#Pure bash, 43

echo $[$1<2?1:$[$1<2?2:$1]#`printf 1%0$1d`]

Try it online.

Not sure if this is bending the rules too much - I'm not using any of the listed banned builtins, but I am using base conversion.

• printf 1%0$1d outputs a 1 followed by n 0s
• $[b#a] is an arithmetic expansion to treat a as a base b number, which gives the required result. Unfortunately base <2 does not work, so the extra ?: bits handle input n=1.

Maximum input is 15, because bash uses signed 64-bit integers (up to 2³¹-1).

Pure bash, 43

echo $[$1<2?1:$[$1<2?2:$1]#`printf 1%0$1d`]

Try it online.

Not sure if this is bending the rules too much - I'm not using any of the listed banned builtins, but I am using base conversion.

• printf 1%0$1d outputs a 1 followed by n 0s
• $[b#a] is an arithmetic expansion to treat a as a base b number, which gives the required result. Unfortunately base <2 does not work, so the extra ?: bits handle input n=1.

Maximum input is 15, because bash uses signed 64-bit integers (up to 2³¹-1).

#Pure bash, 26

echo $[$1#`printf 1%0$1d`]

Try it online.

Not sure if this is bending the rules too much - I'm not using any of the listed banned builtins, but I am using base conversion.

• printf 1%0$1d outputs a 1 followed by n 0s
• $[b#a] is an arithmetic expansion to treat a as a base b number, which gives the required result.

Maximum input is 15, because bash uses signed 64-bit integers (up to 2³¹-1).

#Pure bash, 43

echo $[$1<2?1:$[$1<2?2:$1]#`printf 1%0$1d`]

Try it online.

Not sure if this is bending the rules too much - I'm not using any of the listed banned builtins, but I am using base conversion.

• printf 1%0$1d outputs a 1 followed by n 0s
• $[b#a] is an arithmetic expansion to treat a as a base b number, which gives the required result. Unfortunately base <2 does not work, so the extra ?: bits handle input n=1.

Maximum input is 15, because bash uses signed 64-bit integers (up to 2³¹-1).