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1. Getting working directory info

>>> pwd

2. Getting files and directory info

>>> ls

3. How else can I identify files and directories?

# The shell decides if a path is absolute or
# relative by looking at its first character:
# if it begins with /, it is absolute, and
# if it doesn't, it is relative.

# Getting directory info using relative path
>>> ls  directory/

# Getting directory info using absolute path info
>>> ls /home/directory/

4. How can I move to another directory?

# Using relative path
>>> cd directory/

# using absolute path
>>> cd /home/directory

5. How can I move up a directory?

# Moving up towards current directory
>>>  cd ..

# Moving to same directory
>>> cd .

# Moving to home directory
>>> cd ~

6. How can I copy files?

# Copying a tmp.txt file to new tmp_copy.txt file
>>> cp tmp.txt tmp_copy.txt
# Note : If copy already exists then it will be overwritten

# If last arg in cp is a directory,
# then it will copy all files to that directory
>>> cp seasonal/autumn.csv seasonal/winter.csv backup

# Copying a file to new directory
>>> cp seasonal/summer.csv backup/summer.bck

1. A shell script contains commands that are executed as if you typed them in the terminal.

2. We'll be using Vim for this tutorial

   1. Install Vim :

      sudo apt-get install vim
      

   2. Vim Commands

      1. i : insert mode
      
      2. <ESC> : enter command mode
          i. w : Save / Don't Exit
          ii. wq : Save / Quit
          iii. q! : Quit / Discard Changes
          iv. w : Move to front of next word
          v. b : Move backwards to front of word
          vi. 0 : Move to start of line
          vii. $ : Move to end of line
          viii. G : Jump to last line
      
      3. *Move around with arrows*
      4. *:set number : Displays line numbers*
      5. *:syntax on : Syntax Highlighting*
      6. *:set tabstop=2 : Spaces in tab*
      7. *:set autoindent : Indent new lines*
      8. *Save these in your home/~/.vimrc file*
          a. *Find out what vimrc file you are using with this command in Vim :echo $MYVIMRC*
      

3. Hello World Script

   # The #! shebang tells the system the interpreter to use for the script
   #!/bin/bash
   # Comment
   echo 'Hello World' # Print the string to the screen
   

   1. To make it executable chmod 755 hello_world

   2. Execute with ./hello_world

   3. The numbers after chmod define who can do what with the file

   4. The numbers represent the Owner, the Group and Everyone else

   5. What the numbers mean

      1. 7 : Read, Write & Execute
      2. 6 : Read & Write
      3. 5 : Read & Execute
      4. 4 : Read Only
      5. 3 : Write & Execute
      6. 2 : Write Only
      7. 1 : Execute Only
      8. 0 : None
      

4. We define variables like this myName="Derek"

   1. The variable name starts with a letter or _ and then can also contain numbers

   2. The shell treats all variables as strings

   3. When declaring a variable you can't have whitespace on either side of the =

   4. code snippet:

      #!/bin/bash
      declare -r NUM1=5 # Declare a constant
      num2=4
      
      # Use arithmetic expansion for adding
      num3=$((NUM1+num2))
      num4=$((NUM1-num2))
      num5=$((NUM1*num2))
      num6=$((NUM1/num2))
      
      # Place variables in strings with $
      echo "5 + 4 = $num3"
      echo "5 - 4 = $num4"
      echo "5 * 4 = $num5"
      echo "5 / 4 = $num6"
      echo $(( 5**2 ))
      echo $(( 5%4 ))
      
      # Assignment operators allow for shorthand arithmetic
      # +=, -=, *=, /=
      rand=5
      let rand+=4
      echo "$rand"
      
      # Shorthand increment and decrement
      echo "rand++ = $(( rand++ ))"
      echo "++rand = $(( ++rand ))"
      echo "rand-- = $(( rand-- ))"
      echo "--rand = $(( --rand ))"
      
      # Use Python to add floats
      num7=1.2
      num8=3.4
      num9=$(python -c "print $num7+$num8")
      echo $num9
      
      # You can print over multiple lines with a Here Script
      # cat prints a file or any string past to it
      cat << END
      This text
      prints on
      many lines
      END
      

1. You can use functions to avoid the need to write duplicate code

2. Delete all code in Vim with gg then dG

3. code snippet:

   #!/bin/bash
   # Define function
   getDate() {
   
       # Get current date and time
       date
   
       # Return returns an exit status number between 0 - 255
       return
   }
   
   getDate
   
   # This is a global variable
   name="Derek"
   
   # Local variable values aren't available outside of the function
   demLocal() {
       local name="Paul"
       return
   }
   
   demLocal
   
   echo "$name"
   
   # A function that receives 2 values and prints a sum
   getSum() {
   
       # Attributes are retrieved by referring to $1, $2, etc.
       local num3=$1
       local num4=$2
   
       # Sum values
       local sum=$((num3+num4))
   
       # Pass values back with echo
       echo $sum
   }
   
   num1=5
   num2=6
   
   # You pass atributes by separating them with a space
   # Surround function call with $() to get the return value
   sum=$(getSum num1 num2)
   echo "The sum is $sum"
   

1. Basics

#!/bin/bash

# You can use read to receive input which is stored in name
# The p option says that we want to prompt with a string
read -p "What is your name? " name
echo "Hello $name"

read -p "How old are you? " age

# You place your condition with in []
# Include a space after [ and before ]
# Integer Comparisons: eq, ne, le, lt, ge, gt
if [ $age -ge 16 ]
then
    echo "You can drive"

# Check another condition
elif [ $age -eq 15 ]
then
    echo "You can drive next year"

# Executed by default
else
  echo "You can't drive"

# Closes the if statement
fi

2. Extended integer test

#!/bin/bash

read -p "Enter a number : " num

if ((num == 10)); then
    echo "Your number equals 10"
fi

if ((num > 10)); then
    echo "It is greater then 10"
else
    echo "It is less then 10"
fi

if (( ((num % 2)) == 0 )); then
    echo " It is even"
fi

# You can use logical operators like &&, || and !
if (( ((num > 0)) && ((num < 11)) )); then
    echo "$num is between 1 and 10"
fi

# && and || can be used as control structures

# Create a file and then if that worked open it in Vim
touch samp_file && vim samp_file

# If samp_dir doesn't exist make it
[ -d samp_dir ] || mkdir samp_dir

# Delete file rm samp_file
# Delete directory rmdir samp_dir

3. Testing strings

#!/bin/bash
str1=""
str2="Sad"
str3="Happy"

# Test if a string is null
if [ "$str1" ]; then
    echo "$str1 is not null"
fi

if [ -z "$str1" ]; then
    echo "str1 has no value"
fi

# Check for equality
if [ "$str2" == "$str3" ]; then
    echo "$str2 equals $str3"
elif [ "$str2" != "$str3" ]; then
    echo "$str2 is not equal to $str3"
fi

if [ "$str2" > "$str3" ]; then
    echo "$str2 is greater then $str3"
elif [ "$str2" < "$str3" ]; then
    echo "$str2 is less then $str3"
fi

# Check the file test_file1 and test_file2
file1="./test_file1"
file2="./test_file2"

if [ -e "$file1" ]; then
    echo "$file1 exists"

    if [ -f "$file1" ]; then
        echo "$file1 is a normal file"
    fi

    if [ -r "$file1" ]; then
        echo "$file1 is readable"
    fi

    if [ -w "$file1" ]; then
        echo "$file1 is writable"
    fi

    if [ -x "$file1" ]; then
        echo "$file1 is executable"
    fi

    if [ -d "$file1" ]; then
        echo "$file1 is a directory"
    fi

    if [ -L "$file1" ]; then
        echo "$file1 is a symbolic link"
    fi

    if [ -p "$file1" ]; then
        echo "$file1 is a named pipe"
    fi

    if [ -S "$file1" ]; then
        echo "$file1 is a network socket"
    fi

    if [ -G "$file1" ]; then
        echo "$file1 is owned by the group"
    fi

    if [ -O "$file1" ]; then
        echo "$file1 is owned by the userid"
    fi

fi

4. With extended test [[ ]] you can use Regular Expressions

#!/bin/bash

read -p "Validate Date : " date

pat="^[0-9]{8}$"

if [[ $date =~ $pat ]]; then
    echo "$date is valid"
else
    echo "$date is not valid"
fi

5. Read multiple values

#!/bin/bash

read -p "Enter 2 Numbers to Sum : " num1 num2

sum=$((num1+num2))

echo "$num1 + $num2 = $sum"

# Hide the input with the s code
read -sp "Enter the Secret Code" secret

if [ "$secret" == "password" ]; then
    echo "Enter"
else
    echo "Wrong Password"
fi

6. You can set what separates the values with IFS

#!/bin/bash

# Store the original value of IFS
OIFS="$IFS"

# Set what separates the input values
IFS=","

read -p "Enter 2 numbers to add separated by a comma" num1 num2

# Use the parameter expansion ${} to substitute any whitespace
# with nothing
num1=${num1//[[:blank:]]/}
num2=${num2//[[:blank:]]/}

sum=$((num1+num2))

echo "$num1 + $num2 = $sum"

# Reset IFS to the original value
IFS="$OIFS"

# Parameter expansion allows you to do this
name="Derek"
echo "${name}'s Toy"

# The search and replace allows this
samp_string="The dog climbed the tree"
echo "${samp_string//dog/cat}"

# You can assign a default value if it doesn't exist
echo "I am ${name:-Derek}"

# This uses the default if it doesn't exist and assigns the value
# to the variable
echo "I am ${name:=Derek}"
echo $name

7. Use case to when it makes more sense then if

#!/bin/bash

read -p "How old are you : " age

# Check the value of age
case $age in

# Match numbers 0 - 4
[0-4])
    echo "To young for school"
    ;; # Stop checking further

# Match only 5
5)
    echo "Go to kindergarten"
    ;;

# Check 6 - 18
[6-9]|1[0-8])
    grade=$((age-5))
    echo "Go to grade $grade"
    ;;

# Default action
*)
    echo "You are to old for school"
    ;;
esac # End case

8. Ternary Operator performs different actions based on a condition

#!/bin/bash
can_vote=0
age=18

((age>=18?(can_vote=1):(can_vote=0)))
echo "Can Vote : $can_vote"

1. Strings

#!/bin/bash

rand_str="A random string"

# Get string length
echo "String Length : ${#rand_str}"

# Get string slice starting at index (0 index)
echo "${rand_str:2}"

# Get string with starting and ending index
echo "${rand_str:2:7}"

# Return whats left after A
echo "${rand_str#*A }"

1. While Loop

#!/bin/bash

num=1

while [ $num -le 10 ]; do
    echo $num
    num=$((num + 1))
done

2. Continue and Break

#!/bin/bash

num=1

while [ $num -le 20 ]; do

    # Don't print evens
    if (( ((num % 2)) == 0 )); then
        num=$((num + 1))
        continue
    fi

    # Jump out of the loop with break
    if ((num >= 15)); then
        break
    fi

    echo $num
    num=$((num + 1))
done

3. Until loops until the loop is true

#!/bin/bash

num=1

until [ $num -gt 10 ]; do
    echo $num
    num=$((num + 1))
done

4. Use read and a loop to output file info

#!/bin/bash
while read avg rbis hrs; do

    # printf allows you to use \n
    printf "Avg: ${avg}\nRBIs: ${rbis}\nHRs: ${hrs}\n"

# Pipe data into the while loop
done < barry_bonds.txt

5. There are many for loop options. Here is the C form.

#!/bin/bash
for (( i=0; i <= 10; i=i+1 )); do
    echo $i
done

6. We can cycle through ranges
#!/bin/bash
for i in {A..Z}; do
    echo $i
done

1. Bash arrays can only have one dimension and indexes start at 0

2. Messing with arrays

#!/bin/bash
# Create an array
fav_nums = (3.14 2.718 .57721 4.6692)
echo "Pi : ${fav_nums[0]}"

# Add value to array
fav_nums[4]=1.618

echo "GR : ${fav_nums[4]}"

# Add group of values to array
fav_nums+=(1 7)

# Output all array values
for i in ${fav_nums[*]}; do
    echo $i;
done

# Output indexes
for i in ${!fav_nums[@]}; do
    echo $i;
done

# Get number of items in array
echo "Array Length : ${#fav_nums[@]}"

# Get length of array element
echo "Index 3 length : ${#fav_nums[3]}"

# Sort an array
sorted_nums=($(for i in "${fav_nums[@]}"; do
    echo $i;
done | sort))

for i in ${sorted_nums[*]}; do
    echo $i;
done

# Delete array element
unset 'sorted_nums[1]'

# Delete Array
unset sorted_nums

1. Positional parameters are variables that can store data on the command line in variable names 0 - 9

   1. $0 always contains the path to the executed script
   2. You can access names past 9 by using parameter expansion like this ${10}

2. Add all numbers on the command line

#!/bin/bash

# Print the first argument
echo "1st Argument : $1"

sum=0

# $# tells you the number of arguments

while [[ $# -gt 0 ]]; do

    # Get the first argument
    num=$1
    sum=$((sum + num))

    # shift moves the value of $2 into $1 until none are left
    # The value of $# decrements as well shift
done

echo "Sum : $sum"