C2110 UNIX and programming Lesson 7 -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
Lesson 7
PS / 2020 Distance form of teaching: Rev3
C2110 UNIX and programming Lesson 7 -2Summary
L6
➢ Bash
C2110 UNIX and programming Lesson 7 -3-
Variables
In Bash, a variable means a named location in the memory that contains a value. The value
of a Bash variable is always of string (text) type.
Variable settings:
$ VARIABLE_NAME=value
$ VARIABLE_NAME="value with spaces"
Access to the value of variable:
$ echo $VARIABLE_NAME
To delete a variable:
$ unset VARIABLE_NAME
Overview of all defined variables:
$ set
cannot be a space between the variable name and =
"TEXT${VARIABLE}TEXT"
if the value is to be part of a
text, the variable name is
enclosed in braces
C2110 UNIX and programming Lesson 7 -4Arithmetic
Operations
Possible entries:
(( I = I + 1 ))
(( I++ ))
Arithmetic operation with obtaining the result:
echo "Value I is increased by one : $(( I + 1 ))"
More details: man bash
the value of the result is written
to the position of $ sign
• Arithmetic operations with integers can be performed in ((...)) block.
• Characters in the block are interpreted as variable names. Therefore, it is not necessary
to use $ operator to obtain their value.
• The values ​​of the variables are interpreted as integers. If the conversion fails, a value of
zero is used.
C2110 UNIX and programming Lesson 7 -5-
Input/Output
Input
• Script arguments
• Available in special variables (0, 1, 2, …, 9, #)
• Command read serves for reading text from standard input and storing it in
variables.
Output
• Command echo serves for unformatted printing to standard output current.
• Command printf serves for printing formatted texts and numbers into the standard
output stream.
printf [format] [value1] [value2] ...
echo [options] [string1] [string2] ...
read A # the whole line is stored in variable A
read A B # the first word is stored in variable A
# the rest of the line in variable B
C2110 UNIX and programming Lesson 7 -6New
Subject
➢ Bash
C2110 UNIX and programming Lesson 7 -7Decision
Block
Yes
No
?
Yes
No
block 1
block 2
state?
Yes
No
block 1
change
initialization
Conditional block execution (conditions) Cyclic block execution (loops)
Typical use of decision block
counter (variable)
C2110 UNIX and programming Lesson 7 -8-
Content
➢ Decision block
• conditions, cycles
➢ Decision making
➢ The return value of the process
• exit command
➢ Test command
• comparison operators, logical operators
• simplified notation
➢ Conditions
➢ Loops: while/until