Bash / Shell

Criteria:

• Understand loops, variables, conditionals, functions
• Create a portfolio project
• Write a blog article about what you learned

SHEBANG

Used for specifying the interpreter for executing a shell script. By including this shebang at the beginning of a shell script, you ensure that the script is executed correctly with the appropriate interpreter. It tells the operating system which program should interpret and execute the commands in the script

#!/bin/bash

LOOPS

While Loops:

while [ <test>]
  do
  <command>
done

For Loop:

this is more akin to a forEach() loop in JS since the loop number is automatically set by

for var in <$list>
  do
  <commands>
done

Ranges:

More akin to the vanilla for loop in JS

for value in {1..5}
  do
  echo $value
done

---

VARIABLES

Two actions performed for variables:

1. Setting value for variables
2. Reading value of variables

Setting Variables:

variable="test"

Reading Variables:

echo $variable

Local Variables:

local variables are variables that are defined and accessible only within the scope of a specific function.

Setting Local Variables:

calculate_sum() {
  local a=5
  local b=10
  local sum=$((a + b))
  echo "The sum is: $sum"
}

---

CONDITIONALS

Conditional operators:

Operator	Description
-eq	Returns true if two numbers are equivalent
-lt	Returns true if a number is less than another number
-gt	Returns true if a number is greater than another number
==	Returns true if two strings are equivalent
≠	Returns true if two strings are not equivalent
!	Returns true if the expression is false

If Statement:

if [ conditional ]; then
  <command>
fi

If-else Statement:

if [ conditional ]; then
  <command>
  else
  <command>
fi

If-elif-else Statement:

if [ conditional ]; then
  <command>
    elif [ conditional ]; then 
    <command>
    else
    <command>
fi

---

FUNCTIONS

There are two ways to declare functions in bash:

1. First way to Declare a Function:

function_name() {
  <commands>
}

1. Second way to Declare a Function:

function function_name() {
  <commands>
}

Single-line Function:

function_name() { <commands>; }
# in single line, requires semi-colon after last command

Passing arguments:

add_numbers() {
  a="$1"
  b="$2"
  c=$(($1 + $2))
  echo -e "${GREEN}The sum of $a and $b is $c${NC}"
}
# notice how args are passed
add_numbers 4 6

Return Values:

Unlike in other programming languages, Bash functions don’t allow you to return a value when called. When a bash function completes, its return value is the status of the last statement executed in the function, 0 for success and non-zero decimal number in the 1 - 255 range for failure.`

The return status can be specified by using the return keyword, and it is assigned to the variable $?. The return statement terminates the function. You can think of it like the function’s exit status.

return is used to exit a function and provide a value back to the caller, while exit is used to terminate the entire script or shell session.

Success Return:

# success
success_function() {
  return 0
}

success_function
echo -e "?$"

Failure Return: