September 13, Bash summary

Bash is written on a command program file that is called on the machine.

External commands are provided by each application

Program is by: instruction (algorithm) + data (structure)

Process oriented

Command-centric, design algorithms, data services to algorithms

Object oriented

Data-centric, design structure, program services

Bash's Process Control statement:

Sequential execution: Execute one by one

Select execution: Select one of the multiple bars by criteria to execute

Cyclic execution: Number of executions by condition

Variable: The memory space of the name after the program is run, can be called repeatedly modify

Local variable: The current shell process is valid.

Environment variable: exists in both the current shell process and its child processes. Local variables that are exported

Local variables: Existing during the execution of a function.

Positional parameter variables: parameters passed to the script in the script,

Special variables:

$? (Previous command execution result), 0 is true, 1-255-bit false

$*(),

[Email protected] (),

$#(),

$$()

The value type of the variable:

Values: Shaping and floating-point

Character: ASCII

Bash is a character-type, weakly-typed, and does not need to be declared beforehand.

The following can declare a variable as an integer type

Declare–i name=5

Let Name=5

Single quote: Strong reference, and its internal variables keep the original characters

Double quotes: Weak references, whose internal variables are replaced with variable values

Command reference: ' Command ', $ (command), the execution result of the reference command

Naming method:

Local variables

Variable name = variable value, default is character type

Environment variables:

Export variable Name

Declare–x variable Name

View: Env,printent,export

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Life cycle:

Automatic destruction: Shell process termination is destroyed

Manual destroy: unset variable name

Script: is a text file that is executed by bash to read from.

Startup script:

1,bash Pin Name

2, give the Execute permission pin name first

Shebang syntax format: First line write #!/bin/bash, interpreter path

Note: #内容

The custom status result exit 5 terminates the current process.

Sometimes we just want state results, we don't want state content, so we're going to discard the results.

/dev/null will swallow all the incoming data and discard it.

/dev/zero will swallow the data and spit out a bunch of 0.

To test an expression:

An 1,test expression.

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2, the expression should look like this [expression], if there is no space between the parentheses will be an error.

It can also be written as [[expression]], which is called a keyword.

The following are the parameters of the above command

Integer test:

-GT: Greater Than

-ge: greater than or equal to

-LT: Less than

-le: Less than or equal to

-eq: Equal

-ne: Not equal to

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String test:

: Greater Than

<: Less than

= =: equals

! = does not equal

-Z $a is empty

-n $a is not empty

=~: Tests whether the left string is matched by the pattern on the right

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File testing: Presence Testing

-E (-a) $file: whether the file exists

-F $file: Exists and is normal file

-D $file: Exists and is a directory

-H $file: exists and is a symbolic link

-L $file: Ibid.

-B $file: exists and is a fast device file

-C $file: exists and is a character device file

-S $file: exists and is a socket file

-P $file: exists and is a pipe file

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Permission tests:

-R $file: Whether the current user has read access to the file

-W $file: Whether the current user has write access to the file

-X $file: Whether the current user has execute permissions on the file

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-U $file: Whether the current user has suid to the file

-G $file: Whether the current user owns the file: GUID

-K $file: Whether the current user has sticky on the file

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-O $file: Whether the current user is the owner of the file

-G $file: Whether the current user is a group of files

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$file 1–nt $file 2: Compare the modification timestamp which is close to the current time

$file 1–ot $file 2: Compare the modification timestamp which is far from the current time

$file 1–ef $file 2: Compare two files whether the same inode is executed

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Combination Test conditions:

with: [[Condition 1–a condition 2]]

OR: [[Condition 1–o condition 2]]

Non: [! conditions]

Each condition refers to an expression

With: Condition 1 && Condition 2

Or: Condition 1 | | Condition 2

Non -:! Conditions

Code reuse

Define the method:

Function name ()

{

Content

}

Name ()

{

Content

}

Return can customize the function state return value, once the return is encountered, the function terminates

An array is a contiguous number of independent memory spaces that are referenced by the array name

BASH4.0 supports 2 kinds of following:

Legacy Arrays: Index-time numbers, starting with 0 declare-a name

Associative arrays: Indexes can be customized and indexed using strings declare-a name

Bash supports sparse-format arrays {0,,,, 6}

Assignment value:

1, Single assignment Name[5]=value

2, all copy name= ("value1" "value2" ...)

3. Specify the index to assign the value name= ([0]= "value1" [3]= "Vlue3")

4. You can use read to assign a value read-a name

The length of the array can be used

${#name [*|@]} to view

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$RANDOM get 0-32767 random numbers.

September 13, Bash summary