背景
Linux下的fg和bg命令是進程的前后台調度命令，即將指定號碼（非進程號）的命令進程放到前台或后台運行。比如一個需要長時間運行的命令，我們就希望把它放入后台，這樣就不會阻塞當前的操作；而一些服務型的命令進程我們則希望能把它們長期運行於后台。
命令
進程前后台操作用到以下命令：
# 在前台結束進程 Control+C # 在前台暫停進程 Control+Z # 查看后台執行的進程 查看進程號碼 jobs # 運行命令時，在命令末尾加上&可讓命令在后台執行 & # 將命令進程號碼為N的命令進程放到前台執行 fg processNum # 或者 %processNum # 將命令進程號碼為N的命令進程放到后台執行 bg processNum
實操
我們在操作系統上實際演練一下，這里用的環境是MacOS Catlina + zsh。
啟動一個需要長時間執行的程序，我們這里選擇ping，每隔一定時間向特定網址發送數據包並且輸出詳細信息。
1. 啟動ping，一段時間后Ctrl + Z 暫停程序
yanximin@B000000120095D ~ % ping www.baidu.com PING www.a.shifen.com (61.135.169.125): 56 data bytes 64 bytes from 61.135.169.125: icmp_seq=0 ttl=55 time=0.995 ms 64 bytes from 61.135.169.125: icmp_seq=1 ttl=55 time=1.151 ms 64 bytes from 61.135.169.125: icmp_seq=2 ttl=55 time=1.005 ms 64 bytes from 61.135.169.125: icmp_seq=3 ttl=55 time=1.129 ms ^Z zsh: suspended ping www.baidu.com
2. jobs 查看后台程序
yanximin@B000000120095D ~ % jobs [1] + suspended ping www.baidu.com
3. fg 把程序放在前台恢復運行
yanximin@B000000120095D ~ % fg %1 [1] + continued ping www.baidu.com 64 bytes from 61.135.169.125: icmp_seq=4 ttl=55 time=1.350 ms 64 bytes from 61.135.169.125: icmp_seq=5 ttl=55 time=1.070 ms 64 bytes from 61.135.169.125: icmp_seq=6 ttl=55 time=1.160 ms 64 bytes from 61.135.169.125: icmp_seq=7 ttl=55 time=1.108 ms
另外 這里也可以直接用%1，不用加fg
yanximin@B000000120095D ~ % %1 [1] + continued ping www.baidu.com 64 bytes from 61.135.169.125: icmp_seq=16 ttl=55 time=1.114 ms 64 bytes from 61.135.169.125: icmp_seq=17 ttl=55 time=1.277 ms
4. 再使用Ctrl + Z 暫停程序，然后使用bg， 放在后台繼續運行
yanximin@B000000120095D ~ % 64 bytes from 61.135.169.125: icmp_seq=18 ttl=55 time=1.112 ms 64 bytes from 61.135.169.125: icmp_seq=19 ttl=55 time=1.017 ms 64 bytes from 61.135.169.125: icmp_seq=20 ttl=55 time=1.318 ms 64 bytes from 61.135.169.125: icmp_seq=21 ttl=55 time=1.188 ms 64 bytes from 61.135.169.125: icmp_seq=22 ttl=55 time=0.952 ms 64 bytes from 61.135.169.125: icmp_seq=23 ttl=55 time=0.913 ms 64 bytes from 61.135.169.125: icmp_seq=24 ttl=55 time=0.949 ms 64 bytes from 61.135.169.125: icmp_seq=25 ttl=55 time=1.167 ms
這里沒有重定向標准輸出，所以即使是后台執行的，也會往終端打印。可以使用nohup和重定向，來做到真正的后台執行
nohup ping www.baidu.com >log.txt 2>error.txt &
5. 最后使用ctrl + C 來結束進程
PING www.a.shifen.com (61.135.169.125): 56 data bytes 64 bytes from 61.135.169.125: icmp_seq=0 ttl=55 time=1.120 ms 64 bytes from 61.135.169.125: icmp_seq=1 ttl=55 time=0.981 ms 64 bytes from 61.135.169.125: icmp_seq=2 ttl=55 time=1.214 ms 64 bytes from 61.135.169.125: icmp_seq=3 ttl=55 time=1.003 ms 64 bytes from 61.135.169.125: icmp_seq=4 ttl=55 time=1.173 ms ^C --- www.a.shifen.com ping statistics --- 5 packets transmitted, 5 packets received, 0.0% packet loss round-trip min/avg/max/stddev = 0.981/1.098/1.214/0.092 ms