Standard Linux Text Book

8.5 Repetition

In programming, repetition is also important factor. It repeats same processing and ends it when some conditions are satisfied.

There are three ways for repetition in shell scripts. Let's confirm them surely.

8.5.1 for sentence

'for' sentence enumerates values, repeats processing by using its value.

Format

for variable in list_of_values
do
processing
done

If the result of a command is character strings, you can use its result as list of values.

Example

$ for i in a b c d
> do
> echo $i
> done
a
b
c
d

This is the result by putting 'a b c d' as list of values. In case of 'a', it echo 'a'.(i=a, echo $i) In case of 'b', it echo 'b'.(i=b, echo $i)… If the result of ls command is used as list of values, the 'for' sentence has following description.

for i in `ls`

It gets file and directory names as the ls result, and outputs them to standard output.

8.5.2 while/until Sentence

'while' sentence is used by loop processing that is repeated, while specified conditions are true. (ie. it will end if specified conditions become false) 'until' sentence is entirely reverse. 'until' sentence is used by loop processing that is repeated, while specified conditions are false. (ie. it will end if specified conditions become true)

Format

while conditional_expression
do
processing
done
until conditional_expression
do
processing
done

If you want to implement for-loop process like C language by using shell script, you should use expr command. You can control loop processing, by incrementing (or decrementing) a counting variable.

Example

$ cat loop.sh ↵          loop.sh is created and performed as follows.
#!/bin/bash
count=1                The initial value 1 is set for the counters of variable.
while [ $count -le 10 ]       While shell variable is under 10, the processing is repeated.
do
   echo " This processing was performed $count time(s) "
   count=`expr $count + 1`
done
$ ./loop.sh
This processing was performed 1 time(s) ↵
This processing was performed 2 time(s) ↵
This processing was performed 3 time(s) ↵
This processing was performed 4 time(s) ↵
This processing was performed 5 time(s) ↵
This processing was performed 6 time(s) ↵
This processing was performed 7 time(s) ↵
This processing was performed 8 time(s) ↵
This processing was performed 9 time(s) ↵
This processing was performed 10 time(s)

8.5.3 select Sentence

'select’ sentence prompts numerical value input from user.

Format

select variable in list
do
processing
done

Here is execution example.

select name in "miyahara" "kawai" "okada"
do
echo "You are Mr. $name";
done

Its result is below.

1) miyahara
2) kawai
3) okada
$? 1 Inputted 1
You are Mr. miyahara The scripts from 'do' to 'done' are performed.

8.5.4 Repeat Control

You can control program loop by using 'break' and/or 'continue'. 'break' interrupts program loop, and 'continue' goes back to the top of loop.

Example

while true
do
   echo "Continue? (y/n)"
   read input
   case $input in
      n) break
          ;;
      y) continue
          ;;
      *) echo "Please input y or n."
          ;;
esac
done

Here is the execution result.

$ sample.sh ↵
Continue? (y/n)
y ↵                               Inputted ‘y’
Continue? (y/n)       Goes back to the top of loop
a ↵                               Inputted something except both ‘x’ and ‘y’
Please input y or n.
Continue? (y/n)
n ↵                               Inputted ‘n’
$                                     End this loop

8.5 Repetition

for variable in list_of_values do processing done

$ for i in a b c d > do > echo $i > done a b c d

for i in `ls`

while conditional_expression do processing done until conditional_expression do processing done

select variable in list do processing done

select name in "miyahara" "kawai" "okada" do echo "You are Mr. $name"; done

1) miyahara 2) kawai 3) okada $? 1 Inputted 1 You are Mr. miyahara The scripts from 'do' to 'done' are performed.

while true do echo "Continue? (y/n)" read input case $input in n) break ;; y) continue ;; *) echo "Please input y or n." ;; esac done

$ sample.sh ↵ Continue? (y/n) y ↵ Inputted ‘y’ Continue? (y/n) Goes back to the top of loop a ↵ Inputted something except both ‘x’ and ‘y’ Please input y or n. Continue? (y/n) n ↵ Inputted ‘n’ $ End this loop

for variable in list_of_values
do
processing
done

$ for i in a b c d
> do
> echo $i
> done
a
b
c
d

while conditional_expression
do
processing
done
until conditional_expression
do
processing
done

select variable in list
do
processing
done

select name in "miyahara" "kawai" "okada"
do
echo "You are Mr. $name";
done

1) miyahara
2) kawai
3) okada
$? 1 Inputted 1
You are Mr. miyahara The scripts from 'do' to 'done' are performed.

while true
do
echo "Continue? (y/n)"
read input
case $input in
n) break
;;
y) continue
;;
*) echo "Please input y or n."
;;
esac
done

$ sample.sh ↵
Continue? (y/n)
y ↵ Inputted ‘y’
Continue? (y/n) Goes back to the top of loop
a ↵ Inputted something except both ‘x’ and ‘y’
Please input y or n.
Continue? (y/n)
n ↵ Inputted ‘n’
$ End this loop