bash if

Simple if:

if [ -d /etc]; then
echo "/etc exists"
fi

if with else

if [ -f /etc/fstab ]; then
  echo "/etc/fstab exists"
else
  echo "/etc/fstab does not exists"
fi

if with else - example 2

count=100
if [ $count -eq 100 ]
then
  echo Count is 100
else
  echo Count is not 100
fi

if with else example 3 here - check if file exists:

if [ -e /etc/os-release ]
then
  echo "File exists"
else
  echo "File doesn't exist"
fi

if statement - check if a variable value is met

a=`date | awk '{ print $1 }'`
if [ "$a" == Mon ]
then
  echo Today is $a
else
  echo Today is not Monday
fi

if with elif and else

if [ $1 = "y" ]; then
  echo "yes"
elif [ $1 = "n" ]; then
  echo "no"
else
  echo "other"
fi

if with elif and else example 2 - check the responce and output

clear
echo "What is yout name?"
echo
read name
echo
echo Hello $name
echo
echo "Do you like working in IT? (y/n)"
echo
read like
echo

if [ "$like" == y ]
then
  echo You are cool
elif [ "$like" == n ]
then
echo "You should try IT, it is a good field"
echo
fi

if with test

if test -e /etc/hosts; then
  echo "/etc/hosts exists"
fi

if with || and [[ ]]
|| - other option is &&
[[ ]] - ability to use the entire expression within the brackets themselves, also fives possiblity to use regex and file globbing within expression

if [[ -f /etc/fstab || -d /var2 ]]; then
  echo "true"
else
  echo "false"
fi

if with && and [ ] [ ] - a single expression

if [ -f /etc/fstab ] && [ -d /var2 ]; then
  echo "true"
else
  echo "false"
fi