Logical Constructs

• Use test conditions, which decides what to do next based on the true or false status of the condition.

The shell offers two logical constructs: if-then-fi case

Exit Codes (exit values)

• Refer to the value returned by a command when it finishes execution.
  • Value is based on the outcome of the command.
• If the command runs successfully, you typically get a zero exit code(return code), otherwise you get a non-zero value.
• Return code is stored in a special shell parameter called ? (question mark).

Let’s look at the following two examples to understand their usage:

[root@server30 ~]# pwd
/root
[root@server30 ~]# echo $?
0
[root@server30 ~]# man
What manual page do you want?
For example, try 'man man'.
[root@server30 ~]# echo $?
1

• Exit code was returned and stored in the ?.
• Non-zero exit code was stored in ? with an error.
• Can define exit codes within a script at different locations to help debug the script by knowing where exactly it terminated.

Test Conditions

• Used in logical constructs to decide what to do next.
• Can be set on integer values, string values, or files using the test command or by enclosing them within the square brackets [].

Operation on Integer Value

integer1 -eq (-ne) integer2

• Integer1 is equal (not equal) to integer2

integer1 -lt (-gt) integer2

• Integer1 is less (greater) than integer2

integer1 -le (-ge) integer2

• Integer1 is less (greater) than or equal to integer2

Operation on String Value

string1=(!=)string2

• Tests whether the two strings are identical (not identical)

-l string or -z string

• Tests whether the string length is zero

string or -n string

• Tests whether the string length is non-zero

Operation on File

-b (-c) file

• Tests whether the file is a block (character) device file

-d (-f) file

• Tests whether the file is a directory (normal file)

-e (-s) file

• Tests whether the file exists (non-empty)

-L file

• Tests whether the file is a symlink

-r (-w) (-x) file

• Tests whether the file is readable (writable) (executable)

-u (-g) (-k) file

• Tests whether the file has the setuid (setgid) (sticky) bit

file1 -nt (-ot) file2

• Tests whether file1 is newer (older) than file2

Logical Operators

!

• The logical NOT operator

-a or && (two ampersand characters)

• The logical AND operator. Both operands must be true for the condition to be true. Syntax: [ -b file1 && -r file1 ]

-o or || (two pipe characters)

• The logical OR operator. Either of the two or both operands must be true for the condition to be true. Syntax: [ (x == 1 -o y == 2) ]

if-then-fi Construct

• Evaluates the condition for true or false.
• Executes the specified action if the condition is true
• otherwise, it exits the construct.
• Begins with an if and ends with a fi
• can execute an action only if the specified condition is true. It quits the statement if the condition is untrue.

The general syntax of this statement is as follows:

if condition > then > action > fi

Script07: The if-then-fi Construct

Create if_then_fi.sh to determine the number of arguments and print an error message if there are none provided:

[root@server30 ~]# vim /usr/local/bin/if_then_fi.sh

#!/bin/bash 
if [ $# -ne 2 ] # Ensure there is a space after [ and before ] 
then 
        echo "Error: Invalid number of arguments supplied" 
        echo "Usage: $0 source_file destination_file" 
exit 2 
fi 
echo "Script terminated"

[root@server30 ~]# chmod +x /usr/local/bin/if_then_fi.sh

[root@server30 ~]# if_then_fi.sh
Error: Invalid number of arguments supplied
Usage: /usr/local/bin/if_then_fi.sh source_file destination_file

This script will display the following messages on the screen if it is executed without exactly two arguments specified at the command line:

[root@server30 ~]# if_then_fi.sh
Error: Invalid number of arguments supplied
Usage: /usr/local/bin/if_then_fi.sh source_file destination_file

Return code value reflects the exit code in the script .

[root@server30 ~]# echo $?
2

Return code will be 0 if you supply a pair of arguments:

[root@server30 ~]# if_then_fi.sh a b
Script terminated
[root@server30 ~]# echo $?
0

if-then-else-fi Construct

• Can execute an action if the condition is true and another action if the condition is false.

The general syntax of this statement is as follows:

if condition > then > action1 > else > action2 > fi

Script08: The if-then-else-fi Construct

Create a script called if_then_else_fi.sh that will accept an integer value as an argument and tell if the value is positive or negative:

vim /usr/local/bin/if_then_else_fi.sh

#!/bin/bash
if [ $1 -gt 0 ]
then
        echo "$1 is a positive integer value"
else
        echo "$1 is a negative integer value"
fi

[root@server30 ~]# chmod +x /usr/local/bin/if_then_else_fi.sh
[root@server30 ~]# if_then_else_fi.sh
/usr/local/bin/if_then_else_fi.sh: line 2: [: -gt: unary operator expected
 is a negative integer value
[root@server30 ~]# if_then_else_fi.sh 3
3 is a positive integer value
[root@server30 ~]# if_then_else_fi.sh -3
-3 is a negative integer value

[root@server30 ~]# if_then_else_fi.sh a
/usr/local/bin/if_then_else_fi.sh: line 2: [: a: integer expression expected
a is a negative integer value

[root@server30 ~]# echo $?
0

The if-then-elif-fi Construct

• Can define multiple conditions and associate an action with each one of them.
• The action corresponding to the true condition is performed.

The general syntax of this statement is as follows:

if condition1 > then action1 > elif condition2 > then action2 > elif condition3 > then action3 > else > action(n) > fi

Script09: The if-then-elif-fi Construct (Example 1)

Create if_then_elif_fi.sh script to accept an integer value as an argument and tell if the integer is positive, negative, or zero. If a non-integer value or no argument is supplied, the script will complain. Employ the exit command after each action to help you identify where it exited.

[root@server30 ~]# vim /usr/local/bin/if_then_elif_fi.sh

#!/bin/bash
if [ $1 -gt 0 ]
then
        echo "$1 is a positive integer value"
exit 1
elif [ $1 -eq 0 ]
then
        echo "$1 is a zero integer value"
exit 2
elif [ $1 -lt 0 ]
then
        echo "$1 is a negative integer value"
exit 3
else
        echo "$1 is not an integer value. Please supply an i
nteger."
exit 4
fi

[root@server30 ~]# if_then_elif_fi.sh -0
-0 is a zero integer value

[root@server30 ~]# echo $?
2

[root@server30 ~]# if_then_elif_fi.sh -1
-1 is a negative integer value

[root@server30 ~]# echo $?
3

[root@server30 ~]# if_then_elif_fi.sh 10
10 is a positive integer value

[root@server30 ~]# echo $?
1

[root@server30 ~]# if_then_elif_fi.sh abd
/usr/local/bin/if_then_elif_fi.sh: line 2: [: abd: integer expression expected
/usr/local/bin/if_then_elif_fi.sh: line 6: [: abd: integer expression expected
/usr/local/bin/if_then_elif_fi.sh: line 10: [: abd: integer expression expected
abd is not an integer value. Please supply an i
nteger.>)

[root@server30 ~]# echo $?
4

Script10: The if-then-elif-fi Construct (Example 2)

Create ex200_ex294.sh to display the name of the Red Hat exam RHCSA or RHCE in the output based on the input argument (ex200 or ex294). If a random or no argument is provided, it will print “Usage: Acceptable values are ex200 and ex294”. Add white spaces in the conditions.

[root@server30 ~]# vim /usr/local/bin/ex200_ex294.sh

#!/bin/bash
if [ "$1" = ex200 ]
then
        echo "RHCSA"
elif [ "$1" = ex294 ]
then
        echo "RHCE"
else
        echo "Usage: Acceptable values are ex200 and ex294"
fi

[root@server30 ~]# chmod +x /usr/local/bin/ex200_ex294.sh

[root@server30 ~]# ex200_ex294.sh ex200
RHCSA

[root@server30 ~]# ex200_ex294.sh ex294
RHCE

[root@server30 ~]# ex200_ex294.sh frog
Usage: Acceptable values are ex200 and ex294

Looping Constructs

• Perform certain task on a number of given elements.
• Or repeatedly until a specified condition becomes true or false.
• Examples:
  • if plenty of disks need to be initialized for use in LVM, you can either run the pvcreate command on each disk one at a time manually or employ a loop to do it for you.
  • Based on a condition, you may want a program to continue to run until that condition becomes true or false.

Three looping constructs: for-do-done

• for loop is also referred to as the foreach loop.
• iterates on a list of given values until the list is exhausted. while-do-done
• while loop runs repeatedly until the specified condition becomes false. until-do-done
• until loop does just the opposite of the while loop
• Performs an operation repeatedly until the specified condition becomes true.

Test Conditions

let command

• Used in looping constructs to evaluate a condition at each iteration.
• compares the value stored in a variable against a pre-defined value
• Each time the loop does an iteration, the variable value is altered.
• Can enclose the condition for arithmetic evaluation within a pair of parentheses (( )) or quotation marks (" “) instead of using the let command explicitly.

Operators used in test conditions

!

• Negation

+ / – / * / /

• Addition / subtraction /multiplication / division

%

• Remainder

< / <=

• Less than / less than or equal to

> / >=

• Greater than / greater than or equal to

=

• Assignment

== / !=

• Comparison for equality /non-equality

The for Loop

• Executed on an array of elements until all the elements in the array are consumed.
• Each element is assigned to a variable one after the other for processing.

The general syntax of this construct is as follows:

for VAR in list > do > action > done

Script11: Print Alphabets Using for Loop

Create script for_do_done.sh script that initializes the variable COUNT to 0. The for loop will read each letter sequentially from the range placed within curly brackets (no spaces before the letter A and after the letter Z), assign it to another variable LETTER, and display the value on the screen. The expr command is an arithmetic processor, and it is used here to increment the COUNT by 1 at each loop iteration.

[root@server10 ~]# vim /usr/local/bin/for_do_done.sh

#!/bin/bash
COUNT=0
for LETTER in {A..Z}
do
        COUNT=`/usr/bin/expr $COUNT + 1`
        echo "Letter $COUNT is [$LETTER]"
done

[root@server10 ~]# chmod +x /usr/local/bin/for_do_done.sh

[root@server10 ~]# for_do_done.sh
Letter 1 is [A]
Letter 2 is [B]
Letter 3 is [C]
Letter 4 is [D]
Letter 5 is [E]
Letter 6 is [F]
Letter 7 is [G]
Letter 8 is [H]
Letter 9 is [I]
Letter 10 is [J]
Letter 11 is [K]
Letter 12 is [L]
Letter 13 is [M]
Letter 14 is [N]
Letter 15 is [O]
Letter 16 is [P]
Letter 17 is [Q]
Letter 18 is [R]
Letter 19 is [S]
Letter 20 is [T]
Letter 21 is [U]
Letter 22 is [V]
Letter 23 is [W]
Letter 24 is [X]
Letter 25 is [Y]
Letter 26 is [Z]

Script12: Create Users Using for Loop

Create script create_user.sh script to create several Linux user accounts. As each account is created, the value of the variable ? is checked. If the value is 0, a message saying the account is created successfully will be displayed, otherwise the script will terminate. In case of a successful account creation, the passwd command will be invoked to assign the user the same password as their username.

[root@server10 ~]# vim /usr/local/bin/create_user.sh

#!/bin/bash

for USER in user{10..12}
do
        echo "Create account for user $USER"
        /usr/sbin/useradd $USER
if [ $? = 0 ]
then
        echo $USER | /usr/bin/passwd --stdin $USER
        echo "$USER is created successfully"
else
        echo "Failed to create account $USER"
exit
fi
done

[root@server10 ~]# chmod +x /usr/local/bin/create_user.sh

[root@server10 ~]# create_user.sh
Create account for user user10
Changing password for user user10.
passwd: all authentication tokens updated successfully.
user10 is created successfully
Create account for user user11
Changing password for user user11.
passwd: all authentication tokens updated successfully.
user11 is created successfully
Create account for user user12
Changing password for user user12.
passwd: all authentication tokens updated successfully.
user12 is created successfully

Script fails if ran again:

[root@server10 ~]# create_user.sh
Create account for user user10
useradd: user 'user10' already exists
Failed to create account user10