Merge pull request #4 from Pro-tutorials/03-how-bash-process-commands

add how bash process commands

03-how-bash-process-commands/01.sh

@@ -1,7 +1,50 @@
 #!/bin/bash
 
 # Author: Samir M.
-# Date: 2024-03-04
-# Modified: 2024-03-04
-# Description: Brace expansion
-# Usage: ./01.sh
+# Date: 2024-03-05
+# Modified: 2024-03-05
+# Description: How bash processes command lines
+# Usage: ./01.sh
+
+
+# Bash goes through 5 steps to interpret a command line.
+
+# Step 1: Tokenisation:
+# The command line is broken into tokens. Tokens are the smallest units of a command line.
+# Tokens are separated by metacharacters, such as spaces, tabs, and newlines, | ; & () < >.
+
+# example
+# echo "Hello, World!"  # 4 tokens: echo, "Hello, World!", newline
+
+# What is the differnce between a word and an operator?
+# A word never contain unquoted metacharacters. 
+# An operator always contains unquoted metacharacters.
+
+# example
+# echo "Hello, World!"  # 3 words: echo, "Hello, World!"
+# echo "Hello, World!"  # 1 operator: newline
+
+# Step 2: Command  identification:
+# Bash identifies simple or compound commands. 
+
+# example
+# echo "Hello, World!"  # simple command
+# if [ -f file ]; then echo "File exists"; fi  # compound command
+
+# Step 3: Expansions:
+# Bash do expansions on the tokens. Expansions are the process of replacing a token with its value.
+
+# example
+# echo *  # expansion of * is the list of files in the current directory
+
+# Step 4: Quote removal:
+# Bash removes quotes from the tokens.
+
+# example
+# echo "Hello, World!"  # "Hello, World!" is removed
+
+# step 5: Redirection:
+# Bash performs redirections and then executes the command.
+
+
+exit 0

03-how-bash-process-commands/02.sh

@@ -0,0 +1,32 @@
+#!/bin/bash
+
+# Author: Samir M.
+# Date: 2024-03-05
+# Modified: 2024-03-05
+# Description: Quoting 
+# Usage: .\02.sh
+
+# Quoting: is about removing the special meaning of characters.
+# There are three types of quotes in bash: single quotes, double quotes, and back slashes.
+
+# Single quotes:
+# Single quotes are used to prevent the interpretation of all of the special characters.
+
+# example
+filepath1='\home\sam\My Documents'
+echo $filepath1  # \home\sam\My Documents
+
+# Double quotes:
+# Double quotes are used to prevent the interpretation of most of the special characters, except $ ,  back ticks ` and \.
+
+# example
+filepath2="\home\\${USER}\My Documents"
+echo $filepath2  # \home\sam\My Documents
+
+# Back slashes:
+# Back slashes are used to prevent the interpretation of the next character.
+
+# example
+echo Hello John \& Jane  # Hello John & Jane
+
+exit 0

03-how-bash-process-commands/03.sh

@@ -0,0 +1,40 @@
+#!/bin/bash
+
+# Author: Samir M.
+# Date: 2024-03-05
+# Modified: 2024-03-05
+# Description: Tokenisation
+# Usage: .\02.sh
+
+# Tokenisation:
+# The process of breaking a command line into its a smaller parts called tokens using metacharacters.
+
+# What are metacharacters?
+# Metacharacters are characters that have a special meaning to the shell.
+# The metacharacters are: space, tab, newline, ;, (, ), <, >, |, and &.
+
+# The token can be either a word or an operator.
+# A word  doesn't contain any unquoted metacharacters.
+# An operator does contain at least one unquoted metacharacters.
+
+# Example:
+
+name="Sam"
+echo $name > file.txt 
+# we have 5 metacharacters here: space, > and newline.
+# we have 4 tokens: echo, $name and file.txt all are words, because they don't contain any unquoted metacharacters.
+# > is an operator, because it contains one unquoted metacharacter.
+
+
+
+
+
+
+
+
+
+
+
+
+
+exit 0