Linux learning

linux Process

Process concept
From the perspective of users: a process is a running program.
From the perspective of operating system: when the operating system runs a program, it is necessary to describe the running process of the program through a structural task_struct {}, collectively referred to as PCB, so for the operating system, the process is the PCB(process control block) program control block
The process description information includes: identifier PID, process status, priority, program counter, context data, memory pointer, IO status information and accounting information. All need to be scheduled by the operating system.

Displays the processes executed by the system

Display the process executed by the system:
ps -a # Display all process information of the current terminal
ps -u # Display process information in user's format
ps -x # Displays the parameters of the background process

Instruction description

instructions	meaning
System V	Display style
USER	User name
PID	Process number
%CPU	CPU usage
%MEM	Percentage of physical memory used
VSZ	Occupied virtual memory size
RSS	Occupied physical memory size
TT	Terminal name, abbreviation
STAT	Process status, where s-sleep, s-indicates that the process is the leader of the session, N-indicates that the process has a lower priority than the normal priority, R-running, D-short-term waiting, Z-dead process, T-tracked or stopped, etc
STARTED	Start time of the process
TIME	CPU time used by the process
COMMAND	The instructions and parameters used to start the process will be truncated and displayed if they are too long

Displays the current process

Display the current process in full format:
ps -ef # Displays the current process in full format
ps -e # Show current process
ps -f # Full format

Instruction description

instructions	meaning
UID	User id
PID	Process id
PPID	Parent process id
C	The CPU is used to calculate the execution priority. A higher value indicates that the process is CPU intensive, and the execution priority will be reduced
STIME	Time of system startup
TTY	Complete terminal name
TIME	CPU time used by the process
CMD	The instructions and parameters used to start the process

Terminate process

Display the current process in full format:
kill [option] Process number # Kill process by process number
killall Process name # Kill the process through the process name. Wildcards are supported
kill/killall -9 # Force immediate kill of process

Process tree

Display process information in tree format:
pstree [option] # Display the process information in tree format. If the command cannot be found, use yum install psmisc to download it
pstree -p # Display process number
pstree -u # Display user name

Service management

The essence of a service is a process, but when running in the background, it usually listens to a port and waits for requests from other programs, such as mysqld and sshd. Therefore, it is also called a daemon.

service management instruction

After CentOS 7, many services no longer support service, but systemctl
The service managed by the service instruction is in / etc / init D view

service service name [option] # Options include start, stop, restart, and status

Operation level of service

linux services have seven levels of operation

// Run level
0: Shut down
1: single user [Retrieve lost password]
2: There is no network service in multi-user state
3: Multi user network service status
4: The system is not used and reserved for users
5: Graphical interface
6: System restart
// The common operation levels are 3 and 5. You can also specify the default operation level, which will be demonstrated later
init level // Switch level command
systemctl get-default //Get current level
systemctl set-default //Set default level

Service Management (chkconfig)

The chkconfig command can be used to set the self start / shut-down for each running level of the service
After CentOS 7, many services no longer support chkconfig, but systemctl
The services managed by the chkconfig command are in / etc / init D view

chkconfig --list | grep [option] // View services
chkconfig service name --list // View services
chkconfig --level Run level service name on/off // Set whether the service starts automatically at this level

After using the chkconfig command to set the service, you need to reboot the machine to take effect

Service Management (systemctl)

The services managed by the systemctl command can be viewed in / user/lib/systemd/system

// Basic grammar
systemctl [option] service name // Set the service information, such as start, stop, status, restart, and take effect temporarily
// systemctl sets the self start state of the service
systemctl list-unit-file [|grep service name] // Check the service startup status and grep filter
systemctl enable service name // Set service startup
systemctl disable service name // Turn off service and start up
systemctl is-enabled service name // Query whether a service starts automatically

Service management port operation (firewall)

Open or close the specified port

netstat -anp | more // View port information
// firewall instruction
firewall-cmd --permanent --add-port=port/agreement // open port
firewall-cmd --permanent --remove-port=port/agreement // Close port
firewall-cmd --reload // Reload to take effect
firewall-cmd --query-port=port/agreement // Check whether the port is open

Dynamic monitoring system

The top instruction is very similar to the ps instruction. They are both used to display the executing process. The difference is that top can update the executing process after it is executed for a period of time.
Basic syntax:

top [options]

Option description

option	meaning
-d seconds	Specify the top instruction, which is updated every few seconds. The default is 3 seconds
-i	Make top not show any idle or dead processes
-p	Monitor the status of a process by specifying the monitoring process id

Interactive operation instructions:
After using the top instruction to view the process, you can use the interactive instruction to operate

option	meaning
P	Sort by cpu usage. The default is this sort
M	Sort by memory usage
N	Sort by PID
U u ser 1	Monitor the progress of user 1
k	Terminate process
q	Exit top

View network conditions

Basic grammar

netstat [option]

Option description

option	meaning
-an	Arrange the output in a certain order
-p	Displays which process is calling

Software package management

rpm

RPM is the abbreviation of red hat package manager. Although the name of this file format is marked with the logo of red hat, its original design concept is open, including the distribution versions of Linux such as OpenLinux, S.u.S.E. and Turbo Linux, which can be regarded as a recognized industry standard.

// Basic grammar
rpm -qa // Query all installed rpm packages. You can use more paging and grep filtering to view them
rpm -q Package name // Query whether the software package is installed
rpm -qi Package name // Query package information
rpm -ql Package name // Query the files in the package
rpm -qf File full pathname // Query the package to which the file belongs

rpm -e Package name // Uninstall package
rpm -e --nodeps Package name // Force uninstall package

rpm -ivh Package full path name // Install rmp package
// Where i=install, v=verbose prompt, h=hash progress bar

yum

Yum (full name: Yellow dog Updater, Modified) is a Shell front-end package manager in Fedora, RedHat and CentOS. Based on RPM package management, it can automatically download and install RPM packages from the specified server, automatically handle dependencies, and install all dependent software packages at one time without cumbersome downloading and installation at one time.

// Basic grammar
yum list | grep Software name // Query the yum server for the required installed software
yum install Software name // Install the specified software

shell programming

Bash is one of the most popular shell scripting languages in the Linux world. I think (this is my own view) the reason is that by default, the bash shell allows users to easily navigate through historical commands (previously executed), while ksh requires Make some adjustments to the profile, or remember some "magic" key combinations to review the history and correct the commands.
shell scripts are also like interpreters, but they are often used to call externally compiled programs. It then captures the output, exits the code, and processes it as appropriate.

How shell scripts are executed

Script format requirements
The script should be #/ Start with bin/bash
The script needs to have executable permissions

Examples

#!/bin/bash
ehco "hello"
ehco 'hello'
ehco hello

Execution method:

Directly under the script directory/ The file name can be executed, but you need to have execution permission
The full path of sh file can also be executed

shell variable

shell variable introduction:
1.linux is divided into system variables and user-defined variables
2. System variables: $HOME,$PWD,$USER,$SHELL, etc
3. Display all variables of the current system: set
Definition of shell variable:
Define variable: variable name = value
Undo variable: unset variable name
Declare static variables: readonly variables. Static variables cannot be undone
Definition rules of variables:
Variables can consist of numbers, letters, and underscores, but cannot begin with a number.
There must be no spaces on either side of the equal sign
Variable names are generally capitalized
Assign the return value of the command to the variable:
A=`date ` / / in backquotes, run the command inside and assign the return value to a
A=$(date) / / run the command inside and assign the return value to a

environment variable

// Basic grammar
export Variable name=Variable value // Output as environment variable
source configuration file // Make the modified configuration information take effect immediately
echo $Variable name // Output variable

// Multiline comments for shell scripts
<< ! // A comment must start with another line
 Note Content 
! // There is another line at the end

Position parameter variable
When we execute a shell script, we need to obtain the parameter information of the command line, so we can use the location parameter variable.
For example:/ shell-test.sh 1 2, which is a command line for executing shell script. You can use positional parameter variables to obtain parameter information in the script

// Basic grammar
$n // n is a number, $0 represents the command itself, $1 to $9 represent the following 9 parameters in turn, and more than the 10th parameter needs to be contained in braces, such as 
$* // Represents all parameters in the command line, and regards all parameters as a whole
$@ // Represents all parameters in the command line. Treat each parameter differently
$# //This command represents the number of all parameters on the command line

Predefined variables
The designed variables can be used directly in the shell script

// Basic grammar
$$ // Get the id of the current process
$! // Get the id of the last process in the background
$? // Get the return status of the last command. If the value is 0, it will run correctly, and if it is not 0, it will fail
 command & // Use the & symbol in the script to indicate that the command is running in the background

operator
Operation in shell

// Basic grammar
1.$((expression))perhaps $[expression]perhaps expr expression // expression
2.be careful expr There should be spaces between operators. You can use``Assign the return value to the variable
3.expr Operator in the expression of\*,/,%Indicates multiplication and division

Process judgment

// Basic grammar
// First:
if [ Conditional judgment ];then
 program
fi
// Second:
if [ Conditional judgment ]
then 
	program
elif [ Conditional judgment ]
then 
	program
fi
// Notes: (1) there must be a space between the brackets and the conditional expression; (2) if the conditional expression is not empty, return true
// Branch condition judgment
// And
// Mode 1
if [ c1 -a c2 ]; then
...
fi
// Mode 1
if [ c1 ] && [ c2 ]; then
...
fi
// or
// Mode 1
if [ c1 -o c2 ]; then
...
fi
// Mode 2
if [ c1 ] || [  c2 ]; then
...
fi

Common condition judgment

option	meaning
=	Determine whether the string is the same
-lt	less than
-le	Less than or equal to
-eq	be equal to
-gt	greater than
-ge	Greater than or equal to
-ne	Not equal to
-le	Less than or equal to
-eq	be equal to
-gt	greater than
-ge	Greater than or equal to
-r	Do you have the habit of reading documents
-x	Do you have the habit of executing documents
-w	Do you have the habit of writing documents
-f	Whether the file exists and is a formal file
-e	Does the file exist
-d	Does the file exist and is a directory

case in process control

#! /bin/bash
case $1 in
[a-z] | [A-Z])
echo "English characters"
;;
[0-9])
echo "number"
;;
[,.!?])
echo "Symbol"
;;
*)
echo "I don't know"
esac

for process control

for((i=0;i<=10;i++))
do
	S=$(($S+i))
done
echo $S

while process control

for((i=0;i<=10;i++))
do
	S=$(($S+i))
done
echo $S

shell Function

Read read console input

//Basic grammar
read (Options) (parameters)
//Examples
#!/bin/bash
read -t 5 -p "Please enter the first number:" NUM1
read -t 5 -p "Please enter the second number:" NUM2
SUM=$(($NUM1+$NUM2)) // Assign SUM of two numbers to SUM
echo "The sum of the two numbers is:$SUM"

option	meaning
-p	Specifies the indicator when reading
-t	Specifies the read wait time

System function

dirname basic syntax:
Returns the path of the absolute path file and the file name
Basic syntax of basename:
Returns the file name of the absolute path, including the extended name

Custom function

Basic grammar
function name() {
Function body
}
name parameter / / call the function

// Examples
#!/bin/bash
# Calculate the sum of two numbers
function getSum() {
	SUM=$(($1+$2))
	echo "And are: $SUM"
}
getSum $1 $2 // Call function

Backup database

// Example of backing up mysql database
#Directory where backup data is stored
BACKUP=/data/backup/db/

#current time 
DATETIME=$(date +%Y-%m-%d_%H%M%S)
echo $DATETIME

#Database address, user name, password, backed up database
HOST=localhost
USER=root
PWD=123456fls
DATABASE=Students

#Create backup folder
if [ ! -d ${BACKUP}/${DATETIME} ]
then
        mkdir -p "${BACKUP}/${DATETIME}"
        echo "The backup file is created as follows: ${BACKUP}/${DATETIME}"
else
        echo "The backup file is: ${BACKUP}/${DATETIME}"
fi

#Backup database
mysqldump -u$USER -p$PWD -h${HOST} -q -R -B ${DATABASE} | gzip > ${BACKUP}/${DATETIME}/$DATETIME.sql.gz

# Package the backup folder into a tar file
cd ${BACKUP}
tar -zcvf ${DATETIME}.tar.gz ${DATETIME}
# Delete corresponding directory
rm -rf ${BACKUP}/${DATETIME}
# Delete the files backed up 3 minutes ago, - atime 0 indicates the files accessed within 24 hours, - exec indicates the execution of the following commands, and the slash is prevented; No. escape
find $BACKUP -atime 0 -name "*.tar.gz" -exec rm -rf {} \;