Bash Scripting

This room is the walkthrough for learning bash scripting which is one of the default terminal language in linux. Bash terminal can also be used in windows and any other operating systems and can be helpful for many usages and automations.

Task 1 Introduction

This section gives some basic information about bash scripts. Bash scripts can be given in the terminal by one of two ways. They are,

1. Using bash directly in the terminal
2. Writing a bash script file in .sh extension to automate the script

Considering bash is a turing complete language and so we can use this to do almost anything programmatic. One of the most useful resource for bash scripting is https://devhints.io/bash.

Task 2 Our first simple bash scripts

Prining text in terminal using bash

we use echo command to print something in bash terminal.

echo "Hello"

Commenting code or document in bash

To comment we use the # before the tergeted comment.

# This is a comment
# Printing hello
echo "Hello"

Make .sh script executable

To make shell script executable we need to make the change the mode executable by using chmod

# There is a file example.sh converting to executable
chmod +x example.sh
# running the program
./example.sh

Who is the user

To find who is the current user in terminal we use whoami or id in the bash terminal

whoami

id

Task 3 Variables

Defining and using variables

Variables in bash termianls uses a $ before the variable identifier or variable name. For example,

$h = "Hello world!"
echo $h

Would print Hello world! in the terminal.

Debugging

To debug we have to use set - or set + command. we can even use bash -x command. to run a bash script file in debug mode,

# the file is example.sh
bash -x ./example.sh

to debug code in the file but not the entire code

name="Abul"
age=100
set -x
age=$((age + 50))
echo "$name is $age years old"
set +x

Task 4 Parameters

Parameters are inputs given in the shell script command. This is usd in shell script to make variable input and outputs for the given parameter. The general usages of this is,

bash ./hello.sh sadman

which if hello.sh contains the following

echo "Hello $1"

the numbers in the dollar signs are the parameters passed in the shell script call. the ./hello.sh itself is the 0th parameter and the name is the parameter. if we want to know the number of parameter we use # symbol

echo "$#"

Task 5 Arrays

Arrays are the 0 indexed list of values in bash. In bash we define array like following,

cars=('honda' 'audi' 'bmw' 'tesla')

we use access each variable using ${var[]}

# Print the entire array
echo "${cars[@]}"
# Print one variable
echo "${cars[1]}"

To delete a variable we unset it

unset cars[3]

Task 6 Conditionals

there are three conditionals,

1. Basic if statement

    num=15
    if [ $num -gt 10 ]; then
        echo "Number is greater than 10"
    fi
   

2. If…else statement

    num=8
    if [ $num -gt 10 ]; then
        echo "Number is greater than 10"
    else
        echo "Number is 10 or less"
    fi
   

3. If…elif…else statement

    num=10
    if [ $num -gt 10 ]; then
        echo "Number is greater than 10"
    elif [ $num -eq 10 ]; then
        echo "Number is exactly 10"
    else
        echo "Number is less than 10"
    fi
   

4. Nested if statement

    num=5
    if [ $num -gt 0 ]; then
        if [ $num -lt 10 ]; then
            echo "Number is between 1 and 9"
        fi
    fi
   

   Comparisons

   The comparing symbols are generally not some symbols like other programming language in bash.

bash symbol	equivalant or description
-eq	==
-ne	!=
-lt	<
-gt	>
-le	<=
-ge	>=
-f	File exists
-w	File have write access
-r	File have read access
-d	is a directory