Shell script syntax

Shell script:
#! /Bin/Bash must pass the first line #! To specify the Current Shell

I. variables:
"": Allows you to reference values of other variables using the $ symbol.
'': Other variables cannot be referenced. $ is considered as a normal character.
'': Assigns the command execution result to the variable.

1. input variables:
Read-P "input prompt:"
$ A # Call and DISPLAY variable

2. Custom Variables
A = "Hello World"
Export a # Set A as a global variable, suitable for all shells supported by the current environment
Echo $ A # display Variables
Unset A # Clear Variables
A = "Haha: $ A" # append a string to
Expr $ A + $ B # Calculate integer variables using expr

3. System numeric variables:
#$0 ~ $9 is a digital variable defined by the system. $0 indicates the current process name.

4. predefined variables:
$: # Current process ID
$ #:# Number of location parameters in the command line
$? : # Status returned after the previous command is executed: 0 indicates normal, non-0 indicates exception
$ *: # Parameters of all locations
$! : # The last process number running in the background

 

Application:
A. # Number of users logged on to the query system. If there are too many users, the system is overloaded.
['Who | WC-l'-le 10] & Echo "yes" the WC parameter is short for word count.

B. # An alarm is triggered when the space usage of the Search/boot partition exceeds the rated size.
# Awk is used to extract information data from files or strings according to certain rules,
# The complete awk script is used to format text file information
A = 'df-HT | grep "/Boot" | awk '{print $6}' | cut-d "%"-F 1'
[$ A-GT 95] & Echo "/the disk capacity occupied by the boot directory is too large"

# Determine whether the input information on the keyboard is equal to the information in the system
Read-P "Location:" mylocation
[$ Mylocation = "mylocation"] & Echo "yes"

# Check whether MySQL is running. If a prompt is displayed, start MySQL.
Service mysqld Status &>/dev/null
If [$? -EQ 0]
Then "MySQL server is running"
Else/etc/init. d/mysqld restart
Fi

# Add 20 system users in batches with the following accounts: "user1", "user2",... "user20"
I = 1
While [$ I-le 20]
Do
Useradd user $ I
Echo "123456" | password -- stdin user $ I &>/dev/null
I = 'expr $ I + 1'
Done

# Calculate the sum of several numbers
Result = 0
While [$ #-GT 0]
Do
Result expr $ reesult + $1
Shift
Done
Echo "result is $ result"

Logical statement:

Test command:
1. test condition expression
2. [conditional expressions]:
[Operator file or directory]:
Operators include:
-D: whether it is a directory
-E: whether it exists
-F: whether the file is used
-R: whether the current user has the read permission
Exercise: [-D/etc/vsftpd]
Echo $?
Or
[-D/etc/vosftpd] & Echo "yes"

If the disk space used exceeds 80%
Then alarm
Fi

If the disk space used exceeds 80%
Then alarm
Else...
Fi

For TM in "moring" "Noon" "Evening"
Do
Echo "the $ TM Of The Day ."
Done

While available memory <100 MB
Do
Available memory
Done

Function
# Define a function and call it
Adder (){
Echo 'expr $1 + $2'
}
Adder12 34
Adder 56 78