C2110 UNIX and programming Lesson 7 / Module 2 -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: Rev3
Lesson 6 / Module 2
C2110 UNIX and programming Lesson 7 / Module 2 -2-
Input/Output
C2110 UNIX and programming Lesson 7 / Module 2 -3Output:
Command echo
Command echo used for unformatted output to standard output stream.
The command prints the strings in the specified order, separated by a space.
Syntax:
echo [options] [string2] [string2] ...
Useful choices:
-n does not line out the output
Examples:
$ echo “Horse" “jumps" “high."
Horse jumps high.
$ echo “Horse jumps high."
Horse jumps high.
$ echo –n "Enter color: "
$ A=5
$ echo "Value of variable A is $A."
Value of variable A is 5.
C2110 UNIX and programming Lesson 7 / Module 2 -4Exercise
I
1. In interactive mode, try the examples from the previous page.
C2110 UNIX and programming Lesson 7 / Module 2 -5Input:
Command read
Command read is used to read text from the standard input (i.e. for interactive loading of
input) and its storage in variables. The command always reads the whole line, the first
word is stored in the first variable, ..., the rest of the line is stored in the last variable.
Syntax:
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
Example:
echo -n "Enter value:"
read A
echo “Entered value is: $A"
Attention: do not use the command read in combination with pipes
echo "text“ | read A
echo $A
A won’t contain the value "text". See the
validity of variables in child processes.
C2110 UNIX and programming Lesson 7 / Module 2 -6Input:
Script Arguments
$ bash my_bash_script arg1 arg2 arg3
$./my_bash_script arg1 arg2 arg3
#!/bin/bash
echo "Number of entered arguments: $#"
echo "First argument is: $1"
echo "Second argument is: $2"
echo "All assigned arguments are: $*"
echo "Name of script: $0"
3
arg1
arg2
arg1 arg2 arg3
The use and meaning of arguments is determined by the author of the script.
./my_bash_script
C2110 UNIX and programming Lesson 7 / Module 2 -7Script
Arguments - Variables
Script arguments (special variable names):
# number of arguments with which the script was run
0 name of the running script
1 … 9 values ​​of arguments 1 to 9 with which the script was run
* all arguments with which the script was run
If we need to pass more than nine arguments, we must use the command shift. The
command removes the first argument from the argument list.
for ((I=1; I<=$#; I++)); do
echo $1
shift
done
Advanced work with arguments:
Sequentially lists entered script arguments.
"$@“ all arguments with which the script was run enclosed in quotation marks
(handles input where arguments contain spaces)
Different behavior towards $* or "$*"!
C2110 UNIX and programming Lesson 7 / Module 2 -8Exercise
II
1. Write a script that asks the user for his favorite color and then writes it into the
terminal.
2. Write a script that prints the number of entered arguments and the value of the first
argument.
C2110 UNIX and programming Lesson 7 / Module 2 -9Output:
command printf
Syntax:
printf [format] [value1] [value2] ...
Further information: man bash, man printf
"Number %5d has value %03d"
here insert value1 in the given format
Command printf used to print formatted texts and numbers.
here Insert value2 in the given format
C2110 UNIX and programming Lesson 7 / Module 2 -10Command
printf, Format
More information: man bash, man printf
%[flag][length][.accuracy]type
field for total length
number of decimal places
(real numbers)
Flags:
- align left
0 fill the blank space with zeros
+ always indicate the sign
Type:
d integer
s string (text)
f real number
Special characters:
\n end of line
\r return to beginning of line
%% % sign
[] - optional part
C2110 UNIX and programming Lesson 7 / Module 2 -11Command
printf, Examples
$ I=10
$ B=12.345
$ printf "Value of variable I is %d\n" $I
Value of variable I is 10
$ printf “Entered number B is %10.4f\n" $B
Entered number B is 12.3450
$ printf “Entered number B is %010.4f\n" $B
Entered number B is 00012.3450
$ printf “Entered number B is %+010.4f\n" $B
Entered number B is +0012.3450
$ printf "Number I is %-5d and number B is %.1f\n" $I $B
The number I is 10 and the number B is 12.3
C2110 UNIX and programming Lesson 7 / Module 2 -12Exercise
III
1. Write a script that asks the user for his favorite color and then writes it into the
terminal. Print the query so that the color you enter is on the same line as the query.
2. Practice the command printf by executing the commands listed in the examples.
3. Write a script that prints the first script argument entered in %4d format.
4. Write a script that reads a number from standard input and prints it as follows: a sign
will be given, five places will be used for the output, empty spaces will be filled with
zeros:
Entered number is: +0003
5. What happens if you submit the number 123456 to the script from Task 4?
6. Write a script to which takes two numbers as arguments. The script prints these
numbers and then prints their sum.
7. Write a script which in turns asks the user for two numbers. Then it lists their product.