Basics of Shell programming for Linux Learning notes

Programming languages: Machine language, assembly language, advanced languages

Static language: Compiler language, features: strongly typed ( variables must be declared before they are used, or even initialized ), and converted to executables. Contains languages such as: C, C + +, JAVA, C #

Dynamic languages: Explanatory languages, Features: Weak types ( variable time declarations, or even types ); Edge interpretation side Execution. Includes languages such as: ASP, ASP, PHP, SHELL, Python, PERL

Programming Ability:

Process oriented: Shell, C

Object-oriented: Python, Perl, JAVA, C + +

Bash: Variable Type

Environment variables: Shell and its child shell, as domain as the current shell process and its child processes

To see how environment variables are in the current shell:

1. #printenv

2. #env

3. #export

View variables in the current Shell

#set Vrname

Undo Variable:

#unset vrname

Local variables (called local variables):

Example: Vrname=value

#echo "There is Song${animail}s"

Positional variables:

Example: $, $

Shift: Rotate, turn

Special variables (System variables)

$? : The execution status return value of the previous command

$#: Number of parameters

$*: List of parameters

[Email protected]: List of parameters

program execution, there may be two types of return values

Program execution structure

Program code return Code:

0: Correct execution

1-255: Error execution 1, 2, 127 reserved for system

Output redirection

>: output redirection, overwrite output

>>: Append output

2>: Redirect Error output

2>>: Append error output

&>: Redirect standard output or error output to the same file

Script: Command stack, according to the actual needs, combined with the command flow control mechanism to implement the source program

Format:

#!/bin/bash

#注释行

Statement

How to achieve conditional judgment in Bash

Integer test:

-EQ: Test whether 2 integers want to wait for example $A-eq $B

-ne: Tests whether 2 integers are unequal, unequal to true, and equal to False

-GT: Test whether 1 number is greater than the other number

-LT: Test whether a number is less than another number

-ge: greater than or equal to

-le: Less than or equal to

Character test:

= =: Equal, space is required on both sides example: [$A = = $B] or [$A = $B]

! =: Not Equal

: Greater Than

<: less than

- N string: Tests whether the specified string is empty, the vacancy is true, and the empty is false

- Z String: Tests whether the specified string is not empty, no vacancy true, empty false

number of decimal reserved digits: BC scale=2, Example: Echo "SCALE=2;111/22" | BC or BC <<< "SCALE=2;111/22"

echo "Scale=2;$1$2$3" | BC

File test:

-e file: Test files exist

-F file: Test for normal files

-D FILE: Tests whether the specified path is a directory

-R: Whether the file is readable

-W: Whether the file is writable

-X: Whether the file is executable

Expression for condition test L

[Expression]

[[Expression]]2

Expression

The logical relationship between commands:

Logical vs: &&: If the previous is true, then execute the following command

Logical OR: | | : Executes the following command if the previous condition is false

Combination condition test:

-A: Logic and

-O: Logical OR relational

!: Non-relational

Bash's configuration file

Global configuration

/etc/profile,/etc/profile.d/*.sh,/ETC/BASHRC

Personal configuration

~/.bash_profile, ~/.BASHRC

File for Profile class

Setting environment variables

Sign in to run a command or script

Files for bash classes

Setting Local Variables

Defining command aliases

How the login shell reads the configuration file

/etc/profile--->/etc/profile.d/*.sh---> ~/.bash_profile-->~/.bashrc-->/ETC/BASHRC

Non-sign-on shell

~/.bashrc-->/etc/bashrc-->/etc/profile.d/*.sh

The script starts a child shell process when it executes

command-line-clock-initiated scripts inherit current shell environment variables

Scripts that are automatically executed by the system (not command-line startup) require self-defined environment variables

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Basics of Shell programming for Linux Learning notes