C2110 UNIX and programming Lesson 8 / Module 1 -1C2110
UNIX and programming
Petr Kulhanek
kulhanek@chemi.muni.cz
National Center for Biomolecular Research, Faculty of Science
Masaryk University, Kamenice 5, CZ-62500 Brno
PS / 2020 Distance form of teaching: Rev2
Lesson 8 / Module 1
C2110 UNIX and programming Lesson 8 / Module 1 -2-
Loop
C2110 UNIX and programming Lesson 8 / Module 1 -3for
Loop
for(initialization;condition;change))
do
command1
...
done
for((initialization;condition;change)); do
command1
...
done
Compact notation:
A loop is a control structure that repeatedly executes a sequence of commands. Repetition
and end of the loop is controlled by a condition.
performed before starting the loop
(counter initialization)
if the condition is met, the commands
command etc. are executed
counter is updated after
executing commands
C2110 UNIX and programming Lesson 8 / Module 1 -4For
Loop and Flowchart
I < N
Yes
No
I = I + 1
N=10
I=0
writevar I
N=10
for((I = 0; I < N; I = I + 1)); do
echo $I
done
counter (variable)
C2110 UNIX and programming Lesson 8 / Module 1 -5for
versus while loop
for((I=1;I <= 10;I++)); do
echo $I
done
I=1
while [[ I –le 10 ]]; do
echo $I
(( I = I + 1 ))
done
performed before starting the loop
(counter initialization)
if the condition is met, the commands in
block do/done are executed
updating the counter after
executing commands
It is possible to change the
counter anywhere in the
body of the loop (even in
multiple places).
In addition to changing the
counter to end of loop, it is
possible to make other changes
in the body of the loop.
However, this is NOT
RECOMMENDED because it
reduces the readability of the
code.
C2110 UNIX and programming Lesson 8 / Module 1 -6for
Loop, Usage
for((I=1;I <= 10;I++)); do
echo $I
done
for((I=10;I >= 1;I--)); do
echo $I
done
Lists numbers 10 to 1Lists numbers 1 to 10
Condition:
Following comparison operators can be
used:
!= does not equal
== equals
< smaller
<= less than or equal to
> larger
>= greater than or equal to
Initialization follows free rules because the
expression is specified in the (()) block.
Variable I has a role of a counter.
Change:
You can use any expression that can be
interpreted in the (( )) block, e.g.:
++ increases value of variable by one
- decreases value of variable by one
more ...
Can only be applied to integers in (( )).
C2110 UNIX and programming Lesson 8 / Module 1 -7for
Loop, Counter Change
If a variable can be interpreted as an integer, the following arithmetic operators can be
used:
++ increases value of variable by one
I++
-- decreases the value of the variable by one
I--
+ adds two values
A = 5 + 6
A = A + 1
- subtracts two values
A = 5 - 6
A = A - 1
* multiplies two values
A = 5 * 6
A = A * 1
/ divides two values ​​(integer division)
A = 5 / 6
A = A / 1
+= adds a value to variable
A += 3
A += B
-= subtracts value from variable
A -= 3
A -= B
*= multiplies variable by value
A *= 3
A *= B
/= divides variable by value
A /= 3
A /= B
A=A+3
C2110 UNIX and programming Lesson 8 / Module 1 -8Nesting
Loops
loop control groups can be freely nested.
for((I=1;I <= 10;I++)); do
for((J=1;J <= 10;J++)); do
echo "$I $J"
done
done
external loop
internal loop
for((I=1;I <= 10;I++)); do
for((J=1;J <= I;J++)); do
echo "$I $J"
done
done
the outer loop counter can
affect the behavior of the inner
loop
The number of nestings is not
limited. It can be combined with
other loops (while, until, for in)
or conditions.
C2110 UNIX and programming Lesson 8 / Module 1 -9Exercise
1
1. Write bash scripts solving Tasks 1, 2 and 3. Replace while loop with for loop. Make user
enter the dimension of rendered shape as the first argument of the script. The script
tests to see if the correct number of arguments is specified and whether the first
argument is an integer greater than zero.
2. Modify the solution of Task 1 so that a rectangle is drawn. The dimensions of the
rectangle will be entered by the user interactively after running the script.
3. Write scripts in bash solving Tasks 4 a 5.
C2110 UNIX and programming Lesson 8 / Module 1 -10Task
1
Print a square of characters in the terminal X. The length of the side of the square is
entered by the user.
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
Ignore the fact that it is not visually a square.
However, number of X characters for a line and
the number of lines must be the same. Possibly
use "X " - X and a space.
C2110 UNIX and programming Lesson 8 / Module 1 -11Task
2
Print a right triangle with X characters in the terminal, so that one perpendicular is located
at the top and the other on the left. The length of the perpendicular is entered by the user.
X X X X X X X X X X
X X X X X X X X X
X X X X X X X X
X X X X X X X
X X X X X X
X X X X X
X X X X
X X X
X X
X
C2110 UNIX and programming Lesson 8 / Module 1 -12Task
3
Print a right triangle with X characters in the terminal, so that one perpendicular is located
at the bottom and the other on the left. The length of the perpendicular is entered by the
user.
X
X X
X X X
X X X X
X X X X X
X X X X X X
X X X X X X X
X X X X X X X X
X X X X X X X X X
X X X X X X X X X X
C2110 UNIX and programming Lesson 8 / Module 1 -13Task
4
In the terminal, print the outline of square with X characters. The length of the side of the
square is entered by the user.
X X X X X X X X X X
X X
X X
X X
X X
X X
X X
X X
X X
X X X X X X X X X X
A suitable solution is to use the script
from Task 1 and check the printing of
characters using appropriately selected
condition.
C2110 UNIX and programming Lesson 8 / Module 1 -14Task
5
In the terminal , print the outline of the square and its diagonals with X characters. The
length of the side of the square is entered by the user.
X X X X X X X X X X
X X X X
X X X X
X X X X
X X X X
X X X X
X X X X
X X X X
X X X X
X X X X X X X X X X
A suitable solution is to use the script
from Task 1 and check the printing of
characters using appropriately selected
condition.