R, 39 33 30 bytes

c(a<-scan(),setdiff(a:a[2],a))

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Thanks for saved bytes to user2390246 and J.Doe.

05AB1E, 4 bytes

Ÿ¦¨«

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Explanation

    Ÿ      # inclusive range [a ... b]

     ¦¨    # remove the first and last element

       «   # append to input

Python 3, 52 48 47 42 bytes

lambda a,b:[a,b,*range(a,b,(a<b)*2-1)[1:]]

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Combined former implementations.

Python 2 (Cython), 36 35 bytes

lambda x:x+range(*x,-cmp(*x)|1)[1:]

Thanks to @nwellnhof for golfing off 1 byte!

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Python 2, 37 bytes

lambda x:x+range(*x+[-cmp(*x)|1])[1:]

Thanks to @JonasAusevicius for the port to CPython!

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Perl 6, 26 22 bytes

{|@_,|[...^](@_).skip}

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Explanation

{                    }

 |@_,   # Slip args a,b into result

      [...^](@_)  # Reduce args a,b with ...^ operator, same as a...^b

                .skip  # Skip first element

     |  # Slip into result

Python 2, 40 bytes

lambda x,y:[x,y]+range(x,y,-(y<x)|1)[1:]

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Python 3, 64 62 51 bytes

lambda a,b:[a,b]+[*range(a+1,b)]+[*range(a-1,b,-1)]

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Python 2, 58 45 bytes

lambda a,b:[a,b]+range(a+1,b)+range(a-1,b,-1)

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Python 2, 47 41 bytes

lambda a,b:[a,b]+range(a,b,(a<b)*2-1)[1:]

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Here's mine, now that a lot of other Python answers have been posted

_{-6 bytes, thanks to G B}

Japt, 8 bytes

cUr!õ kU

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             :Implicit input of array U

c            :Concatenate

 Ur          :  Reduce U by

   !õ        :   Inclusive range

      kU     :  Remove all elements in original U

JavaScript (ES6), 51 bytes

Takes input as (a)(b).

a=>g=(b,c=b)=>(b+=b<a|-(b>a))-a?[...g(b,c),b]:[a,c]

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Commented

a =>                // main function, taking a

  g = (             // g = recursive function

    b,              //     taking b

    c = b           // we save a backup of the original value of b into c

  ) =>              //

    (b +=           // add to b:

      b < a |       //   +1 if b is less than a

      -(b > a)      //   -1 if b is greater than a

    )               //   (or 0 if b = a)

    - a ?           // if the updated value of b is not equal to a:

      [             //   generate a new array:

        ...g(b, c), //     prepend all values generated by a recursive call

        b           //     append the current value of b

      ]             //

    :               // else:

      [a, c]        //   stop recursion and return the first 2 values: a and c

Java 10, 109 108 104 102 93 62 bytes

Using a space-delimited String:

b->a->{var r=a+" "+b;for(;a<b?++a<b:--a>b;)r+=" "+a;return r;}

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Using a List:

b->a->{var r=new java.util.Stack();for(r.add(a),r.add(b);a<b?++a<b:--a>b;)r.add(a);return r;}

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(a<b?++a<b:--a>b can be ++a<b||(a-=2)>b for the same byte-count: Try it online for the String or Try it online for the List.)

Old (109 108 104 102 101 bytes) answer using an array:

a->b->{int s=a<b?1:-1,i=a!=b?(b-a)*s+1:2,r=new int[i];for(r[0]=a,r[1]=b;i>2;)r[--i]=b-=s;return r;}

-7 bytes thanks to @nwellnhof.

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Explanation:

a->b->{                // Method with 2 int parameters & int-array return-type

  int s=               //  Step integer, starting at:

        a<b?1          //   1 if the first input is smaller than the second

        :-1;           //   -1 otherwise

      i=               //  Array-index integer, starting at:

        a!=b?          //   If the inputs aren't equal:

         (b-a)*s+1     //    Set it to the absolute difference + 1

        :              //   Else:

         2,            //    Set it to 2

      r=new int[i];  //  Result-array of that size

  for(r[0]=a,          //  Fill the first value with the first input

      r[1]=b;          //  And the second value with the second input

      i>2;)            //  Loop `i` downwards in the range [`i`,2):

    r[--i]=            //   Decrease `i` by 1 first with `--i`

                       //   Set the `i`'th array-value to:

           b-=s;       //    If the step integer is 1: decrease `b` by 1

                       //    If the step integer is -1: increase `b` by 1

                       //    And set the array-value to this modified `b`

  return r;}           //  Return the result-array

Haskell, 34 bytes

a#b=a:b:[a+1..b-1]++[a-1,a-2..b+1]

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Jelly, 4 bytes

,œ|r

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How it works

,œ|r  Main link. Left argument: a. Right argument: b



,     Pair; yield [a, b].

   r  Range; yield [a, ..., b].

 œ|   Perform multiset union.

J, 26 bytes

,,[|.@]^:(>{.)<.+1}.i.@|@-

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Explanation:

A dyadic verb (takes left and right argument)

                         -    subtracts the arguments

                       |@     and finds the absolute value

                    i.@       and makes a list 0..absolute difference

                 1}.          drops the fist element

                +             adds to the entire list

              <.              the smaller of the arguments

   |.@]                       reverses the list

       ^:                     only if

  [                           the left argument

         (>{.)                is greater than the first item of the list

 ,                            appends the list to

,                             the right argument appended to the left one

Octave, 45 bytes

@(a,b)[a b linspace(a,b,(t=abs(a-b))+1)(2:t)]

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Batch, 107 bytes

@echo %1

@echo %2

@for %%s in (1 -1)do @for /l %%i in (%1,%%s,%2)do @if %1 neq %%i if %%i neq %2 echo %%i

Takes input as command-line arguments. Explanation:

@echo %1

@echo %2

Output the two integers.

@for %%s in (1 -1)do

Try both ascending and descending ranges.

@for /l %%i in (%1,%%s,%2)do

Loop over the inclusive range.

@if %1 neq %%i if %%i neq %2

Exclude the two integers.

echo %%i

Output the current value.

Pyth, 5 bytes

+QtrF

Input is a two-element list, [input 1, input 2]. Try it online here, or verify all the test cases at once here.

+QtrFQ   Implicit: Q=eval(input())

         Trailing Q inferred

   rFQ   Generate range [input 1 - input 2)

  t      Discard first element

+Q       Prepend Q

Red, 75 bytes

func[a b][s: sign? d: b - a prin[a b]loop absolute d - s[prin[""a: a + s]]]

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Clean, 49 bytes

import StdEnv

@a b=init[a,b:tl[a,a+sign(b-a)..b]]

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Python 2, 52 47 41 bytes

lambda i,j:[i,j]+range(i,j,(i<j)*2-1)[1:]

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-5 with thanks to @JoKing

-6 by slicing the first element from the range (idea stolen from and with credit to @TFeld)

Non-lambda version...

Python 2, 51 49 47 bytes

i,j=input();print[i,j]+range(i,j,(i<j)*2-1)[1:]

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-2 with thanks to @JoKing

APL (Dyalog Classic), 29 bytes

{⍺,⍵,(⌽⍣(⍺>⍵))(⍺⌊⍵)+¯1↓⍳|⍺-⍵}

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A port of my J solution

J, 13 bytes

,,<.+i.@-~-.=

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     i.@-~       range [0 .. |difference|-1], reverse if the difference is positive

          -.=    remove the zero (either "=" is 0 or there’s nothing to remove)

  <.+            to each element add the smaller of the args

,,               prepend args

TI-BASIC, 35 34 bytes

-1 byte from Misha Lavrov

Prompt A,B

Disp A,B

cos(π(A>B

For(I,A+Ans,B-Ans,Ans

Disp I

End

Charcoal, 15 bytes

ＩＥ²ＮＩ…⊕θηＩ⮌…⊕ηθ

Try it online! Link is to verbose version of code. Explanation:

ＩＥ²Ｎ

Print the inputs on separate lines.

Ｉ…⊕θη

Print the ascending range, if any.

Ｉ⮌…⊕ηθ

Print the reverse ascending reverse range, if any.

Dart, 85 84 bytes

f(a,b)=>[a,b]+((a-b).abs()>1?List.generate((a-b).abs()-1,(i)=>(a>b?-i-1:i+1)+a):);

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QBASIC, 39 53 bytes

INPUT a,b

?a

?b

FOR q=a+1TO b-1 STEP SGN(b-a)

?q

NEXT

Added the STEP parameter to account for a>b, and that uses the SGN() function to get a -1 or a +1 as increment. This however breaks the REPL because the SGN() function isn't implemented there...

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Ruby, 33 40 bytes

->a,b{[a,b]+[*a..b,*a.downto(b)][1..-2]}

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Temporary fix, trying to find a better idea

C (gcc), 65 bytes

f(a,b){for(printf("%d %d",a,b);a<b?++a<b:--a>b;)printf(" %d",a);}

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Not very exciting. The loop increment is borrowed from an early version of Kevin Cruijssen's Java answer.

Powershell, 41 bytes

param($a,$b)$a;$b;$a..$b|?{$_-notin$a,$b}

Less golfed test script:

$f = {



param($a,$b)

$a                # push $a to a pipe

$b                # push $b to a pipe

$a..$b|?{         # push to pipe all integers from $a to $b

    $_-notin$a,$b # ...except $a and $b itself

}



}



@(

    ,( 0,  5  ,   0, 5, 1, 2, 3, 4)

    ,(-3,  8  ,   -3, 8, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7)

    ,( 4,  4  ,   4, 4)

    ,( 4,  5  ,   4, 5)

    ,( 8,  2  ,   8, 2, 7, 6, 5, 4, 3)

    ,(-2, -7  ,   -2, -7, -3, -4, -5, -6)

) | % {

    $a,$b,$expected = $_

    $result = &$f $a $b

    "$("$result"-eq"$expected"): $result"

}

Output:

True: 0 5 1 2 3 4

True: -3 8 -2 -1 0 1 2 3 4 5 6 7

True: 4 4

True: 4 5

True: 8 2 7 6 5 4 3

True: -2 -7 -3 -4 -5 -6

Explanation:

The basic concept of Powershell is the pipe. Pipe is an array. All results that push into the pipe fall into the array. So we should just push the values into the pipe in the correct order.

Java, 739, 555 bytes

 public static List<Integer> firstLastAndEverythingBetween(final int a, final int b) { if (a == b) { return addAB(a, b); } final List<Integer> result = addAB(a, b); int initial = getInitial(a, b); for (int n = 1; n < Math.abs(b - a); n++) { result.add(initial); if (b > a) { initial++; } else { initial--; } } return result; } private static int getInitial(int a, int b) { return (b > a) ? (a + 1) : (a - 1); } private static List<Integer> addAB(int a, int b) { final List<Integer> result = new ArrayList<>(); result.add(a); result.add(b); return result; }