Unix / Linux Shell Scripting Tutorial

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:

for.sh

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

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

for2.sh

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

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:

Looping .... number 1
Looping .... number 2
Looping .... number 3
Looping .... number 4
Looping .... number 5

and, for the second example:

Looping ... i is set to hello
Looping ... i is set to 1
Looping ... i is set to (name of first file in current directory)
    ... etc ...
Looping ... i is set to (name of last file in current directory)
Looping ... i is set to 2
Looping ... i is set to goodbye

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... )

while.sh

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

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 preferrable 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:

while2.sh

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

Another useful trick is the while read f loop. This example uses the case statement, which we'll cover later. It reads from the file myfile, and for each line, tells you what language it thinks is being used. Each line must end with a LF (newline) - if cat myfile doesn't end with a blank line, that final line will not be processed.

while3a.sh

#!/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 < myfile

On many Unix systems, this can be also be done as:

while3b.sh

#!/bin/sh
while f=`line`
do
  .. process f ..
done < myfile

But since the while read f works with any *nix, and doesn't depend on the external program line, the former is preferable. See External Programs to see why this method uses the backtick (`).
Had I referred to $i (not $f) in the default ("Unknown Language") case above - you will get no warnings or errors in this case, even though $i has not been declared or defined. For example:

$ i=THIS_IS_A_BUG
$ export i
$ ./while3.sh something
Unknown Language: THIS_IS_A_BUG
$

So make sure that you avoid typos. This is also another good reason for using ${x} and not just $x - if x="A" and you want to say "A1", you need echo ${x}1, as echo $x1 will try to use the variable x1, which may not exist, or may be set to B2.

I recently found an old thread on Usenet which I had been involved in, where I actually learned more ... Google has it here..

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

mkdir rc{0,1,2,3,4,5,6,S}.d

instead of the more cumbersome:

for runlevel in 0 1 2 3 4 5 6 S
do
  mkdir rc${runlevel}.d
done

And this can be done recursively, too:

$ cd /
$ ls -ld {,usr,usr/local}/{bin,sbin,lib}
drwxr-xr-x    2 root     root         4096 Oct 26 01:00 /bin
drwxr-xr-x    6 root     root         4096 Jan 16 17:09 /lib
drwxr-xr-x    2 root     root         4096 Oct 27 00:02 /sbin
drwxr-xr-x    2 root     root        40960 Jan 16 19:35 usr/bin
drwxr-xr-x   83 root     root        49152 Jan 16 17:23 usr/lib
drwxr-xr-x    2 root     root         4096 Jan 16 22:22 usr/local/bin
drwxr-xr-x    3 root     root         4096 Jan 16 19:17 usr/local/lib
drwxr-xr-x    2 root     root         4096 Dec 28 00:44 usr/local/sbin
drwxr-xr-x    2 root     root         8192 Dec 27 02:10 usr/sbin

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