![Loop
#2) while loop:
Linux scripting while loop is similar to C language while loop. There is a condition in
while. And commands are executed till the condition is valid. Once condition becomes
false, loop terminates.
Syntax: while [condition]
do
commands
done
Example: printing number in reverse order. [10 to 0]
#!/bin/sh
i=10;
while [ $i -le 0 ]
do
echo “$i”
let i-- ;
done
while infinite loop:](https://image.slidesharecdn.com/8-221112091703-b8d9e8c3/85/Case-Loop-Command-line-args-un-Unix-8-320.jpg)
![Loop
#3) Until loop:
It is similar to while loop. The only difference is that until statement executes its code
block while its conditional expression is false, and while statement executes its code
block while its conditional expression is true.
Syntax: until [ condition ];
do
commands
done
Example: example to display number from 5 to 15.
#!/bin/sh
i=5;
Until [ $i –gt 15 ];
do
Echo “$i”
i=$(( i+1 ))
done](https://image.slidesharecdn.com/8-221112091703-b8d9e8c3/85/Case-Loop-Command-line-args-un-Unix-10-320.jpg)
Case, Loop & Command line args un Unix
- 1. Bash Case Statement The Bash case statement is the simplest form of IF-THEN-ELSE with many ELIF elements. Using the case statement makes our bash script more readable and easier to maintain. These are generally applied to simplify the complex conditions having multiple different choices. The Bash case statement follows a similar logic as the Javascript or C switch statement. There is a slight difference, as follows: The Bash case statement takes a value once and tests that value multiple times. It stops searching for a pattern once it has found it and executed the statement linked with it, which is almost opposite in case of the C switch statement.
- 2. Bash Case Statement Conti… Case Statement Syntax Patterns A pattern and its commands make a clause, which ends with ;;. Patterns support special characters. The ) terminates a pattern list. The | operator separates multiple patterns. case expression in pattern_1) statements ;; pattern_2) statements ;; pattern_3|pattern_4|pattern_5) statements ;; pattern-n) statements ;; *) statements ;; esac
- 3. Bash Case Statement Conti… There are some key points of bash case statements: Each case statement in bash starts with the 'case' keyword, followed by the case expression and 'in' keyword. The case statement is closed by 'esac' keyword. We can apply multiple patterns separated by | operator. The ) operator indicates the termination of a pattern list. A pattern containing the statements is referred to as a clause, and it must be terminated by double semicolon (;;). An asterisk symbol (*) is used as a final pattern to define the default case. It is used as a default case when used as the last case.
- 4. Bash Case Statement Conti… Example: #!/bin/bash echo "Do you know Unix Operating System?" read -p "Yes/No? :" Answer case $Answer in Yes|yes|y|Y) echo "That's amazing." echo ;; No|no|N|n) echo "It's easy. Let's start learning from Online Platform." ;; esac #!/bin/bash echo "enter a and b" read a
- 5. Bash Case Statement Conti… Example:To Perform Arithmetic Operation #!/bin/sh echo "enter a and b" read a read b echo "enter operation to be performed" read op case $op in +) c=`expr $a + $b` ;; -) c=`expr $a - $b` ;; /) c=`expr $a / $b` ;; *) c=`expr $a * $b` ;; *) echo "no valid operation specified" ;; esac echo Result after performing operation on a and b is echo $c
- 6. Loop #1) Unix For loop statement Sytax: For varname in list do Echo “Statement” done Keywords are for, in, do, done List is a list of variables which are separated by spaces. If list is not mentioned in the for statement, then it takes the positional parameter value that were passed into the shell. Varname is any variable assumed by the user. Example: Print table of 2. using for loop #!/bin/sh for num in {2..20.2} do Echo “Table of 2 : $num” done
- 7. Loop Syntax of for like C programming language. Syntax: for (( cond1; cond2; cond3 )) do echo “Statement” done Look at the above snapshot, condition1 indicates initialization, cond2 indicates condition and cond3 indicates updation. #!/bash/sh For (( num=1; num<=10; num++ )) do echo “$num” done
- 8. Loop #2) while loop: Linux scripting while loop is similar to C language while loop. There is a condition in while. And commands are executed till the condition is valid. Once condition becomes false, loop terminates. Syntax: while [condition] do commands done Example: printing number in reverse order. [10 to 0] #!/bin/sh i=10; while [ $i -le 0 ] do echo “$i” let i-- ; done while infinite loop:
- 9. Loop while infinite loop: Infinite loop is also called endless loop. It is made with while true (it means condition will always be true) or while : (it means an empty expression), where colon (:) is equivalent to no operation. Example: #!/bash/sh while true or while : do Echo “This is infinite loop. Please ctrl + c to Exit.” done
- 10. Loop #3) Until loop: It is similar to while loop. The only difference is that until statement executes its code block while its conditional expression is false, and while statement executes its code block while its conditional expression is true. Syntax: until [ condition ]; do commands done Example: example to display number from 5 to 15. #!/bin/sh i=5; Until [ $i –gt 15 ]; do Echo “$i” i=$(( i+1 )) done
- 11. Command line argument The Unix shell is used to run commands, and it allows users to pass run time arguments to these commands. These arguments, also known as command line parameters, that allows the users to either control the flow of the command or to specify the input data for the command. While running a command, the user can pass a variable number of parameters in the command line. Within the command script, the passed parameters are accessible using ‘positional parameters’. These range from $0 to $9, where $0 refers to the name of the command itself, and $1 to $9 are the first through to the ninth parameter, depending on how many parameters were actually passed. Example: $ Hello student this is command line argement Here $0 would be assigned sh $1 would be assigned Hello $2 would be assigned student And so on … We will now look at some additional commands to process these parameters. #1) set
- 12. Command line argument We will now look at some additional commands to process these parameters. #1) set This command can be used to set the values of the positional parameters on the command line. Example: $ set Welcome to Unix World $ echo $1 $2 Welcome to Here, “Welcome” was assigned to $1 and “to” was assigned to $2 and so on. #2) shift This command is used to shift the position of the positional parameters. i.e. $2 will be shifted to $1 all the way to the tenth parameter being shifted to $9. Note that if in case there are more than 9 parameters, this mechanism can be used to read beyond the 9th. Example: $ set hello good morning how do you do welcome to Unix tutorial. Here, ‘hello’ is assigned to $1, ‘good’ to $2 and so on to ‘to’ being assigned to $9. Now the shift command can be used to shift the parameters ‘N’ places.
- 13. Command line argument Example: $ shift 2 $ echo $1Now $1 will be ‘morning’ and so on to $8 being ‘unix’ and $9 being ‘tutorial’.