C2110 UNIX and programming 8th lesson -1C2110
UNIX and programming
Petr Kulhánek, Jakub Štěpán
kulhanek@chemi.muni.cz
National Centre for Biomolecular Research, Faculty of Science
Masaryk University, Kotlářská 2, CZ-61137 Brno
CZ.1.07/2.2.00/15.0233
8th Lesson
C2110 UNIX and programming 8th lesson -2-
Contents
 Special variables
• Script arguments
 Input / Formatted output
• Commands printf, read
 Arithmetic operations
• Command expr
 Command return value
 Conditions
• Commands test, exit
C2110 UNIX and programming 8th lesson -3Special
variables
 Script arguments
C2110 UNIX and programming 8th lesson -4Script
arguments
$ bash my_bash_script arg1 arg2 arg3
$./my_bash_script arg1 arg2 arg3
#!/bin/bash
echo "Number of script arguments: $#"
echo "First argument is: $1"
echo "Second argument is: $2"
echo "All script arguments are: $*"
echo "Name of running script is: $0"
3
arg1
arg2
arg1 arg2 arg3
Usage and meaning of arguments designes script author.
./my_bash_script
C2110 UNIX and programming 8th lesson -5-
Overview
Script arguments:
# number of arguments, that were given on script start
0 script name
1 … 9 values of arguments 1 to 9, that were given on script start
* all arguments, that were given on script start
Processes:
? return value of last processed command (process)
$ process identifier (PID)
If script needs more than 9 arguments, it is neccessary to use command shift. This
command removes first argument from list of arguments.
for((I=1;I <= $#;I++)); do
echo $1
shift
done
Advanced arguments management:
Prints all script arguments sequentially.
C2110 UNIX and programming 8th lesson -6-
Exercise
1. Write script, that print number of arguments that were given on script
start.
2. Write script, that print symbols A one next to other. Number of symbols
will be given as first script argument.
3. Write script, that print its process identifier PID. Then pause script
execution for 5 minutes by command sleep. Use command kill in another
terminal to terminate your paused script.
C2110 UNIX and programming 8th lesson -7Input
/ output
 Formated output - printf
 Input - read
C2110 UNIX and programming 8th lesson -8Command
printf
Syntax:
printf [format] [value1] [value2] ...
Further information: man bash, man printf
"Number %5d has value %03d"
This is replaced by value1 in selected format
Command printf prints formatted text and numbers.
This is replaced by value2 in selected format
C2110 UNIX and programming 8th lesson -9Command
printf, příklady
$ I=10
$ B=12.345
$ printf "Variable I value is %d\n" $I
Variable I value is 10
$ printf "Given number B is %10.4f\n" $B
Given number B is 12.3450
$ printf "Given number B is %010.4f\n" $B
Given number B is 00012.3450
$ printf "Given number B is %+010.4f\n" $B
Given number B is +0012.3450
$ printf "Number I is %-5d and number B is %.1f\n" $I $B
Number I is 10 and number B is 12.3
C2110 UNIX and programming 8th lesson -10Command
printf, format
Further information: man bash, man printf
%[flag][length][.precision]type
Total field length
Number of decimal places
(real numbers)
Flag:
- zarovnat doleva
0 prázdné místo zaplnit nulami
+ vždy uvést znaménko
Type:
d integer
s string (text)
f real number
Special symbols:
\n line break
\r return to line begin
%% symbol %
[] – optional part
C2110 UNIX and programming 8th lesson -11Command
read
Command read read text from standard input and save it to variable. Command read while
line and save first word to first variable, ..., rest of line is saved to last variable.
Syntax:
read A # whole line is saved to variable A
read A B # first word is saved to variable A
# rest of line is saved to variable B
Further information: man bash
Example:
echo -n "Write number: "
read A
echo "You wrote number: $A"
Attention: do not use command read together with pipes
echo "text" | read A
echo $A
A does not hold value "text"
C2110 UNIX and programming 8th lesson -12Arithmetic
operations
C2110 UNIX and programming 8th lesson -13Arithmetic
operations
Arithmetic operations with whole numbers may be done in block (( ... )).
Possible input:
(( I = I + 1 ))
(( I++ ))
I=$(( I + 1 ))
echo "Value I incremented by 1 : $(( I + 1 ))"
Operators:
= assignment
+ addition
- sustraction
* multiply
/ division
% division remainder
++ increment by 1
–- decrement by 1
Further information: man bash
Result value is printed to
standard output
C2110 UNIX and programming 8th lesson -14Command
expr
Command expr solve mathematical expressions, results are printed to standard output.
Further information: man expr
Examples:
$ expr 1 + 2
3
$ expr 2 \* 3
6
I=`expr $I + 1`
Result is assigned to variable I
\ prevent special symbol expansion * to file and
directory names in current directory
Variable value is given
C2110 UNIX and programming 8th lesson -15-
Exercise
1. Wtite script, that print first argument in format %4d.
2. Write script, that reads number from standard input and prints it iín following
format: including sign, length will be 5 symbols, empty space will be filled by zeros:
Given number is : +0003
3. What happens if script from exercise 2 is given number: 123456?
4. Write script, that will accept 2 numbers as arguments. These numbers will then be
printed together with their sum.
C2110 UNIX and programming 8th lesson -16-
Conditions
 Command return value
 Conditions
C2110 UNIX and programming 8th lesson -17Command
return value
A process may at the end inform its parent process about its run by return value. Return
value is whole number with values in range 0-255.
Variables
Process: pid, ppid
Process: pid, ppid
Variables
Parent process
Child process
Return value
Return value:
0 = successful run – no error
> 0 = error occured, return value may be used to identify error
Return value of last process command may be obtained by special variable ?.
C2110 UNIX and programming 8th lesson -18Return
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 -19Command
test, whole numbers
Whole numbers comparison:
test number1 operator number2
Operator:
-eq equal to
-ne not equal
-lt less than
-le less or equal
-gt greater than
-ge greater or equal
Further info: man bash, man test
Command test is used to compare values and testing types. If test is successful, then
return value of test command is set to 0.
C2110 UNIX and programming 8th lesson -20Command
test, strings
String testing
test operator string1
Operator:
-n test if length of string is non-zero
-z test if length of string is zero
-f test if string is existing file
-d test if string is existing directory
String comparison
test string1 operator string2
Operator:
== strings are identical
!= strings differ
C2110 UNIX and programming 8th lesson -21-
Conditions
if command1
then
command2
...
fi
if command1; then
command2
...
fi
Compact form:
if command1
then
command2
...
else
command3
...
fi
if command1; then
command2
...
else
command3
...
fi
If command1 has return value 0, then command
command2 is processed. If return value is non-zero,
command3 is processed.
C2110 UNIX and programming 8th lesson -22Command
exit
Command exit end script or interactive session. Optional argument is return value.
#!/bin/bash
if test "$1" –le 0; then
echo "Whole number is not lower then zero!"
exit 1
fi
echo "Number is higher then zero."
exit 0
$ ./my_script 5
Number is higher than zero.
$ echo $?
0
$ ./my_script -10
Number is not lower then zero!"
$ echo $?
1
C2110 UNIX and programming 8th lesson -23-
Nesting
Control structures may be nested.
for((I=1;$I <= 10;I++)); do
for((J=1;$J <= 10;J++)); do
echo $(($I+$J))
done
done
Outer cycle
Inner cycle
condition
cycle
By script design it is advantageous to avoid unneccesary nesting (main reason is better
orientation in script text).
! condition
cycle
exit
Appropriate design to test user
input.
C2110 UNIX and programming 8th lesson -24-
Exercise
1. Write script, that read two numbers from standard input. These numbers will be
printed with information which one is higher (script autor may design output form).
2. Write script, that will accept two numbers as arguments. Script prints these numbers
together with their quotient. Use condition to prevent division by zero.
3. Save list of files and directories that are in your home directory to file list.txt
4. Write script, that accepts file name as argument. Script test if file exists and if yes, then
prints its contents and number of lines in file. Test your script on file list.txt.
C2110 UNIX and programming 8th lesson -25Home
work
1. Print filled square from symbols "X" to terminal. Size of square gives user as script
argument.
2. Rewrite previous script so that it prints only square outlines.
3. Write script, that prints two right-angled triangles in following orientations. Leg length
will be read from user from standard input.
X
X X
X X X
X X X
X X
X
4. Print circle or ring from "X" symbols. Radius and circle / ring option will be read from
user from standard input.