C2110 UNIX and programming Lesson 7 / Module 2 -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 7 / Module 2
C2110 UNIX and programming Lesson 7 / Module 2 -2-
Loops
state
? Yes
No
block 1
change
initialization
Loop block execution
counter (variable)
C2110 UNIX and programming Lesson 7 / Module 2 -3Loop
Using while/until ...
state
? Yes
No
block 1
change
initialization
state
?
Yes
No
block 1
change
initialization
evaluates conditions
at the end loop
evaluates conditions in the
beginning of the loop
C2110 UNIX and programming Lesson 7 / Module 2 -4Loop
using while/until ...
This algorithm has no direct support in
the control structures of bash, its
implementation is possible, but at cost
of poorer readability of resulting code.
complicated
implementation in bash
state
? Yes
No
block 1
change
initialization
state
?
Yes
No
block 1
change
initialization
evaluates conditions
at the end loop
evaluates conditions in the
beginning of the loop
C2110 UNIX and programming Lesson 7 / Module 2 -5Loop
using while/until ...
easy implementation in bash
state
? Yes
No
block 1
change
initialization
state
?
Yes
No
block 1
change
initialization
evaluates conditions
at the end loop
evaluates conditions in the
beginning of the loop
This algorithm has no direct support in
the control structures of bash, its
implementation is possible, but at cost
of poorer readability of resulting code.
C2110 UNIX and programming Lesson 7 / Module 2 -6Loop
Using while/until
while command1
do
command2
...
done
while command1; do
command2
...
done
Compact notation:
the loop is in progress while command1 returns
return value 0 (no error)
A loop is a control structure that repeatedly executes a sequence of commands. Repetition
and end of the loop is controlled by a condition.
until command1; do
command2
...
done
the loop is in progress until command1 will not
return a return value of 0
C2110 UNIX and programming Lesson 7 / Module 2 -7Practical
Example - Loop
I < N
Yes
No
I = I + 1
N = 10
I= 0
writestr
"X"
counter (variable)
N=10
I=0
while [[ I -lt N ]]; do
echo "X"
((I = I + 1))
done
C2110 UNIX and programming Lesson 7 / Module 2 -8Practical
Example - Loop
N=10
I=0
while test "$I" -lt "$N"; do
echo "X"
((I = I + 1))
done
N=10
I=0
while [[ I -lt N ]]; do
echo "X"
((I = I + 1))
done
$ must be used
$ optional, if [[ ]] or (( )) block is used
I < N
Yes
No
I = I + 1
N = 10
I= 0
writestr
"X"
counter (variable)
N=10
I=0
while [[ "$I" –lt "$N" ]]; do
echo "X"
((I = I + 1))
done
C2110 UNIX and programming Lesson 7 / Module 2 -9Exercise
I
1. Write a bash script, which prints N "X" characters to the terminal. The number of
characters will be entered by the user as the first argument of the script. The script
prints an error message if the specified number of characters does not exceed two.
C2110 UNIX and programming Lesson 7 / Module 2 -10Complex
Constructions - Nesting
Bash language does not have a labels and command goto, or their equivalents. Thus, more
complex constructions can be implemented only by immersing loops and conditions into
each other. The level of immersion is not limited.
condition
loop
However, when designing an algorithm/script, we try to avoid unnecessary nesting (mostly
for easier orientation in the script).
! condition
loop
exit
More suitable arrangement,
e.g., for testing input data
from users.
loop
condition
loop I
loop II
C2110 UNIX and programming Lesson 7 / Module 2 -11Nesting
Loops - Example
outer loop counter can affect
behavior of the inner loop
N=10
I=0
while [[ I –lt N ]]; do
J=0
while [[ J –lt I ]]; do
echo –n "X"
((J = J + 1))
done
echo ""
((I = I + 1))
done
For nested structures, we pay attention to indentation of text blocks which increases
clarity and readability of the code. Text editors have built-in support that makes
indentation easier, such as in gedit editor, the indentation of the selected text block can be
achieved with the TAB or Shift + TAB key.
C2110 UNIX and programming Lesson 7 / Module 2 -12Exercise
II
1. Write bash scripts for the following tasks. Make user enter the dimension of the
rendered shape after running the script. While working on the script, employe the
algorithm created from the homework.
C2110 UNIX and programming Lesson 7 / Module 2 -13Task
1
Print a square of X characters into the terminal. 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 on the line and
the number of lines must be the same.
C2110 UNIX and programming Lesson 7 / Module 2 -14Task
2
Print a right triangle with X characters into 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 7 / Module 2 -15Task
3
Print a right triangle with X characters into 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