output different modes when the 144 variable is set:

Valiable_name=value setting the actual value into Variable_name

Valiable_name+value if Variable_name is set, reset its value

Valiable_name:?value If Variable_name is not set, the undefined user error message is displayed first

Valiable_name?value If Variable_name is not set, a system error message is displayed

Valiable_name:=value If Variable_name is not set, its value is set

Valiable_name-value, but the value is not set to the variable_name condition test

The test command is used for testing strings, file states, and numbers, and expr tests and executes numeric outputs.

Test format: Test condition or [condition] (it is important to note that there is a space on both sides of the condition, otherwise it will be an error), the test command returns 0 to indicate success.

The three tests of test are described below, respectively:

N File status test (Common)

-D test whether the folder

-F Tests whether generic files

-l test whether to link files

-R test File is readable

-W test file is writable

-X test whether the file is executable

-S test file is not empty

N String Test

Five formats: Test "string"

Test String_operator "String"

Test "string" String_operator "string"

[String_operator "string"]

["String" String_operator "string"]

Where string_operator can be: = two strings are equal

! = Two String unequal

-Z Empty string

-N Non-empty string

N Numerical Test

Two formats: "Number" Number_operator "number"

["Number" number_operator "number"]

Where: Number_operator can be:-eq,-ne,-gt,-lt,-ge

Example: number=130

["990" le "995" –a "number"-GT "133"]

(Where-a means "and" before and after results)

The L expr command is generally used for integer values, but can also be used for strings.