C2110 UNIX and programming Lesson 7 / 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: Rev4
Lesson 7 / Module 1
C2110 UNIX and programming Lesson 7 / Module 1 -2Decision
Making
C2110 UNIX and programming Lesson 7 / Module 1 -3I.
Return value of the process
The ending process can communicate information about its run to the parent process with
the use of return values. The return value is an integer that takes values ​​from 0-255.
Variables
Process: pid, ppid
Process: pid, ppid
Variables
parental process
daughter process
return value
Return value:
0 = everything was successful (true)
> 0 = an error has occurred, return value then usually identifies the error (false)
Return value of the last executed command can be found using a variable ?.
C2110 UNIX and programming Lesson 7 / Module 1 -4Return
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 Lesson 7 / Module 1 -5Command
exit
Command exit is used to end a script run or interactive session. The optional argument of
the command is the return value.
#!/bin/bash
if test "$1" -lt 0; then
echo "Number is smaller than zero!"
exit 1
fi
echo "Number is larger or equal zero."
exit 0
$ ./my_script 5
Number is larger or equal to zero.
$ echo $?
0
$ ./my_script -10
Number is smaller than zero!
$ echo $?
1
C2110 UNIX and programming Lesson 7 / Module 1 -6II.
Command test
Binary operators (requiring two arguments):
test argument1 operator argument2
Preferred "alternative" notation:
[[ argument1 operator argument2 ]]
Unary operators (requiring one argument):
test operator argument1
Preferred "alternative" notation:
[[ operator argument1 ]]
Command test is used to compare values ​​and test file and directory types (man bash, man
test). If the test is passed, the return value of the command is set to 0 (true).
there must be spaces
there must be spaces
there must be spaces
there must be spaces
C2110 UNIX and programming Lesson 7 / Module 1 -7Command
test, Integers
Additional information: man bash, man test
Comparing integers:
[[ number1 operator number2 ]]
Operator:
-eq equals
-ne does not equal (not equal)
-lt smaller than (less than)
-le less than or equal to (less or equal)
-gt greater than
-ge greater than or equal to (greater or equal)
!= does not equal
== equals
< smaller
<= less than or equal to
> larger
>= greater than or equal to
Examples:
[[ I -eq 5 ]]
[[ J -le K ]]
is value of variable I equal to 5?
is value of the variable K less than or equal than value of variable K?
when using [[…]] and operators for comparing integers, it is not
necessary to use the $ operator to get the value of a variable
C2110 UNIX and programming Lesson 7 / Module 1 -8Command
test, Strings
String comparison
test string1 operator string2
[[ string1 operator string2 ]]
Operator:
== the strings are identical (= can also be used)
!= strings vary
Examples:
[[ $A == "Hi" ]]
[[ $J != $K ]]
does variable A contain the string “Hi"?
does the variable K contain a different string than the variable K?
The $ operator MUST be used to get the value of a variable.
C2110 UNIX and programming Lesson 7 / Module 1 -9Command
test, Strings II
String testing
test operator string1
[[ operator string1 ]]
Operator:
-n tests whether string does not have zero length
-z tests whether string has zero length
-f tests whether string is the name of an existing file
-d tests whether string is the name of an existing one directory
Examples:
[[ -n $I ]]
[[ -f $K ]]
does the variable I contain a value?
does the K variable contain name of an existing file?
The $ operator MUST be used to get the value of a variable.
C2110 UNIX and programming Lesson 7 / Module 1 -10Manipulating
File/Directory Names
Commands:
basename - prints the file name, potentially removes the extension from the name
dirname - prints the directory name
Commands work with plain text, the names may not
refer to existing files.
Examples:
basename test.txt .txt
basename directory/test.txt
NAME=`basename "$FILE" .doc`
dirname directory/test.txt
DIR=`dirname "$FILE"`
prints "test"
prints "test.txt"
inserts the file name without the .doc extension
from FILE variable into NAME variable
prints "directory"
inserts the directory name from FILE variable
into DIR variable
C2110 UNIX and programming Lesson 7 / Module 1 -11Command
test, Logical Operators
Logical operators:
|| logical or
&& logical and
! negation
Examples:
[[ (num1 operator num2) || (num3 operator num4) ]]
[[ (num1 operator num2) ]] || [[ (num3 operator num4)]]
• More complex conditions can be created using logical operators.
• If we do not know priority of operators or we are not sure, then we use parentheses.
• Bash uses lazy evaluation of conditions, which manifests in evaluating only that part of
the logical condition that must be evaluated to determine the resulting logical value.
same result, different way of interpretation!
I do not recommend using
C2110 UNIX and programming Lesson 7 / Module 1 -12Lazy
evaluation
[[ expr1 || expr2 ]] <-> [[ expr1 ]] || [[ expr2 ]]
If the first expression is true (T), so the result is always
true. Therefore, expr2 is evaluated only if the first
expression is not true.
[[ expr1 && expr2 ]] <-> [[ expr1 ]] && [[ expr2 ]]
If the first expression is false (F), the result is always false.
Therefore, expr2 is evaluated only if the first expression is
true.
F || F = F
F || T = T
T || F = T
T || T = T
F && F = F
F && T = F
T && F = F
T && T = T
Trick:
mkdir directory || exit 1
if command mkdir fails (F), the exit command
is called and the script is terminated
C2110 UNIX and programming Lesson 7 / Module 1 -13Command
test, Examples
[[ (I -ge 5) && (I -le 10) ]]
Is the value of variable I in the interval <5;10>?
[[ (I -lt 5)||(I -gt 10) ]] OR [[ ! ((I -ge 5)&&(I -le 10)) ]]
Is the value of variable I outside the interval <5;10>?
[[ I -ne 0]]
Is the value of the variable I different from zero?
[[ $A == "test“ ]]
Does variable A contain the string "test"?
[[ $A ! = "test“ ]]
Does variable A contain a string other than "test"?
[[ -z $A ]]
Does variable A contain an empty string?
[[ -f $NAME ]]
Is there a file whose name is in the NAME variable?
[[ ! (-d $NAME) ]]
Isn't there a directory whose name is in the NAME variable?
C2110 UNIX and programming Lesson 7 / Module 1 -14[[…]],
test, […]
[[ (I -ge 5) && (I -le 10) ]]
test $I -ge 5 && test $I -le 10
[ ($I -ge 5) && ($I -le 10) ]
[[ -f $I ]]
test -F "$I"
[ -f "$I"]
preferred notation
preferred notation
requires more complicated
notation, the use of the $
operator, and possibly quotation
marks
requires more complicated notation,
the use of the $ operator, and possibly
quotation marks
Details:
• man test
• man bash (CONDITIONAL EXPRESSIONS)
C2110 UNIX and programming Lesson 7 / Module 1 -15-
Conditions
?
Yes
No
block 1
block 2
Conditional block execution
C2110 UNIX and programming Lesson 7 / Module 1 -16-
Conditions
if command1
then
command2
...
fi
if command1; then
command2
...
fi
Compact notations:
if command1
then
command2
...
else
command3
...
fi
if command1; then
command2
...
else
command3
...
fi
If command1 ends with a return value 0, command2
will be performed. Otherwise, command3 will be
executed.
C2110 UNIX and programming Lesson 7 / Module 1 -17Practical
Example - Condition
ok?
No
Yes
create a
directory
E
error
mkdir directory 2> /dev/null
if [[ $? –ne 0 ]]; then
echo "I can't create directory!"
exit 1
fi
if ! mkdir directory 2> /dev/null; then
echo "I can't create directory!"
exit 1
fi
functionally identical
notations
mkdir directory 2> /dev/null
if test $? -ne 0; then
echo "I can't create directory!"
exit 1
fi
C2110 UNIX and programming Lesson 7 / Module 1 -18Exercise
I
1. Try the examples from the previous page. Use the command ls to monitor the
existence of the directory and change commands mkdir and rmdir.
2. Write a script that prints the result of the ratio of two numbers. The user enters the
values ​​interactively after running the script. The script handles possible division by
zero.
3. Write a script that asks for the file name. The script prints an error message if the
file does not exist. Otherwise, it writes it to standard output.