Shell Programming – If…else, Array, Loops, Function…

Tuesday, 14/04/2020

Tram Ho

Hello, continue with the Shell programming series, this article we will learn about conditional statements, arrays, loops, functions …

1. Conditional statement

If you already know conditional statements in other programming languages, the same is true for Shell. It is different only in syntax.
There are 5 conditional statements in Shell programming:
- if statement
- if-else statement
- if..elif..else..fi statement
- if..then..else..if..then..fi..fi .. (Nested if)
- switch statement

Syntax

If statement

if [ expression ]
then
   statement
fi

if [ expression ]

then

statement

if-else statement

if [ expression ]
then
   statement1
else
   statement2
fi

if [ expression ]

then

statement1

else

statement2

if..elif..else..fi statement

if [ expression1 ]
then
   statement1
   statement2
   .
   .
elif [ expression2 ]
then
   statement3
   statement4
   .
   .
else
   statement5
fi

if [ expression1 ]

then

statement1

statement2

elif [ expression2 ]

then

statement3

statement4

else

statement5

if..then..else..if..then..fi..fi .. (Nested if)

if [ expression1 ]
then
   statement1
   statement2
   .
else
   if [ expression2 ]
   then
      statement3
      .
   fi
fi

if [ expression1 ]

then

statement1

statement2

else

if [ expression2 ]

then

statement3

switch statement

case  in
   Pattern 1) Statement 1;;
   Pattern n) Statement n;;
esac

case in

Pattern 1) Statement 1;;

Pattern n) Statement n;;

esac

2. Array in Shell

In the previous article we learned about shell variables used to store a value, called a scalar variable. Shell also supports another variable called array variable . Variable naming rules also apply to array naming.

Array definition

Define: array_name[index]=value
- array_name: The name of the array
- index: is the index of the item in the array
- value: value at that index
For example:

NAME[0]="Zara"
NAME[1]="Qadir"
NAME[2]="Mahnaz"
NAME[3]="Ayan"
NAME[4]="Daisy"

NAME[0]="Zara"

NAME[1]="Qadir"

NAME[2]="Mahnaz"

NAME[3]="Ayan"

NAME[4]="Daisy"

If we are using KornShell array initialization syntax: set -A array_name value1 value2 ... valuen
If we are using BashShell the syntax of initializing arrays: array_name=(value1 ... valuen)

Accessing array values

Syntax for accessing the value of an array: ${array_name[index]}
Consider the following example, create file test.sh :

#!/bin/bash

NAME[0]="Zara"
NAME[1]="Qadir"
NAME[2]="Mahnaz"
NAME[3]="Ayan"
NAME[4]="Daisy"
echo "First Index: ${NAME[0]}"
echo "Second Index: ${NAME[1]}"

#!/bin/bash

NAME[0]="Zara"

NAME[1]="Qadir"

NAME[2]="Mahnaz"

NAME[3]="Ayan"

NAME[4]="Daisy"

echo "First Index: ${NAME[0]}"

echo "Second Index: ${NAME[1]}"

Example results above:

$./test.sh
First Index: Zara
Second Index: Qadir

$./test.sh

First Index: Zara

Second Index: Qadir

We can also access all items in the array with the following syntax:

${array_name[*]}
${array_name[@]}

${array_name[*]}

${array_name[@]}

See the following example:

#!/bin/bash

NAME[0]="Zara"
NAME[1]="Qadir"
NAME[2]="Mahnaz"
NAME[3]="Ayan"
NAME[4]="Daisy"
echo "First Method: ${NAME[*]}"
echo "Second Method: ${NAME[@]}"

#!/bin/bash

NAME[0]="Zara"

NAME[1]="Qadir"

NAME[2]="Mahnaz"

NAME[3]="Ayan"

NAME[4]="Daisy"

echo "First Method: ${NAME[*]}"

echo "Second Method: ${NAME[@]}"

Result:

$./test.sh
First Method: Zara Qadir Mahnaz Ayan Daisy
Second Method: Zara Qadir Mahnaz Ayan Daisy

$./test.sh

First Method: Zara Qadir Mahnaz Ayan Daisy

Second Method: Zara Qadir Mahnaz Ayan Daisy

3. The loop

The loop is a powerful programming tool that allows us to execute a script on a continuous basis. We will explore the loops Shell supports for programmers:
- The while loop
- The for loop
- The until loop
- The select loop
It is possible to change the state of the loop using the break, continue statement as in other languages.
We can also use nested loops.

While loop

First, we will learn about the while loop. About using similar languages like C ++, java …
Syntax:

while command
do
   Statement(s) to be executed if command is true
done

while command

Statement(s) to be executed if command is true

done

Example: Print out integers smaller than 10.

#!/bin/bash

a=0
while [ $a -lt 10 ]
do
   echo $a
   a=`expr $a + 1`
done

#!/bin/bash

a=0

while [ $a -lt 10 ]

echo $a

a=`expr $a + 1`

done

Result:

For loop

The second loop we will find out is the For loop.
Syntax:

for var in word1 word2 ... wordN
do
   Statement(s) to be executed for every word.
done

for var in word1 word2 ... wordN

Statement(s) to be executed for every word.

done

For example:

#!/bin/bash

for var in 0 1 2 3 4 5 6 7 8 9
do
   echo $var
done

#!/bin/bash

for var in 0 1 2 3 4 5 6 7 8 9

echo $var

done

Result:

Until loop

A strange sounding name loop, let’s see how it is used.
Until loop allows to execute a script until a condition becomes true. This means that if the condition is false, then the script will be executed, and if the condition is true, then no statement will be executed. Here you can see the until loop is like a do … while loop in C ++, Java …
Until loop always executes at least 1 time.
Syntax:

until command
do
   Statement(s) to be executed until command is true
done

until command

Statement(s) to be executed until command is true

done

Example: Displays numbers from 0 to 9

#!/bin/bash
a=0
until [ ! $a -lt 10 ]
do
   echo $a
   a=`expr $a + 1`
done

#!/bin/bash

a=0

until [ ! $a -lt 10 ]

echo $a

a=`expr $a + 1`

done

Select loop

Again a fairly new name.
The Select loop helps create a numbered menu in order, giving the user a choice.
Syntax:

select var in word1 word2 ... wordN
do
   Statement(s) to be executed for every word.
done

select var in word1 word2 ... wordN

Statement(s) to be executed for every word.

done

For example:

#!/bin/bash

PS3='Please enter your choice: '
select DRINK in tea cofee water juice appe all none
do
   case $DRINK in
      tea|cofee|water|all) 
         echo "Go to canteen"
         ;;
      juice|appe)
         echo "Available at home"
      ;;
      none) 
         break 
      ;;
      *) echo "ERROR: Invalid selection" 
      ;;
   esac
done

#!/bin/bash

PS3='Please enter your choice: '

select DRINK in tea cofee water juice appe all none

case $DRINK in

tea|cofee|water|all)

echo "Go to canteen"

;;

juice|appe)

echo "Available at home"

;;

none)

break

;;

*) echo "ERROR: Invalid selection"

;;

esac

done

Result:

1) tea
2) cofee
3) water
4) juice
5) appe
6) all
7) none
Please enter your choice: 1
Go to canteen
Please enter your choice:

1) tea

2) cofee

3) water

4) juice

5) appe

6) all

7) none

Please enter your choice: 1

Go to canteen

Please enter your choice:

4. Functions

Using functions to perform repetitive tasks is a great way to generate code reuse. This is an important part of modern object-oriented programming principles.

Creating Functions

function_name () { 
   list of commands
}

function_name () {

list of commands

}

For example:

#!/bin/bash
# Define your function here
hello () {
   echo "Hello World"
}
# Invoke your function
hello

#!/bin/bash

# Define your function here

hello () {

echo "Hello World"

}

# Invoke your function

hello

Pass Parameters to a Function

We can define a function that will accept parameters while calling the function, the parameters will be denoted by $ 1, $ 2 …

#!/bin/bash
# Define your function here
hello () {
   echo "Hello World $1 $2"
}
# Invoke your function
hello Zara Ali

#!/bin/bash

# Define your function here

hello () {

echo "Hello World $1 $2"

}

# Invoke your function

hello Zara Ali

Returning values from Functions

To return a value from a function we use the return statement
Example: Returns the value 10

Live Demo
#!/bin/bash
# Define your function here
hello () {
   echo "Hello World $1 $2"
   return 10
}
# Invoke your function
hello Zara Ali
# Capture value returnd by last command
ret=$?
echo "Return value is $ret"

Live Demo

#!/bin/bash

# Define your function here

hello () {

echo "Hello World $1 $2"

return 10

}

# Invoke your function

hello Zara Ali

# Capture value returnd by last command

ret=$?

echo "Return value is $ret"

Note: The function must be defined before the location to call that function.

5. Shell Substitution

Shell performs the replacement when encountering special characters. For better understanding see the example below

#!/bin/bash
a=10
echo -e "Value of a is $a n"

#!/bin/bash

a=10

echo -e "Value of a is $a n"

The -e option allows executing backslash escapes Value of a is 10 . And this is the result without -e Value of a is 10n .
Some escapes are used in the echo command:

: backslash
a : alert
b : backspace
c : suppress trailing newline
f : form feed
n : newline
r : carriage return
t : horizontal tab
v : vertical tab

You can use the -E option to disable backslash, -n to disable newline.
Command substitution : The mechanism by which the shell executes a given script and then replaces its output where it is called.
Syntax: command . Use backquote, not single quote.
Consider the following example:

#!/bin/bash
DATE=`date`
echo "Date is $DATE"
USERS=`who | wc -l`
echo "Logged in user are $USERS"
UP=`date ; uptime`
echo "Uptime is $UP"

#!/bin/bash

DATE=`date`

echo "Date is $DATE"

USERS=`who | wc -l`

echo "Logged in user are $USERS"

UP=`date ; uptime`

echo "Uptime is $UP"

Result:

Date is Thu Jul  2 03:59:57 MST 2009
Logged in user are 1
Uptime is Thu Jul  2 03:59:57 MST 2009
03:59:57 up 20 days, 14:03,  1 user,  load avg: 0.13, 0.07, 0.15

Date is Thu Jul 2 03:59:57 MST 2009

Logged in user are 1

Uptime is Thu Jul 2 03:59:57 MST 2009

03:59:57 up 20 days, 14:03, 1 user, load avg: 0.13, 0.07, 0.15

Variable Substitution : allows manipulating the value of a variable based on its state.

$ {var}: Replace the value of var
$ {var: -word}: If var is null or unset, word is substituted for var. The value of var does not change.
$ {var: = word}: If var is null or unset, var is set to the value of word.
$ {var:? message}: If var is null or unset, the message is printed as a standard error. This checks for variables that are set correctly.
$ {var: + word}: If var is set, word is substituted for var. The value of var does not change.

For example:

#!/bin/bash

echo ${var:-"Variable is not set"}
echo "1 - Value of var is ${var}"

echo ${var:="Variable is not set"}
echo "2 - Value of var is ${var}"

unset var
echo ${var:+"This is default value"}
echo "3 - Value of var is $var"

var="Prefix"
echo ${var:+"This is default value"}
echo "4 - Value of var is $var"

echo ${var:?"Print this message"}
echo "5 - Value of var is ${var}"

#!/bin/bash

echo ${var:-"Variable is not set"}

echo "1 - Value of var is ${var}"

echo ${var:="Variable is not set"}

echo "2 - Value of var is ${var}"

unset var

echo ${var:+"This is default value"}

echo "3 - Value of var is $var"

var="Prefix"

echo ${var:+"This is default value"}

echo "4 - Value of var is $var"

echo ${var:?"Print this message"}

echo "5 - Value of var is ${var}"

Result:

Variable is not set
1 - Value of var is
Variable is not set
2 - Value of var is Variable is not set

3 - Value of var is
This is default value
4 - Value of var is Prefix
Prefix
5 - Value of var is Prefix

Variable is not set

1 - Value of var is

Variable is not set

2 - Value of var is Variable is not set

3 - Value of var is

This is default value

4 - Value of var is Prefix

Prefix

5 - Value of var is Prefix

summary

At this point, we have the basic knowledge for Shell programming, depending on each problem we will apply.
Read more about Shell here

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Shell Programming – If…else, Array, Loops, Function…

1. Conditional statement

Syntax

2. Array in Shell

Array definition

Accessing array values

3. The loop

While loop

For loop

Until loop

Select loop

4. Functions

Creating Functions

Pass Parameters to a Function

Returning values from Functions

5. Shell Substitution

summary

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1. Conditional statement

Syntax

2. Array in Shell

Array definition

Accessing array values

3. The loop

While loop

For loop

Until loop

Select loop

4. Functions

Creating Functions

Pass Parameters to a Function

Returning values ​​from Functions

5. Shell Substitution

summary

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The fastest depreciating after 9 months of launch, iPhone 14 Pro Max continues to break the bottom in Vietnam

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Returning values from Functions