Shebang

  • #!/bin/bash
  • It tells the system to use /bin/bash as the interpreter for this script

    Variables

  • Can contain a number, a character or a string.
  • Can contain ‘_’.
  • Case sensitive.
  • Value assignment through ‘=’. No space permitted between =.
  • eg. 
    name='Kush'
age=26
h_w='Hello World'
  • ((...)) tells Bash to interpret the expression as arithmetic operation 
    a=1
b=4
echo $((a+b))
  • ’ \ ‘ is used to escape special character meaning. 
    PRICE_PER_APPLE=5
echo "The price of an Apple today is: \$HK $PRICE_PER_APPLE"

Output : The price of an Apple today is: $HK 5

  • ”${}” is used to avoid ambiguity.

  • `` can assign a command’s output to a variable eg.

    FILELIST=`ls`

Use of $

  • access value of a variable
    • echo "hello, $name"
  • execute a command $(command)
  • performing arithmetic operations $((a+b))
  • positional reference $1 $2

Passing Arguments to the Script

  • Inside the script, the $1 variable references the first argument in the command line, $2 the second argument and so forth.
  • The variable $0 references to the current script.

eg.

#!/bin/bash
echo "Current file :"$0
echo "Name : $1"
echo "Age : $2"

Output : user1:~$ ./temp.sh Kush 20
Current file :./temp.sh
Name : Kush
Age : 20

  • The variable $# holds the number of arguments passed to the script

  • The variable $@ holds a space delimited string of all arguments passed to the script

Arrays

  • my_array=(apple banana "Fruit Basket" orange)
  • echo ${#array[@]} : given the size of the array (4)

  • ``echo ${my_array[3]}` : orange

  • my_array[4]="carrot" : value assignment without a $ and curly brackets
  • echo ${#my_array[@]} : 5
  • echo ${my_array[${#my_array[@]}-1]} : carrot

Piping

  • |
  • used to pass the output of one command as input to another command. This allows you to chain commands together efficiently.
  • Syntax : command1 | command2 | command3
  • ls -l | grep "txt"
    • ls -l lists files in long format.
    • grep "txt" filters the list to show only files containing “txt” in their names.
    • List Files and Search for a Pattern

Output Redirection

  • > write to a file
  • >> append to a file

Input Redirection

  • < redirects input from a file
  • cat > file.txt = cat file.txt
  • cat > file.txt
    • Bash opens file.txt for reading.
    • It redirects the content of file.txt into cat as if you typed it from the keyboard.
    • cat reads from standard input (stdin) and prints it.
  • << redirects a block of text as input
  • Syntax : 
    command <<EOF
text line 1
text line 2
EOF
  • <<< redirects a single string as input
  • cat <<< "Hello, World!" o/p Hello, World!

wc

  • Syntax : wc [options] [file]
  • options :
    • -l count lines
    • -w count words
  • wc -w < textfile.txt o/p = 3
  • wc -w <<< "hello world I am Kush" o/p = 6

if, elif, else statement

#!/bin/bash
if [ ${1,,} = kush ]; then
        echo "welcome"
elif [ ${1,,} = help ]; then
        echo "enter your username"
else
        echo "I don't know you"
fi
  • [ ] = test command
    • use -gt , -eq , -lt , [...] && [...]
    • $a , $b are used because a and b are treated like string
  • ${1} = first command line argument
  • ${1,,} = converts the argument to lowercase
  • fi to close the if block
  • ((...)) can be used for numeric comparisions
    • this allows use of < , > , = , && can be used inside the parenthesis

    • a and b can be used without $ because they are treated like variables

  • = is used for comparing string
  • -eq is used for numbers

CASE

#!/bin/bash
case ${1,,} in
        kush | administrator)
                echo "hello"
                ;;
        help)
                echo "enter username"
                ;;
        *)
                echo "enter valid username"
esac
  • | is used as OR
  • ;; is used to close a case
  • xyz) ‘ ) ‘ is used to end a case name
  • *) = wildcard
    • it matches anything not covered above
  • esac is used to close case block

Arrays

  • names=("Bob" "Peter" "$USER" "Big Bad John")
  • names=([0]="Bob" [1]="Peter" [20]="$USER" [21]="Big Bad John")
  • names=() creates empty array
  • echo ${names[@]} prints the content
  • echo ${names} prints only the first element
  • ` echo ${names[3]}` prints element at index 3
  • 0 indexing ### Associative Array
    • Like map in C++ or hashmap in java
    • declare -A freq 
        for num in "${arr[@]}"
  do
  ((freq[$num]++))
  done
    • To print 
        for key in "${!freq[@]}"
  do
  echo "$key: ${freq[$key]}"
  done
  • Input array : read -a arr

for loop

#!/bin/bash
names=(one two three four five six)
for item in ${names[@]}; do
        echo $item | wc -c
done
  • prints the size of each word in the array

Functions

#!/bin/bash
up="before"
since="function"
echo $up
echo $since
showuptime(){
        local up=$(uptime -p | cut -c4-)
        local since=$(uptime -s)
        cat << EOF
----
this machine has been up for ${up}
It has been running since ${since}
----
EOF
}
showuptime
echo $up
echo $since
  • local <varname> allows to use a variable locally in a function without altering the global variable

while loop

while [ condition ]; do
    # commands to execute
done