Shell Entry Basics & Common commands and usage

Shell

The shell is a command interpreter, which is actually a program, and all the commands in/bin/bash,linux are explained by it and have their own syntax

shell Script

End With. sh

Shell Syntax +linux command

Comments:

Single-line Comment: #
Multiline Comment:: <<c c

Shell Execution Mode

Bash/path/to/script-name or/bin/bash/path/to/script-name (highly recommended)
/path/to/script-name or./script-name (Execute script under current path)
SOURCE Script-name or. Script-name (note ".") There are spaces behind the dot number)

The first two executions will open a new process execution script
Source does not open a new process

When using source, you can share a process with the current terminal, sharing variables (emphasis)

variables

1, Name=itcast
2, name= ' Itcast$age ' will not parse the inside of the traversal
3, name= "Itcast$age" after parsing the variable, and then stitching into a new string

Command Variables

1. name= ' ls '
2, name=$ (LS)

The parentheses must be a Linux command that logs the result of the command execution to the variable

Global Variables

Method One:

Variable name = value

Export variable

Method Two: (most commonly used)

Export variable name = value

If the global variable is defined in the terminal, the scope is the current terminal and the child process


If you want to define a global variable that is valid in all environments as a system global variable, you need to define it in a ~/.BASHRC or/etc/profile file:

After modifying the ~/.BASHRC, the new terminal is opened directly, and the defined global variable takes effect, which is only valid for the current user.
When modifying/etc/profile, the operating system needs to be restarted, and the defined global variables will take effect, which is valid for all users
In both ways, if you want the global variable to take effect at the current terminal after modifying the file, you need to execute the command source ~/.BASHRC or Source/etc/profile

View Variables

Standard usage: "${variable name}"


built-in

$ A Gets the currently executing shell script file name
$$ Gets the process number that executes the shell script
$n gets the nth parameter value of the currently executing shell script, n=1..9, which represents the file name of the script when n is 0, and if n is greater than 9 is enclosed in curly braces ${10}
$# get the total number of parameters in the current shell command line
$? Gets the return value that executes the last instruction (0 for success, not 0 for failure)


$?: The previous command executes a file and returns the result of the last command in the file

Default Value

1.

Variable A If there is content, then output A's variable value

? Variable A If there is no content, then output the default content

? Format:

???? ${variable Name:-Default Value}

2.

Output default value regardless of whether variable a has content

? Format:

???? ${variable name + default value}

Test Statement

There must be a space around the equals sign, a space around the brackets

Test A = 1

[a = 1]

Logical Expressions

&& symbols
Command 1&& Command 2
If command 1 executes successfully, execute command 2
If command 1 fails, then command 2 is not executed

|| Symbol
Command 1 | | Command 2
1 if command 1 executes successfully, do not execute command 2
2 If command 1 fails to execute, then execute command 2


file Expression

Determine if an executable file

[-F ABC] && [-X ABC]

Evaluating Expressions

a=$ ((a+1))
Let A=a+1


redirect

Commands > Documents

The result returned by the command is correct and error two, 1 is correct, 2 is error

Bash Chongdingxiang.sh>/dev/null 2>&1 &

Output the correct and incorrect results to a black hole file, and the command executes in the background

Linux Four Swordsman

grep

GREP-NR keyword.

Find

Find. -name "*sh"

sed

Line editing Tools

-I real modification

S: replace
A: Append
I: Insert
D: Delete

Sed-i "S#sed#sed#g" Sed.txt

Each line finds the SED, replacing all of them with SED

Sed-i "2s#sed#sed#2" Sed.txt

The second sed in the second row is replaced by SED

Sed-i "1,4a\hello2" Sed.txt

Append a row after each row from 1 to 4 rows

Sed-i "1i\hello3" Sed.txt

Insert a row before line 1th

Sed-i "2d" sed.txt

Delete Line 2nd

awk

Analysis tools, row-by-line processing

awk ' begin{fs= ': "; ofs="--"} {print $, $NF} ' Awk2.txt

The default is to split the data by a space or TAB key, specifying the delimiter for the output by OFS

Process Control

　　

　　If statement

if [condition]
Then
Directive 1
elif [Condition 2]
Then
Directive 2
Else
Directive 3
Fi

# !/bin/bash  " Please enter gender:"  sexif"$sex""  nan"" input is male "fi

　　Case statement

#!/bin/bash Case" $" inch        "Start") echo"Start"        ;; "Stop") echo"Stop"        ;; "Restart") echo" Restart"         ;; *) echo"....  "         ;; Esac

　　For loop

Traverse Folder

# !/bin/bash               mkdir . /bak for file with    $     (LS)do#  echo "file: $file"    "$file" . /bak/"${file}-bak" Done

seq Command

# !/bin/bash                   for  in $ (seq 5       )does"num: $num" Done

While loop

# !/bin/bash               Count=1 While [$count-lt  5 ]do     "count: $count "     Let Count=count+1done

Until cycle

# !/bin/bashcount=1until [$count-ge 5   ] do     "count: $count" Let     count=count+1done

　　Function

 #  !/bin/bash  echo "  jiaoben: $1,$2,$3   "" Span style= "COLOR: #000000" >dayin () { if  [$ #   = 3]  then  echo "  hanshu: $1,$2,$3   "" Span style= "COLOR: #0000ff" >else  echo  "  Requires 3 parameters   fi   " #  dayin E F G  Dayin $ $ 

Scripts are executed using source, and variables and functions defined in the script can be called at the current terminal

Allows the root user to use the Python virtual environment

exportworkon_home=/home/python/.virtualenvs/
source/usr/local/bin/virtualenvwrapper.sh

Shell Entry Basics & Common commands and usage