C2110 UNIX operating system and programming basics 7th 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
7th lesson
Bash - scripting language
C2110 UNIX operating system and programming basics 7th lesson -2-
Contents
➢ Scripting in bash
• basic syntax, comparison with flowcharts, script execution
➢ Variables
• setting and accessing the values, variables and processes,
string enterpretation
➢ Input and Output
• read, echo, printf
➢ Arithmetic operations
• operations with integers
C2110 UNIX operating system and programming basics 7th lesson -3Scripting
in Bash
C2110 UNIX operating system and programming basics 7th lesson -4Script
in Bash
#!/bin/bash
# this is a comment
echo 'This is a script in Bash interpreter!'
echo “The content of`pwd` is:"
ls # displey content of directory
A=6 # sets the value of variables A
echo "The value of variable A is $A"
echo “command one"; echo “command two"
./mycommand first_argument second_argument \
third_argument
Orderofcommandexecution
• blank lines are ignored
• text prefixed by # is ignored (used to comment on the script functionality)
• one-line can include multiple commands, commands are separated by a semicolon ;
• one command can be written to multiple lines using backslash \
immediately
followed
by new line
C2110 UNIX operating system and programming basics 7th lesson -5Script/Program
editors
Since the scripts and program source codes are text files, any text editor can be used to
save code in plain text form (without formatting metadata).
Text editors:
⚫ vi
⚫ kwrite
⚫ kate
⚫ gedit
You can also use the specialized development environment - IDE (Integrated Development
Environment) to write script or source code. In addition to the plain editors, the IDE
includes project manager, debugging tools (debugger) and more. It is mostly available for
more complex languages: C, C++, Fortran, JavaScript, Python, PHP, etc.
Development environment:
⚫ Kdevelop
⚫ qtcreator
⚫ NetBeans
⚫ Eclipse
C2110 UNIX operating system and programming basics 7th lesson -6Flowchart
and scripts in bash
Z
K
#!/bin/bash
echo 'This is a script in bash!'
echo “Content of directory:"
ls
exec ls
Commands execution
order
writestr 'This is a script in bash!'
writestr “Content of directory:"
C2110 UNIX operating system and programming basics 7th lesson -7Execution
of scripts
1) Indirect execution
The script is given as argument of language interpreter.
$ bash my_bash_script
Scripts may not have flag x (executable).
2) Direct execution
Script is executed directly (shell automatically starts the interpreter).
$ chmod u+x my_bash_script
$./my_bash_script
Scripts must have x flag (executable) and set interpreter (part of the script).
#!/bin/bash
echo 'This is a script in Bash!'
C2110 UNIX operating system and programming basics 7th lesson -8Interpreter
specification
#!/bin/bash
echo " This is bash script!"
Script in Bash
#!/absolute/path/to/script/interpreter
Specifications of interpreter (the first line of the script):
⚫ If the interpreter of script is not specified during the direct execution, the system shell
interpreter (bash) is used.
⚫ Interpreter specified in the script is ignored during the indirect script execution.
⚫ Interpreter specification is always appropriate in the script because it is used by text
editors to highlight syntax. (file name is specified without suffix or with suffix .sh)
#!/usr/bin/gnuplot
set xrange[0:6]
plot sin(x)
pause -1
Script in gnuplot
C2110 UNIX operating system and programming basics 7th lesson -9Exercise
I
1. Write a bash script that prints the text "current directory is:" followed by an absolute
path to the current directory. Execute the script indirectly by bash.
2. Extend the functionality of the previous script that in addition to the above, it prints
"Contents of directory is: " followed by a list of files and directories in long format.
Execute the script directly.
C2110 UNIX operating system and programming basics 7th lesson -10-
Variables
C2110 UNIX operating system and programming basics 7th lesson -11-
Variables
In Bash, variable is named memory location that contains a value. The value of the Bash
variable is always string (text).
Setting variables:
$ VARIABLE_NAME=value
$ VARIABLE_NAME =“value with space"
Accessing the variable value:
$ echo $VARIABLE_NAME
Deleting variables:
$ unset VARIABLE_NAME
List of all defined variables:
$ set
There must be no spaces between the variable name and a =
"TEXT ${VARIABLE}TEXT"
if the value of the variable is a
part of the larger text, the
variable name is put into
double quotation marks.
C2110 UNIX operating system and programming basics 7th lesson -12Set
variables
$ VARIABLE_NAME=“value with space"
$ VARIABLE_NAME =“value with space"
$ VARIABLE_NAME= “value with space"
$ VARIABLE_NAME=“value with space" program [arg1...]
interpreted as
a program name
space
interpreted as
an program argument
space interpreted as
a program name
VARIABLE_NAME is set
to empty string,the variable value is
available only to executed program.
Several variables with theirs values can be specified (entries VAR=VALUE are separated
by space), that are only available for the running program.
if program name contains an equal sign, the name must be given in quotation marks
C2110 UNIX operating system and programming basics 7th lesson -13-
Strings
Four types of the string can be used in Bash
• without quotation marks
A=magic
B=*
C=$A
• with quotation marks
A="magic trick"
B="* $A"
• with apostrophes
A='magic trick'
B='* $A'
• With grave accents
A="`ls –d`"
B="number : `ls | wc –l`"
Wild card expansion is context dependent.
replaced by the value of the variable A
is replaced by the value of the variable A, the
asterisk is not expanded (in quotation marks)
Exact text without any expansion or transfomation
the value of the variable contains two words
separated by a space
the output of the quoted commas is put
in the inverted quotes
C2110 UNIX operating system and programming basics 7th lesson -14Expansion
of string/command line
Order of string expansion/command line:
text input (string)
expansion of variables
intermediate result
expansion of wild cards
result
The text is not expanded
when it is in quotation
marks(“) or is not included
in a word that could be
expanded, so it does not
concern the assignment of
values ​​to variables
The text is not
expanded, when
it is in apostrophes(‘).
C2110 UNIX operating system and programming basics 7th lesson -15-
Processes
Variables
Process: pid, ppid
Process: pid, ppid
Variables
Parent process
daughter process
• The first process executed after system launch is "init"
• Each command executed in the shell (command line) is command
pid = process identifier
ppid = parent process
identifies
Process is instance of a running program
memory reserved for variables
C2110 UNIX operating system and programming basics 7th lesson -16Proměnné
a procesy
Variables
Process: pid, ppid
Process: pid, ppid
Variables
parent process
daughter process
Each process has a dedicated part for
storing the variable values.
Daughter process at the moment of execution gets the copy
of variables (exported) and theirs values from parent process.
These variables can be changed or removed. The new
variables can be also set or remove new variables. However
all these changes after end of the daughter process are
removed. Changes will not affect the values of the original
variables or parent process.
Export proměnné:
$ export VARIABLE_NAME
$ export VARIABLE_NAME=“value"
export
export with assigning
C2110 UNIX operating system and programming basics 7th lesson -17Exercise
II
1. Set the variable A to value 55
2. Print the value of variable A (echo command)
3. Display all environment variables in the terminal. Is among them A variable? Use the
command less or more to make the listing more transparent.
4. Use grep command and display only the record that contains the variable A. For search,
use the variable value independent pattern.
5. Display all 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. Remove the variable A
9. Verify that the variable was removed (procedure in 4. task).
10. Gradually assign the variables A, B and C according to the examples listed on page 13.
Gradually validate their values by set and echo commands. Analyze any contradictions.
Work in interactive shell mode shell.
Work in shell interactive mode.
C2110 UNIX operating system and programming basics 7th lesson -18Cvičení
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 does the change affect command line functionality? Try
to run ls and pwd. Explain the behavior.
2. When is expanded the wild card * in the following example:
3. Write a script called print_C that prints the value of the variable C. Explain behavior of
script in the following examples:
$ B=“Content of directory is *"
$ echo $B
Work in new terminal
C2110 UNIX operating system and programming basics 7th lesson -19-
Input/Output
C2110 UNIX operating system and programming basics 7th lesson -20Command
read
Command read is used to read text from standard input (ie. to read interactive input) and
to assign it to variables. The command always reads the entire line, first variable stores first
word, …, last variable stores the rest of the line
Syntax:
read A # entire line is stored in the variable A
read A B # first word is stored to variable A
# rest of the line is stored in the variable B
More information: man bash
Příklad:
echo -n “Enter value: "
read A
echo “Entered value is: $A"
Caution: Do not use read and pipe in one command
echo "text" | read A
echo $A
it would not store the value "text"
C2110 UNIX operating system and programming basics 7th lesson -21Script
arguments
$ bash my_bash_script arg1 arg2 arg3
$./my_bash_script arg1 arg2 arg3
#!/bin/bash
echo “The number of given arguments: $#"
echo "The first argument is: : $1"
echo "The second argument is $2"
echo "All given arguments are: $*"
echo “Name of the launched script is: $0"
3
arg1
arg2
arg1 arg2 arg3
Use and meaning of arguments is determined by the author of the script.
./my_bash_script
C2110 UNIX operating system and programming basics 7th lesson -22Script
arguments- variables
Script arguments(special variables):
# the number of arguments with which the script was executed
0 name of the executed script
1 … 9 the values of arguments 1-9, with which the script was executed
* all the arguments with which the script was executed
When more than nine arguments is needed, it is necessary to use shift command. The
command removes first from the given list of arguments
for((I=1;I <= $#;I++)); do
echo $1
shift
done
Advanced work with arguments:
Gradually prints the arguments of the script.
"$@" all the arguments with which the script was started embedded in the
quotation marks (it handles the input where the arguments contain spaces)
Different behavior towards $* or "$*"!!
C2110 UNIX operating system and programming basics 7th lesson -23Exercise
IV
1. Write a script that will ask user for his favorite color, which is then printed on the
screen.
2. Write a script that prints the number of given arguments, and the value of the first
argument
C2110 UNIX operating system and programming basics 7th lesson -24Command
echo
Command echo is used to print of unformatted text to the standard output stream.
The command prints the string in a specified order separated by space.
Syntax:
echo [options] [string1] [string2] ...
Useful options:
-n do not move to the next line
Příklady:
$ echo "Ema" "grinds" "meat."
Ema grinds meat.
$ echo "Ema grinds meat."
Ema grinds meat.
$ echo –n “Enter color: "
$ A=5
$ echo “Value of variable A is $A."
Value of variable A is 5.
C2110 UNIX operating system and programming basics 7th lesson -25Command
printf
Syntax:
printf [format] [value1] [value2] ...
“Number %5d has value %03d"
in this place, it puts value1 in the given format
Command printf is used to print formatted text and number.
in this place, it puts value2 in the given format
C2110 UNIX operating system and programming basics 7th lesson -26Příkaz
printf, příklady
$ I=10
$ B=12.345
$ printf "The value of the variable I is %d\n" $I
The value of the variable I is 10
$ printf " The value of the variable B is %10.4f\n" $B
The value of the variable B is 12.3450
$ printf " The value of the variable B is %010.4f\n" $B
The value of the variable B is 00012.3450
$ printf " The value of the variable B is %+010.4f\n" $B
The value of the variable B is +0012.3450
$ printf “Number I is %-5d and number B is %.1f\n" $I $B
Number I je 10 a number B is 12.3
C2110 UNIX operating system and programming basics 7th lesson -27Command
printf, format
More information: man bash, man printf
% [flags][length][.precision]type
Total length of array
Number of decimal places after
decimal point (real numbers)
Flag:
- align to left
0 empty space filled with zeros
+ always print sign
Type:
d integer
s string (text)
f real number
Wild cards:
\n end of line
\r go back to the beginning of line
%% character %
an optional part
C2110 UNIX operating system and programming basics 7th lesson -28Cvičení
V
1. Write a script that asks the users for their favorite color, which is then printed to the
terminal. Question should be printed in the way that user would enter the color on the
same line as question.
2. Practice printf command by executing the commands listed in examples.
3. Write a script that prints the first argument of the script in the format %4d.
4. Write a script that reads from standard input a number, that will be printed by
following specifications: prints sign, total length of number will be five, empty space
will be filled with zeros:
The entered number is: +0003
5. What if to the script from the exercises 4 we submit number: 123456?
C2110 UNIX operating system and programming basics 7th lesson -29Home
works
C2110 UNIX operating system and programming basics 7th lesson -30Arithmetic
operations
C2110 UNIX operating system and programming basics 7th lesson -31Arithmetic
operations
Arithmetic of integer operations is possible to do in the block (( ... )).
Possible entries:
(( I = I + 1 ))
(( I++ ))
I=$(( I + 1 ))
echo “Value I is increased by one : $(( I + 1 ))"
Operators:
= assignment
+ addition
- subtraction
* multiplication
/ division
% division remainder
++ increment (increase value by 1)
–- decrement (decrease value by 1)
More information: man bash
prints the value of the result
in the place of the entry
C2110 UNIX operating system and programming basics 7th lesson -32Command
expr
Command expr interpretes mathematical expressions, the results are printed to
stadart output.
More information: man expr
Example:
$ expr 1 + 2
3
$ expr 2 \* 3
6
I=`expr $I + 1`
Result is assigned into the variables I
\ prevents the expansion of the special
character * to file and directory names located
in current directory
Work with the values of the variables
Another option is to use the command bc, that can work with real numbers.
C2110 UNIX operating system and programming basics 7th lesson -33Home
work
1. Write a script, which will submit two numbers as arguments. The script prints the
numbers and the sum of the numbers
2. Write a script that asks gradually for two numbers given by user. After entering the
numbers, it prints product of the numbers.