C2110 UNIX and programming Lesson 6 / 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: Rev1
Lesson 6 / Module 1
C2110 UNIX and programming Lesson 6 / Module 1 -2-
Variables
C2110 UNIX and programming Lesson 6 / Module 1 -3-
Variables
In Bash language, a variable means named location in the memory that contains a value.
The value of Bash variable is always of string (text) type.
Variable settings:
$ VARIABLE_NAME=value
$ VARIABLE_NAME="value with spaces"
Access to the value of a variable:
$ echo $VARIABLE_NAME
To delete a variable:
$ unset NAME_PROMENNE
Overview of all set variables:
$ set
can not be a gap between 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 6 / Module 1 -4Variable
Setting
$ VARIABLE_NAME="value with spaces"
$ VARIABLE_NAME ="value with spaces"
$ VARIABLE_NAME= "value with spaces"
$ VARIABLE_NAME="value with spaces" program [arg1...]
interpreted as name of program gap interpreted as argument of program
gap
interpreted as the name of the program
VARIABLE_NAME is set to an empty string,
value of the variable is available only to the
running program
you can specify several variables and their
values ​(pairs NAME=VALUE are separated by
a space), which are only available for the
running program
if the program name contains an equal
sign, the name must be enclosed in
quotation marks
C2110 UNIX and programming Lesson 6 / Module 1 -5-
Strings
In the Bash language, four types of strings can be used:
• without quotes
A=try
B=*
C=$A
• with quotation marks
A=“foo bar"
B="* $A"
• with single quotes (apostrophe)
A=‘foo bar'
B='* $ A'
• with inverted single quotes (inverted apostrophe)
A="`ls –d`"
B=“count : `ls | wc -l`"
there is no expansion (it is not an argument, but the
value of a variable)
is replaced by the value of the variable A
is replaced by the value of variable A, the asterisk is
not expanded (it is given in quotation marks)
text is given exactly, without any expansion or
transformation
value of variable contains two words separated by a space
standard output of command indicated in
inverted quotation marks is placed in
between them
C2110 UNIX and programming Lesson 6 / Module 1 -6String/Command
Line Expansion
String/command line expansion order:
input text (string)
expansion of variables
intermediate result
expansion of special
characters
(wildcards)
result
if the text is enclosed in
standard quotation marks
(") or is not contained in a
word that could be
expanded, the expansion
will not be performed
if the text is enclosed
in single quotation
marks ('),
expansion will not be
performed
More details: man bash
C2110 UNIX and programming Lesson 6 / Module 1 -7Exercise
I
1. Set value of variable A to 55.
2. Print the value of the variable A (command echo)
3. List all variables set in the given terminal. Is there a variable A between them? Use the
command less or more to clarify the statement.
4. Use the grep command to list only the line containing the record for variable A. Choose
a search pattern that is independent of the value of the variable.
5. List all set variables whose names begin with the letter A (grep ^TEXT).
6. Change the value of the variable to "this is a long string".
7. Print the value of the variable A.
8. Delete the variable A.
9. Verify that you dleted the variable (following the procedure in step 4).
10. Set variables A, B and C one by one according to the examples on slide 5. Check their
value step by step by set and echo commands. Analyze any discrepancies.
Work in interactive mode of shell.
C2110 UNIX and programming Lesson 6 / Module 1 -8Arithmetic
Operations
C2110 UNIX and programming Lesson 6 / Module 1 -9Arithmetic
operations
Possible entries:
(( I = I + 1 ))
(( I++ ))
Arithmetic operation with obtaining the result:
I=$(( I + 1 ))
echo "Value I increased by one : $(( I + 1 ))"
More details: man bash
value of the result is place to
the position of the 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 6 / Module 1 -10Arithmetic
Operations
Next information: man bash
more appropriate notationPossible entries:
(( I = I + 1 ))
(( I++ ))
Arithmetic operation with obtaining the result:
I=$(( I + 1 ))
echo "Value I increased by one : $(( I + 1 ))"
value of the result is place to
the position of the 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 6 / Module 1 -11-
Operators
= assignment
+ addition
- subtraction
* multiplication
/ integer division
% rest after integer division (modulo)
++ increment (increase value by 1)
–- decrementation (decrease in value by 1)
Example:
A=5
(( B = A / 3 ))
(( C = A % 3 ))
echo $A $B $C
5 1 2
C2110 UNIX and programming Lesson 6 / Module 1 -12Command
expr
Command expr evaluates mathematical expressions, results are printed to
standard output.
More details: man expr
Examples:
$ expr 1 + 2
3
$ expr 2 \* 3
6
A= `expr $ I + 1`
we insert the result into the variable A
\ prevents the special character * from
expanding to the names of files and directories
located in the current directory
we pass the value of the variable,
necessary to use the operator $
Another option is to use the command bc, which can work with real numbers.
C2110 UNIX and programming Lesson 6 / Module 1 -13Exercises
II
1. Write a script in which you set the variables A and B to the values ​​5 and 6. Next, list
the value of their sum, difference, proportion and multiplication using the echo
command.
C2110 UNIX and programming Lesson 6 / Module 1 -14Variables
and Processes
C2110 UNIX and programming Lesson 6 / Module 1 -15-
Processes
Variables
Process: pid, ppid
Process: pid, ppid
Variables
parental process
daughter process
• The first process started after system starts is the process "init/systemd”
• Every command run in shell (command line) is a process
pid = process identifier
ppid = parent process identifier
Process is an instance of a running program.
memory reserved for variables
C2110 UNIX and programming Lesson 6 / Module 1 -16Variables
and Processes
Variables
Process: pid, ppid
Process: pid, ppid
Variables
parental process
daughter process
Each process has a dedicated section for
storing variable values.
The child process at the time of its start gets a copy of
variables (exported) and their values ​​from the parent process.
You can change or delete these variables as needed. It can
also set or delete new variables. However, all the changes
will disappear after the end of the daughter process.
Changes taking place do not manifest on values of original
variables in the parent process.
Variable export:
$ export VARIABLE_NAME
$ export VARIABLE_NAME="value"
export
export with assignment
C2110 UNIX and programming Lesson 6 / Module 1 -17Exercise
III
$ ./print_C
$ C="value 1" ./print_C
$ echo $C
$ C="value 2 "
$ echo $C
$ ./print_C
$ export C
$ ./print_C
1. Clear the PATH variable. How will the change affect command line functionality? Try
running the command ls and pwd. Explain the behavior.
2. When is the expansion of wildcard * taking place in the following example:
3. Write a script called print_C, which prints the value of the C variable. Explain the
behavior in the following examples:
$ B="Contents of directory is *"
$ echo $B
Work in a new terminal.