exit codes:
0     - success
other - fail

if ..
then ...
elif ..
...
else ...
fi

test, [ - checks a condition
  -f - is the param an existing file
  -d - ... directory ...
  -e - ... file or directory ...

  -n - if the arg is non-empty string

  -gt - greater for numbers
  -ge
  -lt
  -le
  -eq
  -ne


case $1 in
  pat1)
    cmds1
    ;;
  pat2)
    cmds2
    ;;
  ...
esac


for letter in a b c d
           in $HOME/*
do
  echo $letter
done

while [ $num -lt 10 ]
      read line
do
  echo $num $line
  num = $((num + 1))
done