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PS1='${?#0}$ ' 

It uses a special form of parameter expansion, ${?#0}, which means: "Remove the character zero if it is the first character of ${?}, the exit code of the previous command."

You can also change the color of the prompt if the last exit code were not zero:

PS1='\[\e[0;$(($?==0?0:91))m\]$ \[\e[0m\]' 

That uses a if-else ternary expression $(($?==0?0:91)) — with some escaping — that makes the color code 0;91m (red, see color codes) if the last command exits with non-zero, or 0;0m (your default color) otherwise.

PS1='${?#0}$ ' 

It uses a special form of parameter expansion, ${?#0}, which means: "Remove the character zero if it is the first character of ${?}, the exit code of the previous command."

You can also change the color of the prompt if the last exit code were not zero:

PS1='\[\e[0;$(($?==0?0:91))m\]$ \[\e[0m\]' 

That uses a if-else ternary expression $(($?==0?0:91)) that makes the color code 0;91m (red, see color codes) if the last command exits with non-zero, or 0;0m (your default color) otherwise.

PS1="\${?#0}\$ " 

It uses a special form of parameter expansion, ${?#0}, which means: "Remove the character zero if it is the first character of ${?}, the exit code of the previous command."

You can also change the color of the prompt if the last exit code were not zero:

PS1="\[\e[0;\$((\$?==0?0:91))m\]\$ \[\e[0m\]" 

That uses a if-else ternary expression \$((\$?==0?0:91)) — with some escaping — that makes the color code 0;91m (red, see color codes) if the last command exits with non-zero, or 0;0m (your default color) otherwise.

PS1='${?#0}$ ' 

It uses a special form of parameter expansion, ${?#0}, which means: "Remove the character zero if it is the first character of ${?}, the exit code of the previous command."

You can also change the color of the prompt if the last exit code were not zero:

PS1='\[\e[0;$(($?==0?0:91))m\]$ \[\e[0m\]' 

That uses a if-else ternary expression $(($?==0?0:91)) — with some escaping — that makes the color code 0;91m (red, see color codes) if the last command exits with non-zero, or 0;0m (your default color) otherwise.

That's a great idea.

PS1="\${?#0}\$ " 

It uses a special form of parameter expansion, ${?#0}, which means: "Remove the character zero if it is the first character of ${?}, the exit code of the previous command."

Things can get pretty cool by changing the color of the prompt if the last exit code were not zero:

PS1="\[\e[1;\${?/#[!0]*/31}m\]\$ \[\e[0m\]" 

The color code is 1;31 (red, see color codes here) if the last command exits with non-zero, because ${?/#[!0]*/31} substitutes a initial 0 with 31.

PS1="\${?#0}\$ " 

It uses a special form of parameter expansion, ${?#0}, which means: "Remove the character zero if it is the first character of ${?}, the exit code of the previous command."

You can also change the color of the prompt if the last exit code were not zero:

PS1="\[\e[0;\$((\$?==0?0:91))m\]\$ \[\e[0m\]" 

That uses a if-else ternary expression \$((\$?==0?0:91)) — with some escaping — that makes the color code 0;91m (red, see color codes) if the last command exits with non-zero, or 0;0m (your default color) otherwise.