C2110 UNIX and Programming 8th lesson -1C2110
UNIX and Programming
Petr Kulhánek
kulhanek@chemi.muni.cz
National Centre for Biomolecular Research, Faculty of Science,
Masaryk University, Kamenice 5, CZ-62500 Brno
8th Lesson
Bash - Control Structures (Conditions, Cycles)
C2110 UNIX and Programming 8th lesson -2-
Contents
➢ Decision making block
• conditions, loops
➢ Return value of processes
• command exit
➢ Command test
• comparison operators, logical operators, simplified script
C2110 UNIX and Programming 8th lesson -3Decision
making block
yes
no
?
yes
no
block 1
block 2
State
? yes
no
block 1
change
initialization
Conditional execution of a block (conditions) Cyclic execution of a block (loops)
Typical use of decision-making block
counter (variable)
C2110 UNIX and Programming 8th lesson -4-
Conditions
if command1
then
command2
...
fi
if command1; then
command2
...
fi
Compact notation:
if command1
then
command2
...
else
command3
...
fi
if command1; then
command2
...
else
command3
...
fi
If command1 exits with return value 0,
command2 is executed, otherwise
command3 is executed.
C2110 UNIX and Programming 8th lesson -5Process
Return Value
Exiting process can share the information about its execution to its parent process by
passing back return value. The return value is an integer in the range 0-255.
Variables
Process: pid, ppid
Process: pid, ppid
Variables
parent process
child process
return value
Return value:
0 = everything was successful (true)
> 0 = error has occurred
the return value then generally identifies the error (false)
The return value of the last executed command is stored in ? variable.
C2110 UNIX and Programming 8th lesson -6Return
Value, Examples
$ mkdir test
$ echo $?
0
$ mkdir test
mkdir: cannot create directory `test': File exists
$ echo $?
1
$ expr 4 + 1
5
$ echo $?
0
$ expr a + 1
expr: non-integer argument
$ echo $?
1
C2110 UNIX and Programming 8th lesson -7test,
Operators for Comparing
Comparison of Integers:
test number1 operator number2
Operator :
-eq equal to
-ne not equal to
-lt less than
-le less than or equal to
-gt greater than
-ge greater than or equal to
Alternative notation:
[[ number1 operator number2 ]]
Additional information:
man bash, man test
The test command is used to compare values and to test types of files and directories (man
bash, man test). If the test passes successfully, the return value of the command is set to 0
(true).
must be spaces
C2110 UNIX and Programming 8th lesson -8test,
Logical Operators
Logical operators:
|| logical or
&& logical and
! negation
Příklady:
[[ (number1 operator number2) || (number3 operator number4) ]]
[[ (number1 operator number2) ]] || [[ (number3 operator number4) ]]
• Logical operators can be used to create more complex conditions.
• If we do not know priority of the operators, we should use parentheses.
• Bash uses lazy evaluation of conditions, which is based on evaluating only the
component of the logical condition that must be evaluated to determine the logical
value of the whole condition.
same result, but different way of interpretation!
We do not recommend
C2110 UNIX and Programming 8th lesson -9Lazy
Evaluation
[[ expression1 || expression2 ]] <-> [[ expression1 ]] || [[ expression2 ]]
If the expression1 is true (T), the overall result is always
true. Therefore expression2 is evaluated only if first
expression is not true.
[[ expression1 && expression2 ]] <-> [[ expression1 ]] && [[ expression2 ]]
If the expression1 is false (F), the overall result is always
false. Therefore expression2 is evaluated only if first
expression is true
F || F = F
F || T = T
T || F = T
T || T = T
F && F = F
F && T = F
T && F = F
T && T = T
Trick:
mkdir directory || exit 1
If command mkdir fails (F), calls the exit and terminates script
C2110 UNIX and Programming 8th lesson -10Practical
Example - Condition
ok?
no
yes
create
directory
K
Error
mkdir dir 2> /dev/null
if [[ $? –ne 0 ]]; then
echo "Can’t create directory!"
exit 1
fi
if ! mkdir dir 2> /dev/null; then
echo "Can’t create directory!"
exit 1
fi
Functionally identical
notation
mkdir dir 2> /dev/null
if test $? –ne 0; then
echo "Can’t create directory!"
exit 1
fi
C2110 UNIX and Programming 8th lesson -11Cycle
via while/until
while command1
do
command2
...
done
while command1; do
command2
...
done
Compact notation:
cycle runs while return value of command1 is 0 in
return (no error)
Cycle (loop) is a control structure that repeatedly executes a series of commands. Both
repeating and exiting of the cycle is managed by a condition.
until command1; do
command2
...
done
cycle runs until return value of command1 is 0
C2110 UNIX and Programming 8th lesson -12Cycle
via while/until...
state
? yes
no
blok 1
change
initialization
state
?
yes
no
blok 1
change
initialization
Evaluation of the
condition at the end of
cycle
This algorithm has no direct support in bash
control structures, its implementation is
possible, but at the cost of worse readability
of the final code.
easy transformation
complicated implementation
C2110 UNIX and Programming 8th lesson -13Practical
Example - Cycle
I < N
yes
no
I = I + 1
N=10
I=0
writestr
"X"
counter (variable)
N=10
I=0
while test "$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
N=10
I=0
while [[ I –lt N ]]; do
echo "X"
((I = I + 1))
done
$ must be uses
$ is optional when using [[ ]] or (( )) block
C2110 UNIX and Programming 8th lesson -14More
Complex Structures - Nesting
Bash does not embody labels and goto command, nor its equivalent. Thus, it is necessary
to use nested loops or conditions to create more complex structures. The level of nesting is
not limited.
condition
loop
We try to avoid unnecessary nesting in script/algorithm design (mostly for easier
orientation in the script).
! condition
loop
exit
Better arrangement of
blocks, e.g. for testing user
data input.
C2110 UNIX and Programming 8th lesson -15exit
Command
exit command is used to terminate the execution of the script or the interactive session.
Optional argument of the command is return value.
#!/bin/bash
if test "$1" –lt 0; then
echo "The number is less than zero!"
exit 1
fi
echo "The number is greater than or equal to zero."
exit 0
$ ./my_script 5
The number is greater than or equal to zero.
$ echo $?
0
$ ./my_script -10
The number is less than zero!
$ echo $?
1
C2110 UNIX and Programming 8th lesson -16Nesting
Cycles - Example
counter of outer cycle can affect
behavior of the inner cycle
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
Indentation of text blocks leads to increased clarity and readability of the code and should
be maintained especially with nested structures.
Indentation is usually supported in text editors. For example, gedit can offset the marked
text block by shortcut TAB or Shift + TAB.
C2110 UNIX and Programming 8th lesson -17Exercise
I
1. Write bash scripts for the following tasks. Size of the plotted shape is entered
interactively by the user after launching of the script.
C2110 UNIX and Programming 8th lesson -18Task
1
Print a square composed from X characters to the terminal. Side length 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
Please ignore the fact, that it is not visually a
square. Number of X characters per line and the
number of lines must be the same.
C2110 UNIX and Programming 8th lesson -19Task
2
Print a triangle composed from X characters to the terminal. Legs of triangle are placed at
left and top of the triangle. Leg length of the triangle 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 8th lesson -20Task
3
Print a triangle composed from X characters to the terminal. Legs of triangle are placed at
left and bottom of the triangle. Leg length of the triangle 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 8th lesson -21Task
4
Print a square outline composed from X characters to the terminal. Side length 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
Please ignore the fact, that it is not visually a
square. Number of X characters per line and the
number of lines must be the same.
C2110 UNIX and Programming 8th lesson -22Task
5
Print a square outline and its diagonals composed from X characters to the terminal. Side
length of the square is entered by he 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
Please ignore the fact, that it is not visually a
square. Number of X characters per line and the
number of lines must be the same.
C2110 UNIX and Programming 8th lesson -23-
Homeworks
C2110 UNIX and Programming 8th lesson -24-
Homeworks
Instructions:
1. Listed tasks are for advanced students.
2. The goal of the tasks is to develop your ability to solve problems that are seemingly
unsolvable from the point of available options and resources. In case of bash
language, this involves mainly the possibility to work only with integer arithmetic and
limited way of rendering into the terminal
Tasks:
1. Draw a circle using character "X". The radius of the circle is entered by the user after
starting of the script.
2. Draw a circle outline using character "X". The radius of the circle is entered by the user
after starting of the script