Linux scripting while do

You can read a text file using read command and while loop as follows (whilereadfile.sh):

Save and close the file. Run it as follows:

Reading A Text File With Separate Fields

You can store above output in two separate fields as follows (whilereadfields.sh):

Run it as follows:

Another useful example for reading and phrasing /etc/passwd file using the while loop (readpasswd.sh):

Save and close the file. Run it as follows:

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7. Loops

Most languages have the concept of loops: If we want to repeat a task twenty times, we don’t want to have to type in the code twenty times, with maybe a slight change each time.
As a result, we have for and while loops in the Bourne shell. This is somewhat fewer features than other languages, but nobody claimed that shell programming has the power of C.

For Loops

for loops iterate through a set of values until the list is exhausted:

Try this code and see what it does. Note that the values can be anything at all:

This is well worth trying. Make sure that you understand what is happening here. Try it without the * and grasp the idea, then re-read the Wildcards section and try it again with the * in place. Try it also in different directories, and with the * surrounded by double quotes, and try it preceded by a backslash ( \* )

In case you don’t have access to a shell at the moment (it is very useful to have a shell to hand whilst reading this tutorial), the results of the above two scripts are:

and, for the second example:

So, as you can see, for simply loops through whatever input it is given, until it runs out of input.

While Loops

while loops can be much more fun! (depending on your idea of fun, and how often you get out of the house. )

What happens here, is that the echo and read statements will run indefinitely until you type «bye» when prompted.
Review Variables — Part I to see why we set INPUT_STRING=hello before testing it. This makes it a repeat loop, not a traditional while loop.

The colon ( : ) always evaluates to true; whilst using this can be necessary sometimes, it is often preferable to use a real exit condition. Compare quitting the above loop with the one below; see which is the more elegant. Also think of some situations in which each one would be more useful than the other:

Another useful trick is the while read loop. This example uses the case statement, which we’ll cover later. It reads from the file myfile.txt , and for each line, tells you what language it thinks is being used.

(note: Each line must end with a LF (newline) — if cat myfile.txt doesn’t end with a blank line, that final line will not be processed.)

This reads the file » myfile.txt «, one line at a time, into the variable » $input_text «. The case statement then checks the value of $input_text . If the word that was read from myfile.txt was «hello» then it echo es the word «English». If it was «gday» then it will echo Australian . If the word (or words) read from a line of myfile.txt don’t match any of the provided patterns, then the catch-all «*» default will display the message «Unknown Language: $input_text» — where of course «$input_text» is the value of the line that it read in from myfile.txt . while3.sh

Let’s say our myfile.txt file contains the following five lines:

A sample run of this script would go like this:

A handy Bash (but not Bourne Shell) tip I learned recently from the Linux From Scratch project is:

instead of the more cumbersome:

And this can be done recursively, too:

We will use while loops further in the Test and Case sections.

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Bash While Loop Examples

H ow do I use bash while loop to repeat specific task under Linux / UNIX operating system? How do I set infinite loops using while statement? Can you provide me the while loop examples?

The bash while loop is a control flow statement that allows code or commands to be executed repeatedly based on a given condition. For example, run echo command 5 times or read text file line by line or evaluate the options passed on the command line for a script.

bash while loop syntax

The syntax is as follows:

command1 to command3 will be executed repeatedly till condition is true. The argument for a while loop can be any boolean expression. Infinite loops occur when the conditional never evaluates to false. Here is the while loop one-liner syntax:

For example following while loop will print welcome 5 times on screen:

And here is above code as a bash while one liner:
x=1; while [ $x -le 5 ]; do echo «Welcome $x times» $(( x++ )); done
Here is a sample shell code to calculate factorial using while loop:

To run just type:
$ chmod +x script.sh
$ ./script.sh 5

The while loop in action on my Ubuntu Linux desktop

You can easily evaluate the options passed on the command line for a script using while loop:

Redirecting user input for a while Loop

How about reading user input from a file? Say you have a file as follows with various IP address:
cat bad-guys.ips.txt
List of crackers IP address:

Here is a bash while loop that read those IP address separated by Internal Field Separator ( $IFS ) to an octothorpe ( # ):

Click to enlarge

How do I use while as infinite loops ?

Infinite for while can be created with empty expressions, such as:

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Conditional while loop exit with break statement

You can do early exit with the break statement inside the whil loop. You can exit from within a WHILE using break. General break statement inside the while loop is as follows:

In this example, the break statement will skip the while loop when user enters -1, otherwise it will keep adding two numbers:

Early continuation with the continue statement

To resume the next iteration of the enclosing WHILE loop use the continue statement as follows:

Recommended readings

We learned that bash while loop executes while a condition is true. See the following resource

• See all sample shell script in our bash shell directory
• Bash loops from our Linux shell scripting tutorial guide
• man bash
• help while
• help <
• help break
• help continue

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Comments on this entry are closed.

Nice to see the Bash shell,
How do you break out of a loop

is there a break statement, or do you have to use a goto?

Yes there is a break and continue statement. man bash has more information.

Very useful examples, thank you!

Thank you so much .
you define very nicly all exampls regarding while loop secquence
statement….

could u please give the shell script for printing the fibonocci series and to know if the number is palindrome and also if it an armstrong number

Does anyone know how to write a script that echoes three different arguments three times using a loop

shift command to traverse through arguments

does anyone can give an example of a while loop within a loop?

The script “test” should set variable “filter_mode” to FALSE if there are no lines in the file “switches” and to TRUE if there exists at least one line in the file “switches”.
Why do I get “filter_mode : FALSE” eventually?

$ cat test
And my script:

Run as:
gg@GeorgSimon:

$ cat switches
switch
gg@GeorgSimon:

$ sh test
switch : switch
filter_mode : TRUE
filter_mode : FALSE

how to write a program in bash for displaying table using until statement? table should be like
1 2 3 4
2 4 6 8
3 6 9 12
4 8 12 16
plz reply soon….thanks

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