Shell Scripting - Introduction & Shebang

Key Takeaways

[Music] hey guys hackersploit here back again with another video and welcome to the shell scripting course now you might be a little bit confused as to why i'm making this course but this was probably the highest requested series on the channel and uh you know after giving it a lot of consideration i think it is is extremely important for me to cover this so uh without further ado let's get started so for those of you who are new to the whole idea of bash scripts you might be wondering what they are and what they do well simply uh simply put bash scripts allow you to execute commands from a file now the thing that many people don't understand here or the thing that people don't uh they don't really get to understand is the fact that the interpreter is the shell that's the brilliant thing about this now when when i say that the interpreter is the shell you pretty much get to understand that with a bash or a with a bash script you have unlimited functionality when it comes down to the unix tool set all right so what do i mean by this if you're performing uh python scripting the whole idea of modules come comes into play but if i'm using batch scripting to create a script for example it is much more convenient and will work pretty pretty much all the time without me having to install dependencies or additional or additional extensions or modules for that matter okay so that being said i am going to be covering python scripting i have covered it before but i'll be covering that in depth uh and i want to really explain you know functions because i haven't touched upon that for uh you know in a while but that being said why uh why is bash scripting preferred over python scripting of course that is a very uh that's a very opinionated statement and i do agree but if you ask any systems administrator they will go with bash scripting like 100 of the time over python because when you compare them python is less reliable when it comes down to passion again i said that because it's going to use the shell as the interpreter the shell is built in with unix and it will it will enable you to utilize the entire unix tool set that means you can control sockets with just by typing in each command each command on each line you know so it's really even very intuitive for a person to understand it now uh what is batch scripting very good at number one file management all right with bad scripting and as i've mentioned systems administrators they can vouch for this that bash scripting allows you to copy files update files backup files almost on an fully automated basis and i've seen many of the systems administrators as well as when i was one you know creating bash scripts to do anything from all the all the way from querying a data in an excel worksheet uh to uh to compiling data uh getting figures producing mass amounts of data which i know again many of you will argue python is the king of data uh you know in data science but uh bash is pretty good to be honest the only the only area where it falls off a little bit is when it comes down to huge calculations you know calculations that involve statistics and the merging of all other types of data so that's where it kind of lags off and you can understand why it's really focused on a on a system it's focused more on controlling a system so it's really good for running for file management running scripts and programs and for automating tasks now i'm going to be showing you how to write a few cool scripts that allow you to you know whether you want to call them pen testing scripts or whatever uh and the reason again i was forced to make this course is because uh i was having a discussion with one of the subscribers on facebook and they they didn't quite understand how exactly we were like when performing privilege escalation and you're trying to use python to spawn a shell uh or a or a bash instance uh how how that was happening and again this came from uh this question was raised because they did not understand a thing about bash and how the shell how powerful the terminal as we call it is you know in regards to controlling every aspect of the system now that being said what we will be looking at in this series will range from you know from very basic things to an advanced uh type of to an advanced type of videos we'll be focusing on creating onscript and that's how i want to keep it so we'll be focusing on constants input output uh conditional statements like if else loops variables functions etc and then also file management we'll look at all of that so i'll be showing you how to create a lot of awesome scripts even for those of you who who use raspberry pi's uh you know for automation and for various different things i'll show you how to write even cooler scripts for that as well all right so my environment is pretty simple and you can use this on any linux slash unix operating system i really don't know what you want to call it but many have different opinions about it and the editor i'll be using is going to be sublime text now of course you can use whatever you want i just like sublime text for bash uh or for bash scripting is b because of it's it's just really really simple for me and you can use any but for me i just prefer using sublime text all right so i'm gonna be using sublime text and let's get started so before we create our first shell script or uh or our bash script uh our bash script uh it's very important to understand uh how data is uh is handled and how and when i talk about the shell being the uh being the interpreter what i mean by this now of course you might be saying well python also has an interpreter that is based on a command line well that's slightly does its own interpreter which as you see causes problems especially for those of you who really are moving back and forth from python 3 to python python 2. all right so let's say i wanted to just you know create a simple script that will display the word hello you know so to do that as you know i type in echo if you didn't know echo essentially displays the text it's essentially echoing the text back to you all right so i type in echo and i just type in hello now this is uh primarily a shell script what do i mean by this it's simply a command that the shell will interpret and i type in enter you can see that it is interpreted and it has understood what we are trying to say now of course i can i can complicate it as much as i want i can say uh hello uh we can just you can write whatever you want but the whole idea is is that the shell as probably not a lot of you understand can do a lot of things now when i talk about scripting this essentially is the uh it essentially allows you to execute a lot of these commands from a file okay from a file that's the whole idea of a script remember from a file but when talk about the shell the shell can run commands that you want it to the problem is many people have forgotten how powerful it is so let's create our first uh shell script all right so to do that i'm just going to open up sublime text here and uh in i'm i'm just going to be covering how to create it in the next video we'll get started with variables because i really do want to cover this so let me just zoom in all right so when we talk about creating a shell script you know that the extension is a shell extension so let me just create a new file and i will save it to my desktop uh sorry let me just save this to my desktop right here desktop and i'll call this uh simple dot sh all right dot sh is the extent extension we use for shell scripts so dot s dot sh and i'm gonna hit save and as you can see it's right on the desktop now all right but remember it has nothing in it so even if we are to execute it uh nothing will come of it so what's the first thing we need to do the first thing we need to do is we need to declare what interpreter we're going to be using this is where a lot of people don't understand they have seen something like this all right so they've seen um usually what happens is they they they come across something like this and i know you guys are going to be like well what exactly do you mean well uh they come across something like this all right and i i failed to to explain it uh in python but let me explain it now when i type this in this is called the shabang i'll get i know it sounds uh quite funny but i'll get to to what this is in a second this is called the shebang primarily because of um of the uh the different meanings of uh for example this is a sharp and this uh is used for the music uh and the uh the bang annotation in music so from that they essentially determined to call it shebang all right so anything after a shabang in a script uh will essentially be used as the interpreter okay now in this case the bash is going to be the interpreter remember it can also be python python can also be used as the interpreter as we saw in the python scripting uh when we were creating our reverse shells and our clients etc in this case it can be it can be either the bash it can be the bash it can be the shell as you have probably noticed when performing privilege escalation might be saying hey man what's the difference the shell is different from the bash the bastion is considered to be a terminal instance which gives you a much more streamlined um streamlined what's the correct word for this experience when it comes down to using the linux or unix operating system the shell is simply the shell the interpreter of commands and as you know this does not give you full access in terms of functionality you then have the other interpreters that are not really used that much nowadays which are the csh and the zsh or the ksh which i'm not going to go into but they have very very diff they vary in terms of their functionality so we are going to be using the bash all right so once we have said this what this means is i want you to use this as our interpreter fair enough all right so now we can run our simple script here a simple script is going to simply display uh the words hello world now of course i'm going to use the parentheses here just to make sure that everything i type within them is printed out so i can say hello uh oops sorry my bad hello world uh and uh this is the first uh shell script all right there we are fantastic so uh now if we save this uh just give me a second let's save this and uh if we can open up our terminal here uh huh and uh what's my current working directory all right so cd uh sorry about that guys uh cd root and and let's see what's in there all right so now as you probably would have known from my other videos executing a script is quite simple all we do is we select the interpreter when we're executing a python script we can use python and some of you have noticed when you use python it works most of the time and have you realized something that for example if it was a python script a very poorly designed python script and we try and execute it like this this essentially means we are specifying the we're essentially saying use the specified interpreter and in in that script the developer has not specified the interpreter what happens is if we try and use this to execute a python script for example it will not run you'll get an invalid argument specification what that means is that the developer has not specified what to use as the interpreter and again you can understand why that is considered bad practice always specify the interpreter so that people can launch the scripts without specifying the interpreter intentionally remember all right so if we try and uh if we try and run this simple script um for some reason you'll see that um wait give me a second guys uh yeah you'll see that we cannot we cannot uh we can also we we cannot really use the uh the tab auto completion and that's because the permissions in regards to the script oops my keyboard today is really letting me down the permissions in regard in regards to the script clearly determine here that we we can only read the file but we cannot execute so as i've mentioned before we can use the uh we can change the permissions of the executable file now i'll be explaining permissions and why they're important with running uh the uh the scripts because you might want to have a script run by a certain user we'll get to that but for now let me just make sure that we can execute it and we use the chmod plus x and we spent we specified the script and we hit enter and now if you list it you can see that it's going to be highlighted in a green you all right so i'm going to run the script so simple and if i hit enter you can see it's going to display what we wanted it to display now of course this is very simple but what i want to explain is the syntax right from the beginning because i've seen a lot of developers even people you know systems admins that are working for huge companies really don't understand what what the what interpreter they're using and you know they usually find out that their scripts are not executed by certain users file permissions etc it's really a big it's a big task to understand uh you know shell scripting but once you do it really is a powerful a really really powerful tool or language for that matter to know okay so now you might be saying well what if what if alexis what if i wanted to revert the permissions that i've given this script well to do that we run the chmod but instead of the the the plus x we use the minus x and then we say simple dot sh and now if you list the files in here you can see that it is back to having uh the permissions as read only fantastic so now you know how to create your first shell script and you know about file permissions and you know about interpreters and the different interpreters you can use now remember this is global knowledge when it comes down to uh to selecting your interpreter this is quite universal and is extremely important when creating scripts now of course this is an extremely simple script but in the following videos we'll get started with the real real nitty gritty of shell scripting that being said that's gonna be it for this video uh if you have any questions or suggestions let me know in the comments section on my social networks on my website and i'll be sure to leave your reply and i'll be seeing you in the next video [Music] peace