maggicl/theshell.ch
1
0
Fork 0
Code Issues Pull requests Projects Releases Wiki Activity

Team Scripting: for-loop page added

Browse source 
git-svn-id: svn+ssh://atelier.inf.usi.ch/home/bevilj/group-1@166 a672b425-5310-4d7a-af5c-997e18724b81
This commit is contained in:
omenem 2018-11-13 21:59:36 +00:00
parent 60555b37a6
commit f79da2c9fc
2 changed files with 91 additions and 17 deletions

67
site/pages/scripts/for-loop.html Normal file
View file

@ -0,0 +1,67 @@
---
layout: page
category-page: scripts
category:title Scripting
tags: loop for done script scripting
author: Matteo Omenetti
title: For Loop
---
<p>
<!-- Introduction -->
The second type of loops are for loops.
They follow this sintax:
<pre>
for [variable] in [list] do
[code]
done
</pre>
Their purpose is to <i>iterate</i> over a list. Also while loops could do this, you might argue... <br>
Of course they could, but for loops are specifically meant to do this. Therefore, for instance,
you don't have to decleare your counter variable outside the loop. Most importantly,
this variable can be accessed from inside the loop. <br>
<br>
For loops take this form:
<pre>
for VARIABLE in 1 2 3 4 5 .. N
do
command1
command2
commandN
done
</pre>
<!-- End of Introduction -->
<!-- First Example -->
Here is a simple example:
<pre>
for i in 1 2 3 4 5
do
echo "Welcome $i times"
done
</pre>
This first example of code simply displays a welcome message 5 times.
The output of this piece of code is:
<pre>
Welcome 1 times
Welcome 2 times
Welcome 3 times
Welcome 4 times
Welcome 5 times
</pre>
<!-- End of First Example -->
<!-- Explenation of Numerical Range -->
There are also other ways to specify the <i> numerical range </i>. For instance, if your numerical range
is too big, you can simply write: <code> {1..100} </code>. This piece of code means every natural number
between 1 and 100 (both included). <br>
Ranges can also count backward like this: <code>{10..1}</code>.
You can even icrement the numerical value by step of two: <code> {0..10..2} </code>.
This piece of code means every natural number between 0 and 10 with a step of two,
0 2 4 6... 10.
<!-- End of Explenation of Numerical Range -->
</p>

41
site/pages/scripts/while-loop.html
View file

@ -6,12 +6,13 @@ tags: loop while do script scripting read
author: Matteo Omenetti
title: While Loop
---
<p>
<!-- Introduction -->
Loops are an important concept in programming and therefore also in scripting. Thanks to loops you are able to repeat an instruction
automatically several times, until a certain condition turns false. <br>
Two are the main types of loops: while and for. They both generate a repeating piece of code, but with some key differences
that make them suitable for some different needs while programming. <br>
that make them suitable for different needs while programming. <br>
While loops take this form:
<pre>
@ -38,9 +39,10 @@ While loops take this form:
</pre>
In this first example, you simply create a variable called i and evaluate it to 0.
Then you acces the while loop: the condition <code> [$i -lt 4] </code> means while the varibale
i is less than 4. every cycle of this loop, you print out the value of variable i with <code> echo $i </code>
and finally you increase its value by 1 with <code> i=$((i + 1)) </code>.
Then you acces the while loop: the condition <code> [$i -lt 4] </code> means that this while
loop will run until the <code> i </code> varibale is less than 4.
Every cycle of this loop, you print out the value of variable i with <code> echo $i </code>
and finally you increase its value by 1 with <code> i=$((i + 1)) </code>.
Therefore in 4 cycles the value of i will be 4. This will make the condition of the while loop false.
The output of this piece of code is:
<pre>
@ -51,10 +53,10 @@ While loops take this form:
</pre>
<!-- End of First Example -->
<!-- Second Example -->
Sometimes it required to decleare infinite loops for various programming purposes. <br>
Here is an example:
<!-- Infinite Loop First Example -->
Sometimes it is required to decleare infinite loops for various programming purposes. <br>
Here is an example:
<pre>
i=1
@ -76,16 +78,16 @@ While loops take this form:
i=$((i + 1))
done
</pre>
No termination codinition is set for this loop in this example. This type of loop is called infinite loop.
No termination condition is set for this loop in this example. This type of loop is called infinite loop.
The <code> exit </code> statement is used to quit the loop. This loop will iterate for 9 times, then
as soon as i becomes equal to 0, the condtition of the last if statement will evaluate to true and the
as soon as <code> i </code> becomes equal to 0, the condtition of the last if statement will evaluate to true and the
loop will be terminated. <br>
The output of this piece of code is:
<pre>
1: Hello World
2: Hello World
3: Hello World
I Love programming
I love programming
4: Hello World
5: Hello World
I love Bash
@ -95,30 +97,35 @@ While loops take this form:
8: Hello World
9: Hello World
</pre>
<!-- End of Second Example -->
<!-- End of Infinite Loop First Example -->
<!-- Infinite Loop Second Example -->
If you want your shell to hang forever doing nothing you can write out the following infinite loop:
<pre>
while :
do
:
done
</pre>
<!-- End of Infinite Loop Second Example -->
In scripting, while loops, are often used to process files line by line. <br>
<!-- Read -->
In scripting, while loops are often used to process files line by line. <br>
Here is an examaple:
<pre>
while read -r first_name last_name phone;
do
printf '%s\n' "$last_name"
done < "$file"
done <!-- Find out how to write "<" --> "$file"
</pre>
The <code> read </code> command is used to read file line by line. The flag <code> -r </code> is used to tell the
The <code> read </code> command is used to read a file line by line. The flag <code> -r </code> is used to tell the
command read to intepret backslashes (/) literally, instead as escape characters. This command, expect for some few
rare cases, should always be used with this flag.
In this example, <code> < "$file" </code> redirects the loop's input from a file whose name is stored in a variable.
In this example, <code> <!-- Find out how to write "<" --> "$file" </code> redirects the loop's input from a file whose name is stored in a variable.
This file has 3 colums, <code> first_name last_name phone </code>, separated by blank space (or a tab).
This piece of code only prints out the second column.
<!-- End of Read -->
</p>