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Bash Shell Scripting Language - Documentation and Code Snippets

This is a bash shell scripting which can be found on Unix, Linux and Mac. You can install bash on the Linux subsystem on Windows too.

he first line is #! /bin/bash because the bash program is in /bin/bash, you can know the path where the bash is by this command which bash.

print data or text on the screen

You can use echo Hello, World! or echo "Hello, world!".

Variables & Print out them

NAME = "Abanoub"
echo "My name is $NAME"
                

or you can use this

NAME = "Abanoub"
echo "My name is ${NAME}"
                

Get Data From User

read -p "Enter your name: " NAME
echo "Hello, $NAME!"
                

If Statement

The simple if statement syntax of bash script:

if ["$NAME" == "Abanoub"]
then
echo "Yourname is Abanoub!"
fi
                

NOTE THAT: the end of if is fi the reverse letters of if.

The if else statement syntax in bash script:

if ["$NAME" == "Abanoub"]
then
echo "Yourname is Abanoub!"
else
echo "Yourname is NOT Abanoub"
fi
                

The else if (elif) condition statement syntax in bash scripting:

if ["$NAME" == "Abanoub"]
then
echo "Yourname is Abanoub!"
elif ["$NAME" == "Jack"]
then
echo "Yourname is Jack!"
else
echo "Yourname is NOT Abanoub NOR Jack!"
fi
                

Logic Comparisons

You can use those operators:

Logic Operator Meaning
-eq equal to (the same meaning of == in other programming languages)
-ne not equal (the same meaning of != in other programming languages)
-gt greater then (the same meaning of > in other programming languages)
-ge greater than or equal to (the same meaning of >= in other programming languages)
-lt less than (the same meaning of < in other programming languages)
-le less than or equal to (the same meaning of <= in other programming language)

and use them like this:

NUM1 = 3
NUM2 = 5
if ["$NUM!" -gt "$NUM2"]
then
echo "$NUM1 is greater then $NUM2"
fi
                

File Conditions

these are the file condition flags:

symbol meaning
-d is directory?
-e exists? (usually we use -f instead)
-f a file?
-g is group id set?
-r readable?
-s non-zero size?
-u user id is set?
-w writable?
-x executable?

and use them like this:

FILE = "test.txt"
if [ -f "$FILE" ]
then
echo "$FILE is a file"
else
echo "$FILE is NOT a file"
fi
                

Case Statement

Case is called switch case in other languages, and some modern languages call it when case such as Kotlin programming language.

Here is the case statement in bash scripting:

read -p "Are you 25? Y/N" ANSWER
case "$ANSWER" in
    [yY]|[yY][eE][sS])
    echo "Your age is mine :)"
    ;;
    [nN]|[nN][oO])
    echo "Nooo, your age is different than mine :("
    ;;
    *)
    echo "Please enter y/yes or n/no"
    ;;
    esac
                

Note that:

[nN] is a way of giving two probabilities small n or capital N.

[yY][eE][sS] is the word yes or YES or any combination of small and capital letters to compose a YeS word.

*) this is the default option in the case statement which is called default in other programming languages.

esac is the closing of the case statement as it is the reversed letters of case. This is the way of ending statements in bash script.

For Loop

NAMES = "Abanoub Jack John Smith"
for NAME in $NAMES
do
echo "Hello, $NAME"
done
                

Here is a script to rename all text files *.txt at once by a script:

FILES = $(ls *.txt)
NEW = "new"
for FILE in $FILES
do
echo "Renaming $FILE to new-$FILE"
mv $FILE $NEW-$FILE
done
                

While Loop

Here is a while loop to read nfile.txt line by line.

LINE = 1
while read -r CURRENT_LINE
do
echo "$LINE: $CURRENT_LINE"
((LINE++))
done < "./nfile.txt"
                

Bash Script Function Syntax

function sayHello(){
echo "Hello, World!"
}
sayHello
                

We created a function to print out Hello, World! and call it to occur!

Bash Script Functions with Parameters

Here is how to write a functions with params in NAMES = "Abanoub Jack John Smith"bash scripting:

function greet(){
echo "Hello, I am $1 and I am $2"
}
greet "Abanoub" "25"
                

The $1 is the first parameter, and $2 is the second parameter. So when we call the function, we should specify the two parameters in the same order like this greet "Abanoub" "25". This means that "Abanoub" is the first param $1 and "25" is the second param $2.

Final Tips

Now, you learned the syntax of bash scripting, but you need to learn the bash commands. You will use the bash commands in the bash scripting syntax to build the script you want to run automatically (terminal app).