Loops

for loops and while loop in shell scripting

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. we have for and while loops in the shell.

For Loops:

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

#!/bin/sh
for i in 1 2 3 4 5
do
  echo "Looping ... number $i"
done

#!/bin/sh
for i in hello 1 * 2 goodbye 
do
  echo "Looping ... i is set to $i"
done

Try it without the * and grasp the idea.

While Loops:

#!/bin/sh
INPUT=hello
while [ "$INPUT" != "bye" ]
do
  echo "Please type something in (bye to quit)"
  read INPUT
  echo "You typed: $INPUT"
done

The colon (:) always evaluates to true

#!/bin/sh
while :
do
  echo "Please type something in (^C to quit)"
  read INPUT_STRING
  echo "You typed: $INPUT_STRING"
done

While with case

#!/bin/sh
while read f
do
  case $f in
	hello)		      echo English	;;
	howdy)		      echo American	;;
	gday)		        echo Australian	;;
	bonjour)	      echo French	;;
	"guten tag")	  echo German	;;
	*)		         echo Unknown Language: $f ;;
  esac
done < some_file

Tips: To create multiple file at once

mkdir file_name{1,2,3,new,old}.py

And this can be done recursively, too:

$ cd /
$ ls -ld {,usr,usr/local}/{bin,sbin,lib}