if echo "${word}" | grep '[0-9]' >/dev/null; then
    echo 'Invalid input'
fi

string='My string';

if [[ $string =~ "My" ]]; then
   echo "It's there!"
fi

string='My long string'
if [[ $string == *"My long"* ]]; then
  echo "It's there!"
fi

[[ ! $VAR =~ [0-9] ]];

# contains only alphabets and numbers

$VAR =~ ^[[:alnum:]]+$

# Or

$VAR =~ ^[0-9a-zA-Z]+$

#!/bin/bash

while true; do
   read -r -p "Enter a string: " VAR
   if [[ $VAR =~ ^[[:alnum:]]+$ ]];then
      echo "OK: Contains alphabets and numbers"
   else
      echo "NOK: Contains special character"
   fi
done

#!/bin/bash

while true; do
   read -r -p "Enter a string: " VAR
   if [[ $VAR =~ ^[0-9]+$ ]];then
      echo "Input contains number"
   else
      echo "Input contains non numerical value"
   fi
done

reference:

1. link1
2. if statement

reference

#!/bin/bash

today = `date +%m-%d-%Y`

echo $today

sed -i 's/search_string/replace_string/' filename

大于 -gt (greater than)
小于 -lt (less than)
大于或等于 -ge (greater than or equal)
小于或等于 -le (less than or equal)
不相等 -ne （not equal）

if (( $(echo "$num1 > $num2" |bc -l) )); then
  …
fi

############### and

if [ c1 -a c2 ]; then
…
fi

if [ c1 ] && [ c2 ]; then
…
fi

############## or

if [ c1 -o c2 ]; then
…
fi

if [ c1 ] || [  c2 ]; then
…
fi

: '
This is a
very neat comment
in bash
'