Conditionals and Case Statements

Conditional Operators

String Conditionals

[[ -z STRING ]]	Empty string
[[ -n STRING ]]	Not empty string
[[ STRING == STRING ]]	Equal
[[ STRING != STRING ]]	Not Equal
[[ STRING =~ STRING ]]	Regexp

Integer Conditionals

[[ NUM -eq NUM ]]	Equal
[[ NUM -ne NUM ]]	Not equal
[[ NUM -lt NUM ]]	Less than
[[ NUM -le NUM ]]	Less than or equal
[[ NUM -gt NUM ]]	Greater than
[[ NUM -ge NUM ]]	Greater than or equal
(( NUM < NUM ))	Numeric conditions

File conditions

[[ -e FILE ]]	Exists
[[ -r FILE ]]	Readable
[[ -h FILE ]]	Symlink
[[ -d FILE ]]	Directory
[[ -w FILE ]]	Writable
[[ -s FILE ]]	Size is > 0 bytes
[[ -f FILE ]]	File
[[ -x FILE ]]	Executable
[[ FILE1 -nt FILE2 ]]	1 is more recent than 2
[[ FILE1 -ot FILE2 ]]	2 is more recent than 1
[[ FILE1 -ef FILE2 ]]	Same files

Others

[[ ! EXPR ]]	Not
[[ X && Y ]]	And
`[[ X

• String Operators (==, !=, <, >): Used to compare text based on alphabetical (ASCII) order.
• Integer Operators (eq, ne, lt, gt): Used to compare the mathematical value of numbers.

Conditional Statements (if, elif, else)

[] → old stuff, similar to test , can’t be useful for adv. ops like regex

[[]] → new operator, returns 0 → True, else 1

(()) → for evaluating arithmetic expressions as condition

#!/bin/bash

if (( $# != 1 )) || ! [[ $1 =~ ^[0-9]+$ ]]; then
  echo "Plz Check No.of Args and Type of Arg. it may be invalid number"
elif [[ $1 -ge 10  && $1 -le 100 ]]; then
  if [[ $(($1 % 2)) -eq 0 ]]; then
    echo "Valid EVEN"
  else
    echo "valid ODD"
  fi
else
  echo "Make sure Number is between 10 to 100"
fi

if (( ${#1} % 2 == 0 )); then
  echo "EVEN"
else
  echo "ODD"
fi

#!/bin/bash

if (($1 > 0)) && [[ $2 == *.log ]]; then
  cp $2 "$1.log"
  if [[ $? -eq 0 ]]; then
    echo "Valid - $1.log Created"
  else
    echo "Failed to create file!"
  fi
else
  echo "Invalid"
fi

Case Statement

#!/bin/bash

echo "Enter your Option [Yes/No]"
read option;

case $option in
        [Yy] | [Yy][Ee][Ss])
                echo "Accepted!!"
        ;;

        [Nn] | [Nn][Oo])
                echo "Not Accepted!!"
        ;;

        *)
                echo "INcorrect Option!!"
        ;;
esac

Real World Examples….

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