Shell Programming and Scripting

#!/bin/bash
# test-integer2: evaluate the value of an integer.
INT=-5
if [[ "$INT" =~ ^-?[0-9]+$ ]]; then
if [ "$INT" -eq 0 ]; then
echo "INT is zero."
else
if [ "$INT" -lt 0 ]; then
echo "INT is negative."
else
echo "INT is positive."
fi

grep "^[-0-9]\+$" <<<  "$INT"

if grep "^[-0-9]\+$" <<<  "$INT"
  then  if test "$INT" -eq 0 
          then  echo "INT is zero."
          elif  test "$INT" -lt 0 
            then        echo "INT is negative."
            else        echo "INT is positive."
        fi
  else  echo "No int"
fi

if grep -Eq '^-?[0-9]+$' <<< "$INT"; then

is_int(){
# error if $1
#   contains a character that is not a dash or digit
#   is empty
#   is dash only
#   has a dash not at the first position
  case $1 in
  *[!-0-9]*|""|-|*?-*) return 1;;
  esac
  return 0
}

INT=-5
if is_int "$INT"; then
  if [ "$INT" -eq 0 ]; then
    echo "INT is zero."
  elif [ "$INT" -lt 0 ]; then
    echo "INT is negative."
  else
    echo "INT is positive."
  fi
fi

zzz=${INT#-}                  # removes the negative sign, if it exists
if test -z "${zzz//[0-9]/}"   # removes all digits; resulting string should be zero length
then 
   test ${INT} -lt 0 && printf "%s is negative\n" ${INT}
   test ${INT} -eq 0 && printf "%s is zero\n" ${INT}
   test ${INT} -gt 0 && printf "%s is positive\n" ${INT}
fi

expr "$INT" : '-\?[0-9][0-9]*$'

Last login: Wed Jan 23 12:04:15 on ttys000
AMIGA:amiga~> help case
case: case WORD in [PATTERN [| PATTERN]...) COMMANDS ;;]... esac
    Selectively execute COMMANDS based upon WORD matching PATTERN.  The
    `|' is used to separate multiple patterns.
AMIGA:amiga~> _

       case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
              A case command first expands word, and tries to match it against
              each pattern in turn, using the same matching rules as for path-
              name expansion (see Pathname  Expansion  below).   The  word  is
              expanded  using  tilde  expansion, parameter and variable expan-
              sion, arithmetic  substitution,  command  substitution,  process
              substitution  and  quote  removal.   Each  pattern  examined  is
              expanded using tilde expansion, parameter  and  variable  expan-
              sion, arithmetic substitution, command substitution, and process
              substitution.  If the shell option nocasematch is  enabled,  the
              match  is  performed  without  regard  to the case of alphabetic
              characters.  When a match is found, the  corresponding  list  is
              executed.   After  the  first  match,  no subsequent matches are
              attempted.  The exit status is zero if no pattern matches.  Oth-
              erwise,  it  is  the exit status of the last command executed in
              list.