Bash Script Programming

Concept : Using read Command in Shell Scripts
Description : The read command in Linux prompts the user for input and stores the input in a variable for further processing. 
              This is often used to capture menu choices or user-supplied data in scripts.
Purpose : To interactively gather user input in shell scripts and make decisions based on that input.
Linux commands :
read variable_name : Waits for user input and stores it in variable_name.
echo $variable_name : Prints the value stored in a variable.
chmod +x script.sh : Makes the script executable.
./script.sh : Runs the script in the current directory.
Script example: Beginning----
#!/bin/bash
echo "Menu"
echo "1 - List logged on users"
echo "2 - Show kernel version"
echo " "
echo "Please enter your choice:"
read menuchoice
echo "You typed: $menuchoice"
Script example: End----
Additional Info : 
-The value captured can be a string, number, or any text. 
-Proper validation should be done to handle unexpected input.

Concept : Logic Statements (if, elif, else) in Shell Scripts
Description : Logic statements allow conditional execution of commands based on the evaluation of expressions. 
              The if statement checks a condition, elif checks another condition if the first fails,
              and else handles all other cases.
Purpose : To execute different commands depending on user input or system conditions.
Linux commands :
[ condition ] : Tests a condition (must have spaces after [ and before ]).
-gt, -lt, -eq : Greater than, less than, equal for numeric comparisons.
echo "message" : Prints messages depending on the condition outcome.
Script example: Beginning----
#!/bin/bash
if [ $menuchoice -eq 1 ]; then
  clear
  who
elif [ $menuchoice -eq 2 ]; then
  clear
  uname -r
else
  exit
fi
Script example: End----
Additional Info : 
-if ... fi constructs end with fi. 
-Logical operators like && can combine conditions for more complex checks.

Concept : Loops (while and until) in Shell Scripts
Description : Loops allow repeated execution of commands until a condition is met.
              -while executes while a condition is true,
              -until executes until a condition becomes true.
Purpose : Automate repetitive tasks or continuously prompt the user until a specific input is given.
Linux commands :
while [ condition ]; do ... done : Executes commands as long as the condition is true.
until [ condition ]; do ... done : Executes commands as long as the condition is false.
echo "message" : Display messages during each iteration.
Script example: Beginning----
#!/bin/bash
while [ $menuchoice -ne 3 ]; do
  # commands
done

#Or use until 

#!/bin/bash
until [ $menuchoice -eq 3 ]; do
  # commands
done
Script example: End----
Additional Info : 
-Both loops use do ... done to enclose commands. 
-The choice between while and until depends on whether you want to loop on true or false conditions.

Concept : Using test Command
Description : The test command evaluates expressions or file attributes and returns true or false. 
              It is often used in scripts to verify conditions.
Purpose : To check numeric, string, or file conditions before performing actions.
Linux commands :
test 10 -gt 5 && echo True : Checks if 10 is greater than 5 and prints True if true.
test -e filename : Checks if a file exists.
test -w filename : Checks if a file is writeable.
echo $? : Prints exit status of the last command (0 = true, >0 = false).
Script example: Beginning----
#!/bin/bash
# Numeric test
test 10 -gt 5 && echo "True"

# File existence
test -e filename
echo $?  # 0 = exists, >0 = does not exist

# Write access
test -w filename && echo "Writeable"

Script example: End----
Additional Info : 
-[ ] can also be used instead of test. Always include spaces inside square brackets. 
-Useful for validating user input or file system states in scripts.

Concept : Shell Expansion
Description : Expansion replaces typed input with an expanded value before executing the command. 
              This includes wildcard expansion, arithmetic, brace expansion, command substitution, and tilde for home directory.
Purpose : To simplify command input, generate sequences, and store command outputs in variables.
Linux commands :
echo * : Expands to all files in the current directory.
echo {6..8} : Expands to 6 7 8 (sequence generation).
CURRENTDATE=$(date) : Command substitution to store output in a variable.
cd ~ : Tilde expansion to home directory.
Additional Info : 
-Expansion helps automate repetitive tasks, generate sequences, and use command outputs directly in scripts. 
-Arithmetic expansion also allows shell scripts to perform calculations.

Concept : Exit Status in Linux
Description : Exit status indicates whether the last executed command completed successfully (0) or failed (>0). 
              It is stored in the built-in variable $?.
Purpose : To check the result of a command or script execution and handle errors appropriately.
Linux commands :
echo $? : Prints the exit status of the previous command.
[[ 10 -gt 5 ]] : Tests a condition without printing output; exit status can be checked via $?.
if [ $? -eq 0 ]; then ... else ... fi : Executes commands based on the success or failure of the previous command.
Script example: Beginning----
#!/bin/bash
cd /nonexistent_dir
if [ $? -eq 0 ]; then
  echo "No problem."
else
  echo "Anthony, we have a problem."
fi
Script example: End----
Additional Info : 
-Useful in scripts to chain commands, handle errors, or report issues. 
-Can be combined with && and || operators to manage control flow.