==============
Bash Scripting
==============

.. Tip::

    * https://devhints.io/bash
    * https://www.educative.io/blog/bash-shell-command-cheat-sheet
    * https://ryanstutorials.net/bash-scripting-tutorial/bash-script.php

Prerequisites
=============

First you need to find out where is your bash interpreter located. Enter the following into your command line:

.. code-block:: none

    fabgt@fabgt-SAT:~/gitclone/Pico-WiFi-Air-Monitoring-Expansion$ which bash
    /usr/bin/bash

Hello World!
============

Open up you favorite text editor and create file called hello_world.sh. Insert the following lines to a file:

NOTE:Every bash shell script in this tutorial starts with shebang:”#!” which is not read as a comment. First line is also a place where you put your interpreter which is in this case: /bin/bash.

Here is our first bash shell script example:

.. code-block:: bash

    #!/bin/bash
    # declare STRING variable
    STRING="Hello World"
    #print variable on a screen
    echo $STRING

Navigate to a directory where your hello_world.sh is located and make the file executable:

.. code-block:: none

    $ chmod +x hello_world.sh 

Now you are ready to execute your first bash script:

.. code-block:: none

    ./hello_world.sh 

.. Tip::

    https://linuxconfig.org/bash-scripting-tutorial


Variables
=========

In this example we declare simple bash variable and print it on the screen ( stdout ) with echo command.

.. code-block:: bash

    #!/bin/bash
    STRING="HELLO WORLD!!!"
    echo $STRING 


test:
https://linuxhint.com/bash-test-command/
https://www.cyberciti.biz/faq/unix-linux-bash-script-check-if-variable-is-empty/

Empty Variable
--------------
How to Check if Variable is Empty

A popular and simple way to check if a variable is empty is to use the -z option in the condition statement.

The -z $var returns true if a variable is empty and false if not.

The general syntax for such a test is:

.. code-block:: bash

    #!/bin/bash
    if [[ -z $var ]]
    then
        do
    else
    do
    fi

Example Script

Let us illustrate a simple script that emulates the cd command and navigates the specified directory.

Consider the script below:

.. code-block:: bash

    #!/bin/bash
    echo "Enter path to navigate to: "
    
    read _path
    
    while [[ -z $_path ]]; do
        echo "Please provide a path"
    done
    echo "Navigating to $_path"
    cd $_path

The script starts by asking the user to enter the directory to navigate to. It then checks if the variable is empty. If empty, it recursively asks the user for a path until the variable is not empty.

Once the path is available, it navigates to the set directory and prints the status.

Concatenate strings
===================

Concatenating Strings

The simplest way to concatenate two or more string variables is to write them one after another:

VAR1="Hello,"
VAR2=" World"
VAR3="$VAR1$VAR2"
echo "$VAR3"

The last line will echo the concatenated string:

Hello, World

You can also concatenate one or more variable with literal strings:

VAR1="Hello, "
VAR2="${VAR1}World"
echo "$VAR2"

Hello, World

Conditions
==========

unary operator:
https://codefather.tech/blog/bash-unary-operator-expected/

Find files
==========

locate command in bash:

.. code-block:: bash

    sudo apt-get install mlocate

To search a file called xorg.conf, enter:

.. code-block:: bash

    locate xorg.conf

Sample outputs:

/etc/X11/xorg.conf
/etc/X11/xorg.conf.backup
/etc/X11/xorg.conf.failsafe
/home/vivek/Downloads/xorg.conf.txt
/usr/share/man/man5/xorg.conf.5.gz

Instead of writing file names on scree, write the number of matching entries only, enter:

.. code-block:: bash

    locate -c xorg.conf

Sample outputs:

5

Ignore case matching (i.e. match foo.txt, FOO.TXT, foo.Txt and so on):

.. code-block:: bash

    locate -i filename

Only find and limit search to one file at a time:

.. code-block:: bash

    locate -n 1 filename

Only find and limit search to three files at a time:

.. code-block:: bash
    
    locate -n 3 filename

To search for a file named exactly NAME (not *NAME*), use

.. code-block:: bash
    
    locate -b '\FILENAME'

https://www.cyberciti.biz/faq/search-for-files-in-bash/

Add to PATH
===========

https://www.baeldung.com/linux/path-variable


CURL
====

https://www.geeksforgeeks.org/curl-command-in-linux-with-examples/

ZIP
===

Assuming I have zip installed:

.. code-block:: bash

    sudo apt-get install zip

    zip files.zip -@ < zip.lst

Or you could skip the list and just glob

.. code-block:: bash

    zip files.zip *.txt *.jpg