linux : bash script

We can run the bash script directly in the terminal.

$ echo "This is a bash script."
This is a bash script.

We also can store the script in a file, so that we can run it every time we want without typing the script in terminal.

# !/bin/bash
echo "This is a bash script." 

We need to update the file permission if we want to run the bash file. We can run this command, and specify the file permission

$ chmod 700 FILE
$ ./FILE
This is a bash script.

We can run linux command in bash file and run the bash script in the cli.

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The bash file will very useful if we want to use some variable or put logics in the script.

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VARIABLE

• Positional Parameter Variable

# !/bin/bash
echo "This is a $1 $2." 

we can give a value for $1 and $2 after the run command.

$ ./FILE bash script

• User Defined Variable

# !/bin/bash
# define a string value, print as is
print1='script'

# define a string value, we can add variable inside the double quotes
print2="bash $print1"

# define a number
print3=3

# define a command
print4=`clear`

echo $script

• Environment Variable

Usually we use .env file to store the environment variable

PORT=80
ENV=development

operation for variable

# using let
let sum=$number1+$number2

# using expr
sum=`expr $number1 + $number2`
length=`expr length $string`

declaring variable type

# declare an intrger
declare -i number
number=100

---

IF STATEMENT

if [ CONDITION ]; then
    TRUE_COMMANDS
else
    FALSE_COMMANDS
fi

---

WHILE LOOPING

while [ CONDITION ]
do
    COMMANDS
done

---

FOR LOOPING

for VARIABLE in LIST
do
    COMMANDS
done