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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:
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 $i" done
This is well worth trying. Make sure that you understand what is happening
here. Try it without the *
and grasp the idea, then re-read
the Wildcards section and try it again with
the *
in place. Try it also in different directories, and with the *
surrounded by
double quotes, and try it preceded by a backslash (*
)
In case you don't have access to a shell at the moment (it is very useful to have a shell to hand whilst reading this tutorial), the results of the above two scripts are:
Looping .... number 1 Looping .... number 2 Looping .... number 3 Looping .... number 4 Looping .... number 5
and, for the second example:
Looping ... i is set to hello Looping ... i is set to 1 Looping ... i is set to (name of first file in current directory) ... etc ... Looping ... i is set to (name of last file in current directory) Looping ... i is set to 2 Looping ... i is set to goodbye
So, as you can see, for
simply loops through whatever input it is given, until it runs out of input.
While Loops
while
loops can be much more fun! (depending on your idea of fun, and how
often you get out of the house... )
while.sh
#!/bin/sh INPUT_STRING=hello while [ "$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 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:
while2.sh
#!/bin/sh while : do echo "Please type something in (^C to quit)" read INPUT_STRING echo "You typed: $INPUT_STRING" done
Another useful trick is the while read
loop. This example uses the case statement,
which we'll cover later. It reads from the file myfile.txt
, and for each line, tells you what language it thinks is being used.
(note: Each line must end with a LF (newline) - if cat myfile.txt
doesn't end with a blank line, that final line will not be processed.)
This reads the file "myfile.txt
", one line at a time, into the variable "$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
.
while3a.sh
#!/bin/sh while read input_text do case $input_text in hello) echo English ;; howdy) echo American ;; gday) echo Australian ;; bonjour) echo French ;; "guten tag") echo German ;; *) echo Unknown Language: $input_text ;; esac done < myfile.txt
Let's say our myfile.txt
file contains the following five lines:
this file is called myfile.txt and we are using it as an example input. hello gday bonjour hola
A sample run of this script would go like this:
$ ./while3a.sh Unknown Language: this file is called myfile.txt and we are using it as an example input. English Australian French Unknown Language: hola
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.


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