Meaning of eq, ne, le, ge, lt, gt in Shell script
-ne:not equal to
-le:less than or equal to
-ge:greater than or equal to
-gt: greater than
Shell Scripting Basics
Overview: shell is actually an interface between the kernel and the user.
If there is a series of frequently used linux commands, you can store them in a file swell. shenll can read this file and execute the shenll can read this file and execute the commands in it. Such a file becomes a script file.
Executing shell scripts
To create a shell script, you have to write it in a text file using any editor like vi.
In order to use bashshell Rai to execute the script magic, the command is: bashmagic or . /magic
The screen will show back the ” thisisanexampleoftheechocommand!”
Used in shell scripts that can contain commented entries
The second line is an example of a comment. It will be ignored by the shell and no message will be generated
Can be created at any time by simple assignment.
All variables in Linux are treated as strings
Referring to a variable:
The $ symbol is used to refer to the contents of a variable
Reading a value into a variable
When executing shell scripts, the shell also allows the user to read a value into a variable directly from the keyboard, which can also be done using the read command.
Local and global shell variables
When a shell is referenced, only the shell that created it is aware of the variable’s existence
are known as subshells
Variables created in a shell are localized to the shell that created them, unless the shell created them. in the shell that created it, unless specifically noted as global with the export command.
By changing the values of these variables, the user is able to customize the environment
Some examples of environment variables are HOME, PATH, PS1, PS2, LOGNAME, SHLVL, and SHELL
Each user on a Linux system has an associated variable called HOME. Each user on a Linux system has an associated directory called HOME
When a user logs on, he or she enters the corresponding HOME directory
The PATH variable
Contains a list of directory pathnames delimited by colons, which makes it easy for executable programs to search.
The PS1 (PromptString1) variable contains the shell prompt, the $ sign
PS2 variable< /p>
Is the environment variable that sets the value for the second prompt
Contains the user’s registered name
The variable contains the the current working shelllevel
The environment variable stores the user’s default shell
The env command
can be used to view a table of all moved environment variables and their respective values!
Another (non-Pipes) way to use multiple commands on a single command line is through command substitution
echo “thedatais `date`”
Used to find the value of an arithmetic expression. The output of this command is sent to standard output
Will display 9 on the screen
You can bracket an expression in $((…)) and compute its value with the following command;
Write a shell script to count the number of unanswered inquiries at a call center. The script should accept the total number of queries reported for the day and the number of queries answered in order to calculate the number of unanswered queries.
Total of all unanswered inquiries = total of all inquiries – number of answered inquiries
<! –[if!supportLists]–>*<! –[endif]–>※※※※※※※※※※※※※※※※※※※※※※※
test and 
Tests and 
Evaluates expressions and returns true(0) or false()
– eq equals is true
– ne is true if not equal
-gt is true if greater than
-ge is true if greater than or equal to
-lt is true if less than
-le is true if less than or equal to
The Linux shell provides constructs for loops and determinations that can be used in shell scripts
< p>Arithmetic test
Combined with the if construct, it can be used to test the numeric value of a variable
The test command can also be used for strings
=Equal is true
! = not equal is true
-z string length is zero is true
-n string length is non-zero is true
The test command can also be used to check the status of a file
-e file exists is true
-r file exists and is readable is true
-w file true if it exists and is writable
-x file true if it exists and is executable
-s file true if it exists and has at least one character
-d file true if it exists and is a directory
-f file true if it exists and is a normal file
-c file true if it exists and is a character file
True if -b file exists and is a block special file
-a and -o or ! not
To terminate the execution of a shell script and return to the $ prompt
case. . esac
This construct used in shell scripts executes a specific set of commands based on the value of a variable
When the value of a variable matches one of the values. a set of commands written to that value is executed.
Between do and done can only be executed if the condition is true.
until loop constructs the opposite of a while loop
until loop will continue to execute until the condition is true
The for loop takes a list of values as input and executes a loop for each value in the loop
The break and contineu commands
Similar to other languages
The for constructor
Controlling the execution of a process
Requesting background processing
The symbol used to request background processes is (&)
Checking for background process
The ps (process status) command generates one line of entry for each process for each current activity.
Terminating background processes
To terminate background processes, use kill, as follows
Seeing how long it took to complete a command
You can use the time command to see how long it took for a command to complete from start to finish
timefine/etc -name “passwd “2>/dev/null/dev/null indicates that error messages are ignored.
Introduction to piping
The vertical bar (|) is the piping character
It’s just the shell: the output of the command before the “|” is sent as input to the command after the “|”
Combining commands with pipes is powerful!
Shell script to compare number sizes
Your write down is correct, the following is fine
If you are using  or [] for integer testing, you need to use the symbols -eq or -le or -ge for the comparison operators inside, and you can use the >= and <= comparison operators only inside (()). and the logical operations && and || are used in [[and -a and -o are used in [[and && and || are used in (().
Under Linux, write a shell script that reads 5 integers from the keyboard and then displays the maximum, minimum, and average numbers.
#Input any 5 numbers, determine max, min, and sum
read-p “pleaseinput: “num
read-p “pleaseinput: “num
Write it on one line:
Write a shell script that reads in 10 parameters and outputs the maximum and minimum values. (Include execution results)
Basically no more problems
The shell script -d is the catalog file, so what are -e, -f? And "! -e" what does this mean again?
-e means true if filename exists.
-f means true if filename is a regular file.