Update notes.md

Author: OddExtension5

bash shell/notes.md

@@ -156,3 +156,105 @@
 | > | Output redirect |
 | / | Pathname directory separator |
 | ? | Single character wildcard |
+
+
+## Understanding Command Line Parsing
+
+ + When a command is interpreted by the shell, the shell interprets all special characters
+ + This process is known as ***command line parsing***
+ + Commands themselves may interpret parts of the command line as well
+ + To ensure that a special character is interpreted by the command and not by the shell, use quoting
+ 
+## Quoting
+ 
+ + Quoting is used to treat special characters literally
+ + If a string of characters is surrounded with single quotation marks, all characters are stripped of the special meaning they may have
+ + Imagine **echo 2*3 > 5**, whic would be interpreted
+ + Or imagine **find .-name '*.doc'** which ensures that **find** interprets *.doc and not the shell
+ + Double quotes are weak quotes and treat only some special characters as special
+ + A backslash can be used to escape the one following character
+ 
+## Using Double Quotes
+
+ + Double quotes ignore pipe characters, aliases, tilde substitution, wildcard expression, and splitting into words using delimiters
+ + Double quotes do allow parameter substitution, command substitution, and arithmetic expression evaluation
+ + best practice: use single quotes, unless you specifically need parameter, command, or arithmetic substitution
+ 
+## Handling Script Arguments
+ 
+ + Any argument that was used when starting a script, can be dealt with from within the script
+ + Use $1, $2 and so on to refer to the first, the second, etc. argument
+ + **$0** refers to the name of the script itself
+ + Use **${nn}** or **shift** to refer to arguments beyond 9
+ + Arguments that are stored in $1 etc. are read only and cannot be changed from within the script
+ 
+> Try : [simplearg](https://github.com/OddExtension5/Linux-Guide/blob/master/bash%20shell/code/simplearg.sh)
+
+Run Commands : 
+
+ + chmod + simplearg.sh
+ + ./simplearg sushil sneha
+ + ./simplearg sushil sneha debagni
+ + ./simplearg sushil sneha debagni aakansha
+ 
+## Handling Argumets the Smart Way
+ 
+ + The previous example works only if the amount of argumets is known on berforehand
+ + If this is not the case, use **for** to evaluate all possible arguments
+ + Use **$@** to refer to all arguments provided, where all arguments can be treated one by one
+ + Use **$#** to count the amount of argumets provided
+ + Use **$*** if you need a single string that contains all arguments (use in specific cases only)
+ 
+> Try : [nicearg](https://github.com/OddExtension5/Linux-Guide/blob/master/bash%20shell/code/nicearg.sh)
+
+Run commands:
+ 
+ + chmod +x nicearg
+ + ./nicearg a b c
+ 
+Output:
+ 
+ + $* gives a b c
+ + $# gives 3
+ + $@ gives a b c
+ + $0 is path/nicearg
+ + a b c
+ + a
+ + b
+ + c
+ 
+Testing script: **bash -x filename**
+
+## Using Shift
+ 
+ + Shift removes the first argument from a list so that the second argument gets stored in $1
+ + Shift is useful in older shell versions that do not understand ${10} etc.
+ 
+> Try: [shift_arg.sh](https://github.com/OddExtension5/Linux-Guide/blob/master/bash%20shell/code/shift_arg.sh) <br>
+> Try: [shift_arg2.sh](https://github.com/OddExtension5/Linux-Guide/blob/master/bash%20shell/code/shift_arg2.sh)
+
+## Understanding Command Substitution
+
+ + Command susbtitution allows using the result of a command in a script
+ + Useful to provide ultimate flexibility
+ + Two allowd syntaxes:
+ + **`command`** (deprecated)
+ + **$(command)** (preferred)
+ + **ls -l $(which passwd)**
+ 
+## Using String Verification
+ 
+ + When working with arguments and input, it is useful to be able to verify availability and correct use of a string
+ + Use **test -z** to check if a string is empty
+ + **test -z $1 && exit 1**
+ + Use **[[...]]** to check for specific patterns
+ + **[[ $1=='[a-z]*' ]] || echo $1 does not start with a letter**
+ 
+ 
+# Using Here Documents
+ 
+ + In a here document, I/O redirection is used to feed a command list into an interactive program or command, such as for instance **ftp** or **cat**
+ + Use it in scripts to replace echo for long texts that need to be displayed
+ + Use it if in a script a command is called that opens its own prompt, such as an FTP client interface
+ 
+> Try: [heredoc1.sh](https://github.com/OddExtension5/Linux-Guide/blob/master/bash%20shell/code/heredoc1.sh)