Shell Programming Basics must know

Shell programming

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Static language

Dynamic language

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Programming languages: Machine language, assembly language, advanced languages

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Object-Oriented programming language: SEHLL C

Object-Oriented programming language: JAVA Python perl C + +

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Variables: Memory space, naming

Memory: Addressable storage unit

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Variable type: Defines the storage format of the variable

Character type

Numeric type: integer, floating-point

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Logical Operation:& with | | or! Non-XOR or

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Bash Variable type

Environment variables

Local variables (local variables)

Positional variables

Special variables

Variable name rule

Cannot duplicate a variable that exists

See the name of the idea

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Varname=value the entire bash process is working.

Local Varname=value valid for current code

Environment variables:

Current shell processes and child processes are valid

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Undo Variable: Unset VARNAME

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$? : Return value for program execution completion

o Correct

1-255 error return value 1, 2 127 reserved

[[Email protected] ~]# $?

-bash:127:command not found

[[email protected] ~]# echo $?

127

[Email protected] ~]# echo It is

It is

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Output redirection

>

>>

2>

2>>

&> Errors and correct results

Give the return value to/dev/null

[Email protected] ~]# CD/CDD &>/dev/null

[[email protected] ~]# echo $?

1

[Email protected] ~]# CD/&>/dev/null

[[email protected]/]# echo $?

0

/dec/null

[[email protected] ~]# echo $?

0

commands for viewing environment variables

Printenv

Env

Export

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What a script does: a command stack to accomplish a specific task

Scripting: Creating user1 user2 user3 user4 user5 users

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1. Create the. sh file

[Email protected]/]# vim/tmp/useradd.sh

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2, writing format

[Email protected]/]#

#!/bin/bash

#

Useradd user1

Useradd User2

Useradd User3

Useradd User4

Useradd User5

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3. Execute permissions for script

[Email protected] ~]# chmod +x/tmp/useradd.sh

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4, Execute script

[Email protected] ~]#/tmp/useradd.sh

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Conditional judgment

How to achieve conditional judgment in bash

Condition test Type

Integer

Text

File

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Conditional test Expression

[Expression]

[[Expression]]

Test expression

Integer comparison: Compare plus brackets

-eq: = equal [$a-eq $b]

-ne:\= Range

-gt:> Greater than

-lt:< less than

-gt:>=

-lt:<=

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The logical key between commands

Logic and:&& the first one is false, the second does not have to be executed

|| The first one is true and the second one is to execute

Create User7, no presence created USER7, presence hint already exists

[Email protected] ~]# Vi./id.sh

#!/bin/bash

#

! ID user7 &>/dev/null && useradd User7 | | Echo ' User7 is exist '

Or: | | The first one is true

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This article is from the "Jameszhan Linux Advanced ops" blog, so be sure to keep this source http://jameszhan.blog.51cto.com/10980469/1880110

Shell Programming Basics must know