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That’s it. We are done with our Linux journey. I hope it’s been a joyful ride. I hope you have confidence that you have a firm grasp of the basics of Linux and if you need to explore any additional parts of Linux you will have a great sense of where it fits in into the bigger picture.
May the force of the penguin be with you from this point on!
Thank you for reading and I hope you enjoyed the ride!
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Let’s recap what we learned about shell scripting:
#! is called a shebang# is called a comment – it is ignored by the interpreter and serves only for human understanding of the shell script$1 corresponds to the first argument in your script – as well as some other onescase construct instead of a lot of elifsfor is used for iterationread variableNameI hope you refreshed your memory!
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There are things I never used in shell scripting, such as sed, awk, basename etc. Since shell scripts are composed mostly of commands, the commands I never used on the command line I have never used in shell scripts as well.
I covered everything I deemed important for shell scripting, but if you encounter something you don’t understand, feel free to use Google or reference (Shotts, 2019). Although a caveat: (Shotts, 2019) seems to take the stance of “show a whole lot of features of shell scripting” where I am more biased towards “learn only the essentials of shell scripting and use other programming languages for activities other than file manipulation”. Just a difference to keep in mind.
Thank you for reading!
References
Shotts, W. (2019). The Linux Command Line, Fifth Internet Edition. Retrieved from http://linuxcommand.org/tlcl.php. Part 4 – Writing Shell Scripts
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Fixing errors, also known as troubleshooting, is a process or removing errors from your bash script.
If you adhere to the rule which I laid out before, which was “Use single quotes to enclose values unless you have a concrete reason not to”, you will be fine most of the time. For the times when you are not fine, literally copy and paste the error message you’re getting into Google and you will get some suggestions.
There is a finely written piece on this in (Shotts, 2019), which again, let me remind you, is free on the World Wide Web, so go read it.
Thank you for reading!
References
Shotts, W. (2019). The Linux Command Line, Fifth Internet Edition. Retrieved from http://linuxcommand.org/tlcl.php. Pages 454-467
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You can read user input in shell scripts. (Ward, 2014)
Here is an example of a script which reads user input:
#!/bin/bash
read name
echo "My name is $name"
Notice how I used the double quotes to make the shell expand my variable. My variable, by the way, is called name. read name prompts the user for input and when the user inputs the information (and presses Enter), that input is stored in the variable named name.
Here is how it works:
mislav@mislavovo-racunalo:~/Linux_folder$ ./tutorialScript.sh
Mislav
My name is Mislav
As you already know, you can read user input through arguments that you pass to your shell script. I don’t know which way is the better way and which way is more idiomatic to the shell, but I’d opt for passing values as arguments to the script and then doing checks on the arguments at the beginning of the script. You can also do these checks after reading something in a variable (such as “Is the string that the user inputted empty?”), but I think that doing these checks after each read is tedious; much better to do them at the beginning of the script. Again, not certain which way is idiomatic to the shell and both exist so both must be fine, but I am unsure.
Hope you learned something useful!
References
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Page 269
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You can take a result of a command and store it in a variable or use the result of that first command as an argument for another command. This is called command substitution. (Ward, 2014)
Here is an example of command substitution in a sample script:
#!/bin/bash
LINES=$(grep 'a' aba.txt)
for word in $LINES
do
echo $word
done
We first take every line that has the character a in the file aba.txt and then we store it in the variable LINES. Then we iterate over each word in the variable LINES and we print it. Why does LINES contain words, not lines? It contains lines indeed, but words that make up those lines are separated by a space (you can see so yourself by putting echo $LINES just before the for loop) and since each space is interpreted as a delimiter in a sequence, we get individual words.
Here is the output (along with aba.txt contents):
mislav@mislavovo-racunalo:~/Linux_folder$ cat aba.txt
Mustard is how we transfer wealth
Some stuff Abba Mustard
Mustard Mustard
Mustard Mustard
It's the Mustard
In the Rich Man's World
AB
Mustard is how we transfer wealth
Ab
aB
ab
mislav@mislavovo-racunalo:~/Linux_folder$ ./tutorialScript.sh
Mustard
is
how
we
transfer
wealth
…
A simple example of using the result of the first command as an input to another command is echo $(ls). We use the output of ls and we echo it.
Thank you for reading!
References
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 263-264
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while and until loops are used for iteration, the same as for loops. However, I won’t teach you these. Why? Because, as (Ward, 2014) says, if you ever need a while loop (or an until loop), it’s a good time to move to another programming language. I agree.
I will tell you that (“Bash While Loop Examples,” n.d.) and a Google search will teach you, if you really must use them. But again, I will repeat myself: If you need to use a while or an until loop, switch to another programming language.
Thank you for reading!
References
Bash While Loop Examples. (n.d.). Retrieved February 22, 2020, from https://www.cyberciti.biz/faq/bash-while-loop/
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 262-263
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for loops are used for iterating. Iterating means going over a sequence of values.
Here is an example of a for loop, modeled after (“Bash For Loop Examples,” n.d.).
#!/bin/bash
for i in 1 2 3 4 5
do
echo $i
done
This will produce the following output:
mislav@mislavovo-racunalo:~/Linux_folder$ ./tutorialScript.sh
1
2
3
4
5
What just happened? The variable i first took on the value of 1 (the first element in the sequence). Then it echoed its value. Then variable i took the next value in the sequence (the value after the value it currently had). So i was 2. Then 2 got outputted – and so on until the end of the sequence.
You can specify numerical sequences more easily (the above one could have been written as {1..5}) and you can iterate over other sequences (not just numbers, but other things – files for example). I leave you to consult the reference and Google for some further examples.
Hope you learned something useful!
References
Bash For Loop Examples. (n.d.). Retrieved February 22, 2020, from https://www.cyberciti.biz/faq/bash-for-loop/
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Today let’s talk about the case statement. case statement is used to replace a lot of elif statements. Here is our entire script with the same functionality it had before, rewritten using a case statement:
#!/bin/bash
case $1 in
'Hello')
echo 'Hello back to you!'
;;
'Hi')
echo 'Hi!'
;;
*)
echo 'You are rude.'
;;
esac
Here is how the case statement works:
$1)case goes through the list of patterns (each pattern ends in a )) and if it finds a matching pattern, the code between the ) and the ;; is executed and then it skips to esacesac denotes the end of the case statementThe case statement does not evaluate any exit codes, it just matches patterns.
Here are some things to note: (Ward, 2014)
'Hi!'| ‘Hi’)* matches a case which is unmatched by any other caseHope you found this useful!
References
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 261-262
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In our script, we have tested if our first argument equals a certain string, that is:
[ "$1" = 'Hello' ]
there are other tests as well. You can test files (for example, if a file is a regular file), integers and strings. I won’t cover that here and will leave you this resource to look at if you want to learn more about these tests: (“Chapter 7. Tests,” n.d.) Relevant sections are 7.2 and 7.3.
But remember, no matter what test you are using, the gist of it is: “If this is true (if the exit code is 0) then do this, if not, do the other thing”.
Hope you learned something useful!
References
Chapter 7. Tests. (n.d.). Retrieved February 22, 2020, from https://www.tldp.org/LDP/abs/html/tests.html. Sections 7.2 and 7.3