Bash

• Compare integer in bash, unary operator expected error

  if [[ $i -ge 2 ]]
  

• Number tables

  for i in {1..9}; do for j in $(seq 1 9); do echo -ne $i×$j=$((i*j))\\t;done; echo;done
  

• Test of variable is a number in bash

  re='^[0-9]+$'
  if ! [[ $yournumber =~ $re ]] ; then
     echo "error: Not a number" >&2; exit 1
  fi
  

• Remove last character of variable

  something="myname"
  echo ${something%?}
  

• Removing first character of variable

  something="myname"
  echo ${something#?}