perl調用子進程的最簡單的方式是用system。
1. 返回值問題
1> $out = system "date";
print "out: $out\n";
output：
Mon Jul 24 11:25:50 EDT 2017
out: 0 （執行成功的時候，system返回的值是0，如果想要將命令的運行結果放入到變量$out中，要用`date`）
my $out = `date`;
print "$out\n";
output:
test]# perl test.pl
Fri Feb 2 09:13:00 EST 2018
2> $out = system "datea"; (錯誤的命令)
print "out: $out\n";
output：
out: -1（執行成功的時候，system返回的值是非零）
system函數的返回值低8位存Unix Signal，高位存命令執行的返回值，因此，如果需要判斷shell命令是否執行成功，必須判斷返回的值是否爲0.
2. 後臺運行問題
有時想要在後臺運行shell命令，只要用nohup shell命令 &即可。而在perl中應該使用system調用這條命令，而不是反引號。
實驗如下：
1> 使用反引號：
my $cmd = "nohup ping www.baidu.com -c 10";
my $out = `$cmd`;
print "$out\n";
Output:
test]# perl test.pl
nohup: ignoring input and redirecting stderr to stdout (會直至hang在這裏等到ping命令結束後打印出結果纔會退出)
PING www.a.shifen.com (61.135.169.125) 56(84) bytes of data.
64 bytes from 61.135.169.125: icmp_seq=1 ttl=55 time=24.1 ms
64 bytes from 61.135.169.125: icmp_seq=2 ttl=55 time=23.9 ms
64 bytes from 61.135.169.125: icmp_seq=3 ttl=55 time=24.2 ms
64 bytes from 61.135.169.125: icmp_seq=4 ttl=55 time=24.0 ms
64 bytes from 61.135.169.125: icmp_seq=5 ttl=55 time=24.0 ms
64 bytes from 61.135.169.125: icmp_seq=6 ttl=55 time=24.4 ms
64 bytes from 61.135.169.125: icmp_seq=7 ttl=55 time=24.0 ms
64 bytes from 61.135.169.125: icmp_seq=8 ttl=55 time=24.1 ms
64 bytes from 61.135.169.125: icmp_seq=9 ttl=55 time=24.2 ms
64 bytes from 61.135.169.125: icmp_seq=10 ttl=55 time=24.3 ms
--- www.a.shifen.com ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 11684ms
rtt min/avg/max/mdev = 23.986/24.183/24.488/0.229 ms
2> system
my $cmd = "nohup ping www.baidu.com -c 1000 &";
my $out = system $cmd;
print "$out\n";
Output:
test]# perl test.pl
0
[root@TB-Arlene-PC2 test]# nohup: appending output to `nohup.out'
