Meaning of eq, ne, le, ge, lt, gt in Shell script
if [1-ne1];then
…
fi
-eq:equal to
-ne:not equal to
-le:less than or equal to
-ge:greater than or equal to
-lt:less than
-gt: greater than
Shell Scripting
Shell Scripting Basics
Overview: shell is actually an interface between the kernel and the user.
Shell Scripting
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
echo command:
echo “thisisanexampleoftheechocommand!”
The screen will show back the ” thisisanexampleoftheechocommand!”
#Symbol
Used in shell scripts that can contain commented entries
echo “hello”
thisisacommentline.thiswouldnotproceanyoutput!
echo “world!”
The second line is an example of a comment. It will be ignored by the shell and no message will be generated
Variables:
Can be created at any time by simple assignment.
Syntax:
<variablename>-<value>
All variables in Linux are treated as strings
Referring to a variable:
The $ symbol is used to refer to the contents of a variable
variable1=${ variable2}
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.
$readfname
Local and global shell variables
Local variables
When a shell is referenced, only the shell that created it is aware of the variable’s existence
Global variables
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.
Environmental variables:
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
HOME variable
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
$echo$HOME
The PATH variable
Contains a list of directory pathnames delimited by colons, which makes it easy for executable programs to search.
PS1 variable
The PS1 (PromptString1) variable contains the shell prompt, the $ sign
$PS1=”HELLO>”
HELLO>
PS2 variable< /p>
Is the environment variable that sets the value for the second prompt
LOGNAME variable
Contains the user’s registered name
$echo “${LOGNAME}”
SHLVL variable
The variable contains the the current working shelllevel
SHELL variable
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!
Command substitution
Another (non-Pipes) way to use multiple commands on a single command line is through command substitution
echo “thedatais `date`”
expr command
Used to find the value of an arithmetic expression. The output of this command is sent to standard output
$expr4+5
Will display 9 on the screen
Arithmetic Expansion:
You can bracket an expression in $((…)) and compute its value with the following command;
$((expression))
example1
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]–>※※※※※※※※※※※※※※※※※※※※※※※
Conditional execution
test and []
Tests and []
Evaluates expressions and returns true(0) or false()
Numeric tests:
– 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
If Constructs
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
String test
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
File test
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
exit command
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.
example3
Iteration
while construct
while<condition>
do
<command(s)>
done
Command(s)>
Between do and done can only be executed if the condition is true.
until construct
until loop constructs the opposite of a while loop
until loop will continue to execute until the condition is true
for construct
forvariable_namein<list_of_values>
forvariable_namein<list_of_values>
do
…
done
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
forvariable_namein<list_of_variable_namein<list_of_values>
Controlling the execution of a process
Requesting background processing
The symbol used to request background processes is (&)
$wctempfile&
[1]2082
$vinewfile
Checking for background process
The ps (process status) command generates one line of entry for each process for each current activity.
Terminating background processes
To terminate background processes, use kill, as follows
kill278
Seeing how long it took to complete a command
You can use the time command to see how long it took for a command to complete from start to finish
timefine/etc -name “passwd “2>/dev/null/dev/null indicates that error messages are ignored.
Introduction to piping
The vertical bar (|) is the piping character
It’s just the shell: the output of the command before the “|” is sent as input to the command after the “|”
ls -l|more
Combining commands with pipes is powerful!
Shell script to compare number sizes
Your write down is correct, the following is fine
if[[$DEV_SIZE-ge${EXT_LIMIT[0]}&&$DEV_SIZE-le${EXT_LIMIT[1]}]];
if[$DEV_ SIZE-ge${EXT_LIMIT[0]}-a$DEV_SIZE-le${EXT_LIMIT[1]}]]
If you are using [] or [[]] for integer testing, you need to use the symbols -eq or -le or -ge for the comparison operators inside, and you can use the >= and <= comparison operators only inside (()). and the logical operations && and || are used in [[and -a and -o are used in [[and && and || are used in (().
Under Linux, write a shell script that reads 5 integers from the keyboard and then displays the maximum, minimum, and average numbers.
#! /bin/bash
#Input any 5 numbers, determine max, min, and sum
s=0
read-p “pleaseinput: “num
s=$(($s+$num))
max=$num
min=$num
avg=$(($s/5))
foriin`seq4`
do
read-p “pleaseinput: “num
s=$(($s+$num))
if[$num-le$min];then
min=$num
fi
if[$num-ge$max];then
max=$num
fi
done
echomax:$maxmin:$minavg:$avg
< imgclass>
Extended information:
For loop:
General format:
forvarinitem1item2…. .itemN
do
command1
command2
…
commandN
done
Write it on one line:
forvarinitem1item2…. .itemN;docommand1;command2?done;
Write a shell script that reads in 10 parameters and outputs the maximum and minimum values. (Include execution results)
#! /usr/bin/ksh
cal_id=1
num_count=10
max_num=-99999
min_num=-99999
while[$cal_id-le$num_count]
do
readline
if[$cal_id-eq1];then
min_num=$line
fi
if[$line-ge$max_num];then
max_num=$line
< p>fi
if[$line-le$min_num];then
min_num=$line
fi
cal_id=`expr$cal_id+1`
done
clear
echo$ max_num
echo$min_num
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.