This document discusses looping structures in scripting languages. It covers the for loop and while loop, providing examples of how each can be used. The for loop allows executing commands a specified number of times or over a list of values. The while loop executes commands as long as a condition is true. The document also discusses redirecting input/output of loops, and how to use break and continue commands to control loop execution.

1. REPETITION
STRUCTURES

2. QUESTION
• What are some reasons to repeatedly execute commands?
• What are some tasks that we can/do perform on our machines regularly?

3. LOOPING IN SCRIPTS
• Loops are used to perform a set of commands repeatedly until a condition is met to
terminate them.
• Question: What are some programs you wrote in CIS120 that used loops?
• While we can use the loops the same in each language a distinction has to be made
for scripting.
Programming Scripting

4. FOR LOOP
• The For Loop is a looping
structure used to execute a set
of commands a specified
number of times.
– for var in <list>
do
command
command
done
• In programming you would specify a
range to execute your loops. Ex. 1 -
100, 2-10
• In scripting, for loops are much more
powerful than just executing ranges:
– Range: Run for a predefined set of
numbers
– List: A list of values or files
– List/Range: A list with a naming scheme
– Directory: A directory to manipulate

5. EXAMPLE: FOR LOOPS
• Exercise in Learning Materials
– Identify what the for loop is doing:
• for1.txt
• for2.txt
• for3.txt
• for4.txt

6. WHILE LOOP
• The While Loop is a looping
structure that executes as long
as a condition is true(exit status
of 0).
– while [ condition ]
do
command
command
done
• Question: What are some conditions
that we can execute a while loop over?
• There is another type of loop known as
an Until Loop that executes while a
condition is false.
• Question: What is the exit status being
tested for an Until loop?
• Question: How could we alter a while
loop to behave in the same way as an
Until loop?

7. EXAMPLE: WHILE LOOPS
• Exercise in Learning Materials
– Identify what the while loop is doing:
• while1.txt
• while2.txt
• while3.txt

8. I/O REDIRECTION ON A LOOP
REDIRECT OUTPUT
• for i in 1 2 3 4
do
echo $i
done > loopout.txt
• for file
do
echo “Process $file” > &1
done > output
• while [“$endofdata” –ne TRUE ]
do
…
done 2 > errors
REDIRECT INPUT
• while read n
do
run $n
done < installFile.txt
• You can override redirection of the entire loop’s input or output by explicitly using the
redirect commands.
– <, >

9. BREAK AND CONTINUE
BREAK
• The Break command is often used to
terminate the remainder of a loop
• Used to create a condition where if an
undesirable outcome was to happen
you can test for it and break the loop.
– Ex. If disk space is low stop processing
files immediately
• break
CONTINUE
• The Continue command is used to
terminate the current iteration of a
loop.
• Used to test if an undesirable outcome
happens and then to start a new
iteration of the loop.
– Ex. If you are updating files and one
doesn’t exist, proceed to the next one.
• continue