ali@kali:~$ ping -c 400 localhost > ping_results.txt
^Z
[1] + Stopped ping -c 400 localhost > ping_results.txt
kali@kali:~$ find / -name sbd.exe
^Z
[2] + Stopped find / -name sbd.exe
kali@kali:~$ jobs
[1]- Stopped ping -c 400 localhost > ping_results.txt
[2]+ Stopped find / -name sbd.exe
请问倒数第一,二行的加减号是什么意思?