awk.md

• matching groups

gawk '{
    match($0, /ns0:OpenDate>(.+?)<\/ns0:OpenDate.+?ns0:CloseDate>(.+?)<\/ns0:CloseDate/, Arr)
    printf "%s %s\n", Arr[1], Arr[2]
    }' *

linux_snipptes.md

• list top 20 largest file sizes(bytes) in a directory

du -a -b backup/ | sort -n -r | head -n 20

• list top 10 largest file size(bytes) in a directory

find $HOME -type f -printf '%s %p\n' | sort -nr | head -10

shell_scripting

############################################## Notations ############################################
Brackets []
Parentheses ()
Braces {}
(Double) Quotation marks ""
(Single) Quotation marks (apostrophes) ''
Backticks `` (Same as the tilde ~ key)

############################################## VARIABLES ############################################

# Declare the variables using declare 
declare -r read_only=1   # read only variable
declare -i number  # integer variable
declare -a indices
decalre -f function_name
declare -x car="audi" # 
Another use of declare is 
declare | grep HOME  # which returns HOME=/home/bozo

# Variables are declared and assigned without $ and without {}. You have to use
var=10
to assign. In order to read from the variable (in other words, 'expand' the variable), you must use $.

$var      # use the variable
${var}    # same as above
${var}bar # expand var, and append "bar" too # output is 10bar
$varbar   # same as ${varbar}, i.e expand a variable called varbar, if it exists.

You are also able to do some text manipulation inside the braces:
STRING="This is a string"
echo ${STRING// /_}

$(command) - used to store the command result in 
STR2=$(date)
Now STR2 stores the string "Thu May 7 09:32:06 PDT 2015".

$VAR vs ${VAR} and to quote or not to quote:
VAR=$VAR1
VAR=${VAR1}
VAR="$VAR1"
VAR="${VAR1}"  
all the above output will be same
when to quote: let say if we have a directory with spaces like kafka server/, it is better to use quotes for VAR , so it
thinks as a single output. if not treated as a multiple words and we cannot delete the directory using rm
rm -rf $VAR #wont work
rm -rf "$VAR"

############################################## ARRAYS ###############################################

# to declare and initiliaze an array
array=( item1 item2 item3 )
distro=("redhat" "debian" "gentoo")

# we can access the individual elements using
${array[0]}" , "${array[1]}" , "${array[2]}

# length of the array using 
${#array[@]}

# to print all elements using for loop
tLen=${#array[@]}
for (( i=0; i<${tLen}; i++ ));
do
  echo ${array[$i]}
done

# access the array indices using, this need to used in for loop
for $index in ${!array[@]}:
  echo ${array[$index]}

# to split the string based on specified delimiter
IFS=':' read -r -a array <<< "TEST:0:1234"

################################################# ASSOCIATIVE ARRAYS #############################################

# initialize the associative array
declare -A services_mapping=( [AZURE]=azure_service_bus [BLOB]=blob)

# another method to initialize the array
declare -A services_mapping
services_mapping[AZURE]=azure_services_bus
services_mapping[BLOB]=blob

# Display the content of the key:
${services_mapping[AZURE]}

# list all the keys
${!services_mapping[*]} # will return an arrays.

# Looping through keys and values in an associative array
for key in "${!services_mapping[@]}"; do 
  echo $key --- ${services_mapping[$key]}; 
done

# Loop through all values in an associative array
for value in "${services_mapping[@]}"; do 
  echo $value; 
done

# unsetting the associative array
unset services_mapping

# deleting the key
unset services_mapping[BLOB]

# deleting key which has space
unset MYMAP["foo Z"]

# length of the associatuve array
${#services_mapping[@]}



################################################ CONDITIOINAL OPERATORS ############################################
if operator:
if [[ $container_name == "" ]] || [[ $image_name == "" ]] || [[ $json_file == "" ]]; then
    # do something
fi

# for strings comparison we use '=='
# for arithmetic comparision we use '-eq'
# to check whether the variable is empty or not, we use '-z'


################################################## eval command ###################################################
eval - construct command by concatenating arguments.The  args  are read and concatenated together into a single command.  
This command is then read and executed by the shell, and its  exit status is returned as the value of eval. If there are
no args, or only null arguments, eval returns 0.

example:
foo=10 x=foo
y='$'$x
echo $y # output to $foo
eval y='$'$x
echo $y # 10
In the first line you define $foo with the value '10' and $x with the value 'foo'.
Now define $y, which consists of the string '$foo'. The dollar sign must be escaped with '$'.
To check the result, echo $y.
The result will be the string '$foo'
Now we repeat the assignment with eval. It will first evaluate $x to the string 'foo'. Now we have the statement y=$foo which will get evaluated to y=10.
The result of echo $y is now the value '10'.

############################################  ARTITHMETIC EXPRESSIONS ############################
For integers:
Use arithmetic expansion: $((EXPR))
  num=$((num1 + num2))
  num=$(($num1 + $num2))       # also works
  num=$((num1 + 2 + 3))        # ...
  num=$[num1+num2]             # old, deprecated arithmetic expression syntax
  Using the external expr utility. Note that this is only needed for really old systems.
  num=`expr $num1 + $num2`     # whitespace for expr is important

For floating point:
Bash doesn't directly support this, but there's a couple of external tools you can use:
  num=$(awk "BEGIN {print $num1+$num2; exit}")
  num=$(python -c "print $num1+$num2")
  num=$(perl -e "print $num1+$num2")
  num=$(echo $num1 + $num2 | bc)   # whitespace for echo is important
  You can also use scientific notation (e.g.: 2.5e+2)


################################################# GETOPTS ##############################################
package=""  # Default to empty package
target=""  # Default to empty target

# Parse options to the `pip` command
while getopts ":h" opt; do
  case ${opt} in
    h )
      echo "Usage:"
      echo "    pip -h                      Display this help message."
      echo "    pip install <package>       Install <package>."
      exit 0
      ;;
   \? )
     echo "Invalid Option: -$OPTARG" 1>&2
     exit 1
     ;;
  esac
done
shift $((OPTIND -1))

subcommand=$1; shift  # Remove 'pip' from the argument list
case "$subcommand" in
  # Parse options to the install sub command
  install)
    package=$1; shift  # Remove 'install' from the argument list

    # Process package options
    while getopts ":t:" opt; do
      case ${opt} in
        t )
          target=$OPTARG
          ;;
        \? )
          echo "Invalid Option: -$OPTARG" 1>&2
          exit 1
          ;;
        : )
          echo "Invalid Option: -$OPTARG requires an argument" 1>&2
          exit 1
          ;;
      esac
    done
    shift $((OPTIND -1))
    ;;
esac