readded udpated scripts directory
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scripts/tutorials_notes/bash_notes.txt
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scripts/tutorials_notes/bash_notes.txt
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Bash Scripting can be rather difficult at first, so these notes are an attempt to establish some basics in understanding. This won't go into too much details, but rather is a follow along of the most important takeaways from the https://guide.bash.academy/commands/ section thus far.
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# A side note: you can append to the directories your bash shell looks for shell scripts by adding a :/path/to/shell_scripts to your $PATH in your .bashrc
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A standard if statement that checks to see if you cannot remove hello.txt, and if not, then a message is sent to standard error(2), then the exit command is sent, and the if statement ends.
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if ! rm hello.txt; then echo "Couldn't delete hello.txt." >&2; exit 1; file
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A shorter version of the same command using the control operator (||):
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rm hello.txt || { echo "Couldn't delete hello.txt." >&2; exit 1; }
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Create a file called myfiles.ls if any standard output (1) is produced by the command ls, if not, and a standard error(2) is produced, then send the standard error output to a file in this directory(./) called myerrors.txt
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ls >myfiles.ls 2>./myerrors.txt
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Create a file called myfiles_or_myerrors.txt which takes the output of ls sermthing and creates that file with the output of either the successful standard output(1) or the standard error(2)
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ls sermthing >myfiles_or_myerrors.txt 2>&1
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Echo into a file called world in our home directory the phrase Hello(note that if world doesn't exist, it will be created)
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echo Hello >~/world
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If you wished to append to our world file above, rather than rewriting it, use the append (>>) syntax instaed of the write(>) syntax.
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echo World >>~/world
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cat world
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Hello World
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Take the results of ping 127.0.0.1 and write that to a file called results, whether it has a standard output(1) result or a standard error result(2)
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ping 127.0.0.1 >results 2>&1
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A shorter version of this would be written like so
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ping 127.0.0.1 &>results
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Assigning temporary variables in bash is as simple as using the equal syntax
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name="Linus"
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echo "Hello $(name)!"
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Hello Linus!
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We can also use curly braces to refer to this. echo "Hello ${name}!"
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Hello Linus!
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Let's say we wanted to only refer to what followed before or after a specific character, this can be done like so:
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name="Britta" time=23.73
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echo "$name's current record is ${time%.*} seconds and ${time#*.} hundredths."
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Britta's current record is 23 seconds and 73 hundredths.
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Another example is using it with letters to make this more clear:
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echo "$name's name is ${name%i*} before the i and ${name#*i} after the i."
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Britta's name is Br before the i and tta after the i.
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Capitalize the first word of the variable greeting.
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greeting="hello world"
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echo "${greeting^}"
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Hello world
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Replace the first space character with the string " big "
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greeting="hello world"
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greeting="${greeting/ / big }"
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echo $greeting
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hello big world
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Redirect the contents of $greeting into a filename of the same name, but with underscores(_) instaed of spaces( ) ending in .txt
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greeting="hello big world"
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echo $greeting > "${greeting// /_}.txt"
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Show the contents of the variable greeting with the middle word fully upper-cased.
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greeting="hello big world"
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# Assigns the middle variable to everything before the last space.
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middle="${greeting% *}"
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echo $middle
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hello big
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# Reassigns the middle variable to everything in it currently, but everything after the first space is deleted.
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middle="${middle#* }"
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echo middle
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big
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# Now we can capitalize our middle word in the phrase like so:
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echo "${greeting%% *} ${middle^^} ${greeting##* }"
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hello BIG world
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# Note the difference if we omit the double signs:
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echo "${greeting% *} ${middle^^} ${greeting#* }"
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hello big BIG big world
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This is because the double percent(%%) and double hashtags(##) will double omit the spaces, meaning that it will omit the spaces as many times as there is %% or ##.
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# To open a new bash shell and then output echo $1, where 'Hello World!' is passed in as the argument for $1
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bash -c 'echo "$1"' -- 'Hello World!'
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Hello World!
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# To start a bash shell that takes in any number of arguments, and echos back the number of arguments passed.
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bash -c 'echo "$#"' -- 1 2 'The Third'
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3
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# To start a bash shell that shifts a positional parameter away and then ouitputs the first, passing in arguments 1, 2, and The Third
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bash -c 'shift; echo "$1"' -- 1 2 'The Third'
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2
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# To start a bashs hell that outputs the last argument passed in and pass in the arguments 1, 2, and The Third.
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bash -c 'echo "${@: -1}"' -- 1 2 'The Third'
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The Third
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# Arrays
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Arrays in bash are created using parentheses () braces. They can contain any files, variables, numbers, etc. They are separated by spaces and thusly, like a bash command, must be delineated using quotation marks "" in order to delineate specific files, especially when spaces are included in the filename. Here's an example:
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files=( myscript hello.txt "05 Between Angels and Insects.ogg")
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We can then access these elements in the array using the echo command like so:
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echo "${files[0]}"
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myscript
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echo "${files[1]}"
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hello.txt
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echo "${files[2]}"
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'05 Between Angels and Insects.ogg'
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echo "${files[@]}"
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myscript hello.txt '05 Between Angels and Insects.ogg'
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# Note that the above can also be done by substituting @ with *, but because of how dangerous and "greedy" the * argument can be, it is best avoided in favor of @
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# Note that if we just try to echo files, it will only return the first element of the array.
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echo "$files"
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myscript
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We can also append(push) onto the array, as well as use a few other interesting array methods
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files+=( self.png )
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echo "${files[@]}"
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myscript hello.txt '05 Between Angels and Insects.ogg' self.png
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Assigns an array of all txt files to an array called files.
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files=(*.txt)
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Remove a specific element of an array
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echo "${files[@]}"
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myscript hello.txt '05 Between Angels and Insects.ogg' self.png
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unset "files[3]"
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echo "${files[@]}"
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myscript hello.txt '05 Between Angels and Insects.ogg'
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# As noted above the * argument is dubious and should always be used with caution, however in the example given above when comparing it with @, it is rather innocuous, and in certain situations where it is used for trivial purposes, it can be quite useful, such asi n the following example:
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names=("Susan Quinn" "Anne-Marie Davis" "Mary Tate")
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echo "Invites sent to: <$[names[*]]>."
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Invites were sent to: <Susan Quinn Anne-Marie Davis Mary Tate>
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# The following is interesting, the $IFS shell variable is usually set to a space, but in this case we set it to a comma. Note the opening parentheses without any equal operator, this indicates opening up what is called a "subshell" in which the global shell variable is changed within a localized SCOPE, and is only equivalent to the comma within the context of that scope (i.e. when the parentheses close)
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( IFS=','; echo "invites sent to: <${names[*]}>." )
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Invites were sent to: <Susan Quinn,Anne-Marie Davis,Mary Tate.>.
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#### LEFT OFF AT THE END OF VARIABLES, STARTING NEXT AT TESTS AND CONDITIONALS ####
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https://guide.bash.academy/conditionals/
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#### CONDITIONALS ####
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# Here let's just jump right in with a simple example of an if statement that evalutates user input:
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read -p "Would you like some breakfast?[y/n]: "
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Would you like some breakfast?[y/n]: (user inputs y)
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echo $REPLY
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y
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# Now let's evaluate this with a simple if statement
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if [[ $REPLY = y ]]; then
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echo "Here you go, an egg sandwich"
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else
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echo "Here, you should at least have some coffee."
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fi
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# And of course the reponse is based off of whether the user passed y/n to the original read statement
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# This can be chained so that the response is immediately evaluated upon response like so:
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read -p "Breakfast?[y/n]: "; if [[ $REPLY = y ]]; then echo "Here you go, an egg sandwich"; else echo "Here, you shoudl at leats have some coffee."; fi
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#### LOOPS ####
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That's it for conditionals! (short chapter!)
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Next up is Loops, which can be found at https://guide.bash.academy/loops
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This section was not yet completed at the time of this writing, so instead we went to gokicker.com and picked up their Bash for Professionals free book pdf (see ~/Documents/Code/programming_books)
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We picked up for loops at section 10.3:
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A simple for loop can be displayed as such:
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arr=(a b c d e f)
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for i in "${arr[@]}" ; do
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echo "$i"
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done
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# Continue on at 13.1 of Bash Notes for Professionals from GoalKicker.com
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