Create a shifter

APL (Dyalog), 32 bytes

Anonymous function which takes the sift as left argument and the number (as a string) as right argument.

{a←|⍺⋄(≢⍵)↑(a-⍺)↓(a⍴⊃⍵),⍵,a⍴⊃⌽⍵}

Try it online!

{ anonymous function where ⍺ and ⍵ are left and right arguments

 |⍺ the absolute value of the shift

 a← store in a

 ⋄ then

 ⌽⍵ reverse the number

 ⊃ pick the first (i.e. last) digit

 a⍴ reshape it to length a

 ⍵, prepend the number

 (…), prepend the following:

  ⊃⍵ the first digit

  a⍴ reshape it to length a

 (…)↓ drop the following number of characters:

  a-⍺ a minus the shift

 (…)↑ take the following number of characters:

  ≢⍵ the length of the original number

Python 2, 50 bytes

lambda n,s:(n[0]*s+n+n[-1]*-s)[-s*(s<0):][:len(n)]

Try it online!

MATL, 12 bytes

tn:i-yn1&Xl)

Inputs are: number to be shifted as a string; amount of shifting as a number.

Try it online! Or verify all test cases.

Consisder inputs '452 and '-1'.

t     % Implicitly input string. Duplicate
      % STACK: '452', '452'
n     % Number of elements
      % STACK: '452', 3
:     % Range
      % STACK: '452', [1 2 3]
i     % Input number
      % STACK: '452', [1 2 3], -1
-     % Subtract, element-wise
      % STACK: '452', [2 3 4]
y     % Duplicate from below
      % STACK: '452', [2 3 4], '452'
n     % Number of elements
      % STACK: '452', [2 3 4], 3
1     % Push 1
      % STACK: '452', [2 3 4], 3, 1
&Xl   % Clamp, with three inputs. Applies min function, then max
      % STACK: '452', [2 3 3]
)     % Reference indexing. Implicitly display
      % STACK: '522'