Not old to learn

test.sh

• echo
• read
• if statement
• case statement
• sub string

friend.sh

• if statement
• append to file

for.sh

• for statement

• selete statement

• changing file's extension

• [[ -n string ]] - string is non-empty

• [[ -z string ]] - string is empty

• [[ -e file ]] - file exists

• [[ -f file ]] - file is a regular file

• [[ -d file ]] - file is a directory

• [[ -t file ]] - fd is open and refers to a terminal

• LIST=$(<friends.txt) - read line from friends.txt to LIST variable

pyramid.sh

• while statement
• let statement
• if statement in one line
• replace ex. ${val//-/- /} [replace "-" with "- "]

df # Displays the amount of disk space available
-h # human readable format (K, M, G)
-T # print file system type

cut # Remove or "cut out" sections of each line of a file or files.
-c # characters
Ex. 	cut -c 3-12 data.txt -> e two thre
-f # fields
Ex. 	cut -f 3 data.txt -> tree
	cut -f 2-3 data.txt -> two three four
	cut -f 1-2,4-5 data.txt -> one two four five
	cut -f 3- data.txt -> three four five
	cut -f -3 data.txt -> one two three
let # is a builtin function of Bash that allows us to do simple arithmetic.
Ex.	let a=5+4 # 9
	let "a = 5 + 4" # 9
	let a++ # 10
	let "a = 4 * 5" # 20
expr # is similar to let except instead of saving the result to a variable it instead prints the answer. Unlike let you don't need to enclose the expression in quotes. You also must have spaces between the items of the expression.
Ex.	expr 5 + 4 # 9
	expr "5 + 4" # 5 + 4
	expr 5+4 # 5+4
	expr 5 \* 12 # 60 -> Some characters have a special meaning to Bash so we must escape them (put a backslash in front of) to remove their special meaning.
	a=$( expr 10 - 3 ) # 7
Double Parentheses # In the section on Variables we saw that we could save the output of a command easily to a variable. It turns out that this mechanism is also able to do basic arithmetic for us if we tweak the syntax a little.
Ex.	a=$(( 4 + 5 )) # 9
	a=$((3+5)) # 8
	b=$(( a + 3 )) # a and $a are work
	a=$(( 4 * 5 )) # doing multiplication doesn't need to escape *
Length of a Variable # This isn't really arithmetic but it can be quite useful. If you want to find out the lengh of a variable (how many characters) 
you can do the following: ${#variable}
Ex.	a='Hello World' echo ${#a} # 11

Note

$ IFS=$'\r\n' GLOBIGNORE='*' command eval  'XYZ=($(cat /etc/passwd))'
$ echo "${XYZ[5]}"
sync:x:5:0:sync:/sbin:/bin/sync

CR: https://stackoverflow.com/questions/11393817/read-lines-from-a-file-into-a-bash-array

CR: http://www.tldp.org/LDP/Bash-Beginners-Guide/html/sect_08_02.html https://stackoverflow.com/questions/1224766/how-do-i-rename-the-extension-for-a-batch-of-files https://www.cyberciti.biz/faq/bash-for-loop/ http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_10_02.html http://tldp.org/LDP/abs/html/comparison-ops.html https://www.youtube.com/watch?v=uqHjc7hlqd0&t=1602s https://www.computerhope.com/unix.htm https://ryanstutorials.net/bash-scripting-tutorial/bash-arithmetic.php http://workshop-bash.com/7-Brace-Expansion.html

Command Groups

Subshell:

Evaluate list of commands in a subshell, meaning that its environment is distinct from the current shell and its parameters are contained.

(list)

Group command:

Evaluate list of commands in the current shell, sharing the current shell's environment.

{list;}