ObiLab - BASH Scripting

Conditional Statement

Conditional statements are used to perform different actions based on different conditions. Very often when you write code, you want to perform different actions for different decisions. You can use conditional statements in your code to do this.

#!/bin/bash
if [ 1 == 1 ]
        then
                echo 1 is equal to 1
        else
                echo 1 is not equal to 1 #This part of code, will never be executed
fi
echo "program continue with other statements..."

if can be used in order to check if a file exists, or if a path exists.

#!/bin/bash
if [ -e ./testo1.txt ]
        then
                echo File testo1.txt exists
        else
                echo File testo1.txt does not exists
fi
echo "program continue with other statements..."

with unary operator, not

#!/bin/bash
if [ ! -e ./testoCheNonEsiste.txt ]
        then
                echo File testoCheNonEsiste does not exists
        else
                echo File testoCheNonEsiste.txt exists
fi
echo "program continue with other statements..."

With double condition or nested if

#!/bin/bash
if [ 1 == 1 ] && [ 2 == 2 ]
then
        echo This file is useful.
else
        echo "this part of code will never be true (in this case)"
fi

echo script go on...

Comparison between INTEGER and STRINGS, with elif ("else, if")

#!/bin/bash
    # elif statements
    if [ 2 -gt 1 ]
    then
		echo 2 is greater then 1
    elif [ 'yes' == 'yes' ]
    then
		echo yes is always equal to yes, but we will never enter here
    else
		echo "This part of code, will never be executed (in this case)"
    fi

Interger Comparison

-eq

    is equal to

    if [ "$a" -eq "$b" ]
-ne

    is not equal to

    if [ "$a" -ne "$b" ]
-gt

    is greater than
	#with double parentheses
	if (("$a" > "$b"))
	#with square parentheses
    if [ "$a" -gt "$b" ]
-ge

    is greater than or equal to
	
	#with double parentheses
	if (("$a" >= "$b"))
	#with square parentheses
    if [ "$a" -ge "$b" ]
-lt

    is less than
	#with double parentheses
	if (("$a" < "$b"))  
	
	#with square parentheses
    if [ "$a" -lt "$b" ]
-le

    is less than or equal to
	#with double parentheses
	if (("$a" <= "$b"))
	#with square parentheses
    if [ "$a" -le "$b" ]
		

string comparison

=   is equal to

    if [ "$a" = "$b" ]

==

    is equal to

    if [ "$a" == "$b" ]
	
!=

    is not equal to

    if [ "$a" != "$b" ]

    This operator uses pattern matching within a [[ ... ]] construct.
	
<

    is less than, in ASCII alphabetical order

    if [[ "$a" < "$b" ]]

    if [ "$a" \< "$b" ]

    Note that the "<" needs to be escaped within a [ ] construct.
	
>

    is greater than, in ASCII alphabetical order

    if [[ "$a" > "$b" ]]

    if [ "$a" \> "$b" ]

    Note that the ">" needs to be escaped within a [ ] construct.
	
-z

    string is null, that is, has zero length
	
	 String=''   # Zero-length ("null") string variable.
		if [ -z "$String" ]
			then
				echo "\$String is null."
			else
				echo "\$String is NOT null."
		fi     # $String is null.

-n

    string is not null.
	

Loops For

#!/bin/bash
for i in 1 2 3 4 5
do
   echo "Loop number $i"
done

C Style

#!/bin/bash
for (( i=0; i<5; i++ ))
do
   echo "Loop number $i"
done

While

#!/bin/bash
i=0
while [ $i -lt 5 ]
do
  echo "Loop number $i"
  i=$(( $i + 1 ))
done

Sum of 2 numbers, until variable "$a" will be -1

#!/bin/bash
while :
do
        read -p "Enter two numnbers ( - 1 to quit ) : " a b
        if [ $a -eq -1 ]
        then
                break  #when condition is satisfied, the statement ends
        fi
        sum=$(( a + b ))
        echo $sum
done

Let's iterate an array

#!/bin/bash
vet=("Stefano" "Marco" "Serena" "Mario")
# vet is and Array of 4 elements

        for (( i=0; i<4; i++))
        do
            echo ${vet[i]}
                if [ ${vet[i]} == 'Stefano' ]
                then
                    echo "[ Comparazione stringa Avvenuta ]"
                fi
        done

#!/bin/bash
A=("9" "17" "stefano") #Array with no types declaration

#Array iteration with for loop
for i in ${A[@]}
do
    echo ${i}
    #And what would be happened if we'd do a math operation on array element, inside this loop?
done

What will be the value of "sum" at the end of the loop?

#!/bin/bash
A=("9" "17" "stefano") #no types declaration

for i in ${A[@]}
do
  #echo ${i}
  sum=$(($sum+${i}))
done

echo $sum