mislav@mislavovo-racunalo:~/Linux_folder$ ls allla
ls: cannot access 'allla': No such file or directory
mislav@mislavovo-racunalo:~/Linux_folder$ echo $?
If a command executes successfully, its exit code is 0. Anything other than 0 indicates a failure of some sort. To find out what exactly went wrong, you can look at EXIT VALUE or DIAGNOSTICS section in the man pages of the command that failed. (Ward, 2014)
I just wanted to leave a post saying that we will be using variables in shell scripts. We talked about variables and how they are essentially a named object that stores value in a piece of computer memory. We are going to be using them in our shell scripts for that exact purpose – storing values and then doing something based on the values of the variables.
We see we get argument1. Try putting echo $2 instead of echo $1 and then pass two arguments to your script. If the second argument is argument2, you will get argument2 printed out. Try it out! No, really, try it out – it will take less than 1 minute of your time and it will solidify the concept, so it is a worthwhile investment based on time investment / gain ratio.
Here is a list of special variables you should try echoing out as well:
$# – number of arguments
$@ – all arguments
$0 – script name
$$ – process ID (of the shell running the script)
$? – exit code (will be covered later)
You can also shift arguments with the shift command, but I leave you to Google that if you ever need it.
Hope you learned something useful!
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 253-255
Let’s remember what happens here: In the first command, echo A*, A* gets printed out because there is not folder starting with capital a. Then, for echo a*, we get anaconda3, because there is a folder starting with zero or more lowercase a’s – that is anaconda3. Same for Calibre Library, only it starts with capital c.
But get this now:
mislav@mislavovo-racunalo:~$ echo "a*"
So double quotes prevent globbing. However, double quotes don’t prevent variables from expanding:
The first part of a shell script is called a shebang. A shebang tells the kernel what interpreter to use to execute the script that follows. (Shotts, 2019) By interpreter, I mean the shell. (“Does the shebang determine the shell which runs the script?,” n.d.) You will recognize the shebang because it is the line that starts with #!.
A sidenote: shell scripts are interpreted, not compiled.
Let’s now write a shebang in our script. Let’s name our script tutorialScript.sh. In it, write the following as the first line:
That tells the kernel to interpret our script using the bash interpreter. You have different-looking shebangs for different interpreters.
Shell scripts ought to be used for automating tasks and file management. That was an assertion made by (Ward, 2014) and I agree with that.
If you ever need to do something more than manipulate files or automate tasks, shell scripts probably aren’t the ideal choice for it. I used shell scripts for the following tasks:
seeing a difference between multiple files in two folders
automating a task of starting a program
If I were to use shell scripts for some kind of text manipulation or some calculations, I would use a different programming language. It’s not that shell scripts can’t do calculations or manipulate text, it is that it is easier to do so in other programming languages. So keep in mind: If it is not file management or automation, use another programming language. Pick the right tool for the right task.
Hope you learned something useful!
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Page 250
Shell scripts are commands written in a file. (Ward, 2014) If you were to look at a shell script file, it would be composed of commands. Not everything would be a command though, but you would recognize commands.
So in a sense, we aren’t learning nothing new, but we will learn some things which appear in shell scripts and don’t appear outside them.
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Page 249
We are about to delve into shell scripting. Shell scripts are used to automate certain things related to files.
It is good to be acquainted with shell scripts if you are a software developer, but if you are a regular user, read this only if you are interested. The aim of this section will be to get you familiar enough with shell scripting to read and write shell scripts, albeit you will need to Google certain details of the things you are trying to achieve (if you are writing your own shell scripts). We will do this by making a very simple shell script which demonstrates the usage of each of the constructs we will learn to use.