Mycpen

Format: sort options file
-n	Sort by numerical value
-r	Sort in descending order
-k	Specify the column to sort
-t	Specify the delimiter
-u	Remove duplicates

Format: uniq options file
-c	Count occurrences of each line
-u	Show only lines that appear once
-d	Show only lines that are repeated

Format: cut options extract range file
-d	Specify the delimiter
-f	Specify the specific fields to display
-c	Specify specific characters

grep Filter General regular expression analysis program
grep [options]... pattern target file
-i	Case insensitive
-v	Reverse search, do not display lines containing the specified character
-o	Display matching content and show it on a new line
-n	Display the line numbers of the filtered lines
-r	Recursively search all files in the specified directory (including its subdirectories)
-E	Support more extended regular expressions

^aa	Lines starting with aa
Aa$	Lines ending with aa

*	Matches the previous item any number of times
?	Matches the previous item 0 or 1 time
+	Matches the previous item 1 to many times
.	(Placeholder) Matches any character except a newline
{n,m}		Matches n to m times
{,n}		Matches 0 to n times
{m,}		Matches m times or more

[] Set notation
[a-zA-Z]
[0-9]
[^a]		Does not match a

Example
---------------------------------------------------------------------------------------------------------------------------------
# grep -E "a.*c" grep_test.txt 				# Note: .* matches the previous item . 0 times or any number of times
# grep -E "a*c" abc.txt --color=auto		# Note: * matches the previous item a 0 times or any number of times
# grep -E "a+c" abc.txt --color=auto		# Note: + matches the previous item a 1 time or more

[root@sanchuang-linux ~]# cat wc_test.txt 
a b c
aa bb cc
xyz
1234
aa-bb

Example
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# wc -l wc_test.txt 	# Note: Count lines
5 wc_test.txt
[root@sanchuang-linux ~]# wc -w wc_test.txt 	# Note: Count words
9 wc_test.txt
[root@sanchuang-linux ~]# wc -c wc_test.txt 	# Note: Count characters
30 wc_test.txt
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# cat wc_test.txt|wc -c		# Method 2: cat
30
[root@sanchuang-linux ~]# wc -c < wc_test.txt 		# Method 3: Redirection
30

[root@sanchuang-linux ~]# cat diff_1_test.txt
aa
bb
cc
xx
[root@sanchuang-linux ~]# cat diff_2_test.txt
aa
bb
xx

Example 1
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# diff diff_1_test.txt diff_2_test.txt 
3d2							# Note: 3d2 means line 3 of file 1 needs to be deleted to match file 2
< cc						# Note: cc in file 1
============================================================================================

[root@sanchuang-linux ~]# cat diff_1_test.txt
aa
bb
cc
xx
gg
[root@sanchuang-linux ~]# cat diff_2_test.txt
aa
bb
dd
xx
ee

Example 2
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# diff diff_1_test.txt diff_2_test.txt 
3c3											# Note: Line 3
< cc										# Note: cc in file 1
---
> dd										# Note: dd in file 2
5c5											# Note: Line 5
< gg										# Note: gg in file 1
---
> ee										# Note: ee in file 2
--------------------------------------------------------------------------------------------
============================================================================================

Example 3: -u: Will group the differences together for compact readability
[root@sanchuang-linux ~]# diff -u diff_1_test.txt diff_2_test.txt 
--- diff_1_test.txt	2020-10-30 11:50:45.784010843 +0800
+++ diff_2_test.txt	2020-10-30 11:51:11.475010836 +0800
@@ -1,5 +1,5 @@
 aa
 bb
-cc											# Note: Understand as left - right +
+dd											# Note: Or understand as left -cc +dd to match right
 xx
-gg
+ee

Example
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# yum install patch
[root@sanchuang-linux ~]# diff diff_1_test.txt diff_2_test.txt 	
3c3							# Note: Difference content
< cc
---
> dd
5c5
< gg
---
> ee

# Note: The difference file is also called a patch file
# Note: The generated file is a patch file for file 1
[root@sanchuang-linux ~]# diff -u diff_1_test.txt diff_2_test.txt > diff_test.patch # Note: Patch for file 1
[root@sanchuang-linux ~]# cat diff_test.patch 		# Note: Patch file
--- diff_1_test.txt	2020-10-30 11:50:45.784010843 +0800
+++ diff_2_test.txt	2020-10-30 11:51:11.475010836 +0800
@@ -1,5 +1,5 @@
 aa
 bb
-cc
+dd
 xx
-gg
+ee
[root@sanchuang-linux ~]# patch diff_1_test.txt < diff_test.patch 	# Note: Apply patch
patching file diff_1_test.txt
[root@sanchuang-linux ~]# cat diff_1_test.txt 						# Note: After applying patch
aa
bb
dd
xx
ee
[root@sanchuang-linux ~]# cat diff_2_test.txt 		# Note: File 1 and 2 contents are the same
aa
bb
dd
xx
ee

Example
[root@localhost ~]# grep -A 3 quit /etc/passwd		# Note: Find matching lines and the following few lines
[root@localhost ~]# grep -B 3 quit /etc/passwd		# Note: Output matching lines and the preceding few lines

[root@sanchuang-linux ~]# free -g		# Note: Check memory usage, -g unit G, -m unit M
              total        used        free      shared  buff/cache   available
Mem:              1           0           1           0           0           1
Swap:             1           0           1
[root@sanchuang-linux ~]# free -m
              total        used        free      shared  buff/cache   available
Mem:           1800         272        1101           8         426        1363

Implement the following functions
1. Monitor memory usage, if memory usage exceeds 80%, give a reminder
	total free usage rate
2. Scan local area network IPs, check which IP addresses are in use
	ping -c 1		# Note: Send 1 packet
3. Monitor if the file /etc/passwd has been modified, check every 5 minutes
	diff
	md5sum	# md5 value, unique identifier of the file
4. Monitor if the nginx process exists, if not, give a corresponding reminder
	pidof nginx

--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# md5sum abc.txt 			# Note: md5 value
2416b02c3d9d753f48cf49dbb5f1de94  abc.txt
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# pidof nginx				# Note: Display the process ID of the specified program
12767 12766 12765

Example
--------------------------------------------------------------------------------------------
#!/bin/bash
function mem(){
    total=`free -m|grep -i mem|tr -s " "|cut -d " " -f2`
    #free=`free -m|grep -i mem|tr -s " "|cut -d " " -f4`
    used=`free -m|grep -i mem|tr -s " "|cut -d " " -f3`
    used_rate=`echo "scale=4;$used/$total" |bc`
    #used_1=`echo "$total*0.8"|bc `
    result=` echo "$used_rate>0.8"|bc `
    echo $result
    if (( $result  == 1  ))
    then
        echo -e "\e[31mUsage rate exceeds 80%, please timely expand memory to avoid unnecessary losses\e[0m"
    else
        echo  " nothing to do"
    fi
}
mem
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# bash mem_test.sh
0
 nothing to do
============================================================================================

Knowledge Point 7.1.1 bc Command
Beginner's Tutorial: https://www.runoob.com/linux/linux-comm-bc.html
The bc command is an arbitrary precision calculator language, usually used as a calculator in linux
[root@localhost ~]# yum install bc -y
[root@localhost ~]# used=`free -m|grep -i mem|tr -s " "|cut -d " " -f3`
[root@localhost ~]# total=`free -m|grep -i mem|tr -s " "|cut -d " " -f2`
[root@localhost ~]# echo "scale=2;$used/$total" |bc	# Note: Keep 2 decimal places
.16
[root@localhost ~]# echo "scale=3;$used/$total" |bc	# Note: Keep 3 decimal places
.165
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# free -m|grep -i mem|tr -s " "|cut -d " " -f2 # Note: -i case insensitive
1800
[root@sanchuang-linux ~]# use_rate=`echo "scale=4;$used/$total" |bc`
[root@sanchuang-linux ~]# echo "$use_rate>0.8"|bc	# Note: Returns 0 for false
0
[root@sanchuang-linux ~]# echo "0.7>0.8"|bc			# Note: Returns 0 for false
0
[root@sanchuang-linux ~]# echo "0.9>0.8"|bc			# Note: Returns 1 for true
1		# Note: This return should be a boolean value 0 false 1 true, not the value of command execution failure. $? is all 0, command execution successful
############################################################################################
Knowledge Point 7.1.2 Decimal Operations
Decimal operations:
1. Can use bc
[root@sanchuang-linux ~]# echo "scale=3;1/3"|bc		# Note: Keep 3 decimal places
.333
[root@sanchuang-linux ~]# echo "0.7>0.8"|bc			# Note: Returns 0 for false
0
[root@sanchuang-linux ~]# echo "0.9>0.8"|bc			# Note: Returns 1 for true
1

2. awk Options
Syntax: awk options 'pattern+action' file
Common options:
-F	Specify the delimiter

Built-in variables
NR	Represents the line number in awk
NF	Represents the column number in awk

Pattern

Example
--------------------------------------------------------------------------------------------
[root@localhost ~]# free -m
              total        used        free      shared  buff/cache   available
Mem:           3770         195        3274          11         300        3348
Swap:          2047           0        2047
[root@localhost ~]# free -m|awk 'NR==2{print $2}'		# Note: Print the second variable of the second line
3770													# Note: NR line number, $2 second variable
[root@localhost ~]# free -m|awk 'NR==2{print $3}'		# Note: Print the third variable of the second line
194
============================================================================================

[root@sanchuang-linux ~]# free -m|awk '/Mem/{print $3/$2}'		# Note: Calculate decimal, filter out Mem
0.156111														# Note: Filter out this 1 line of Mem
[root@sanchuang-linux ~]# free -m|awk '/Mem/{printf "%.2f\n", $3/$2}'	# Note: Keep 2 decimal places
0.16															# Note: \n newline output

Method 1
--------------------------------------------------------------------------------------------
scan_ip(){
    for ip in `seq 255`
    do
        ( ip_full=192.168.0.$ip
        ping -c 1 $ip_full &>/dev/null && echo $ip_full >>up.txt || echo $ip_full >>down.txt
        ) &	# Note: Execute in a background subprocess
    done
wait # Parent process waits for child processes to complete before exiting
}
scan_ip

Method 2
--------------------------------------------------------------------------------------------
scan_ip(){
    for ip in 192.168.0.{1..255}					# Note: 1-255 can be written this way
    do
        ( 
        ping -c 1 $ip &>/dev/null && echo $ip >>up.txt || echo $ip >>down.txt
        ) & 
    done
wait # Note: Purpose: Parent process waits for child processes to complete before exiting
}
scan_ip
Note: Background processes
Command &	   Creates a child bash process to execute the command task
wait		Parent process waits for child processes to finish before exiting
============================================================================================

[root@sanchuang-linux ~]# ip=45
[root@sanchuang-linux ~]# ip_full=192.168.0.$ip			# Note: Shell string concatenation
[root@sanchuang-linux ~]# echo $ip_full 
192.168.0.45
[root@sanchuang-linux ~]# top							# Note: View CPU

Example
--------------------------------------------------------------------------------------------
check_nginx(){
    pidof nginx && echo "nginx is running" || echo "nginx is down"
    #if [[ $? -eq 0 ]]
    #then
    #    echo "nginx is running"
    #fi
}
check_nginx
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# pidof nginx
12767 12766 12765
[root@sanchuang-linux ~]# echo $?					# Note: Return value 0 indicates success
0

Example
--------------------------------------------------------------------------------------------
check_monitor(){
    check_num=`differ /etc/passwd /lianxi/passwd |wc -l`
    [[ check_num -eq 0 ]] && echo "File has not been modified" || echo "File has been modified"
}
check_monitor
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# cp /etc/passwd /lianxi/passwd
cp: overwrite '/lianxi/passwd'? y
[root@sanchuang-linux ~]# diff /etc/passwd /lianxi/passwd
[root@sanchuang-linux ~]# echo $?	# Note: Even if the file is not modified, the return is also 0 (understand as command execution success)
0									# Note: Therefore, cannot directly use a ternary-like operation to judge
[root@sanchuang-linux ~]# diff /etc/passwd /lianxi/passwd
[root@sanchuang-linux ~]# diff /etc/passwd /lianxi/passwd|wc
      0       0       0
[root@sanchuang-linux ~]# diff /etc/passwd /lianxi/passwd|wc -l
0
# Note: The basis for judgment is whether diff outputs content
	No output, wc -l line count is 0, indicating the file has not been modified
# Note: To check if the file has been modified, think of the diff command