shell 脚本中，程序的标准输出重定向到 FIFO, 需要注意的问题 - yanbin's Blog
shell 脚本中，程序的标准输出重定向到 FIFO, 需要注意的问题
FIFO, 又称为命名管道 (named pipes)。与 PIPE 不同和相同之处：
1. PIPE 只能用于关联进程之间，比如： 父子进程之间，两个子进程之间。
2. 关于读写:
(a) 写一个没有读端的 PIPE，会触发 SIGPIPE(Broken pipe: write to pipe with no readers).
(b) 写一个没有读端的 FIFO，没有设置 O_NONBLOCKING 会阻塞，直到有进程读;
设置了 O_NONBLOCKING, 将会返回 -1.
(c) 没有写端时读 PIPE/FIFO 都是会阻塞，而设置了 O_NONBLOKING 读都会返回 -1.
由于 FIFO 没有读端则写端会阻塞的特性。
在 shell 脚步中，一个程序的标准输出重定向到 FIFO, 即写 FIFO 的程序启动之时或之后，
确保读 FIFO 的程序是否存在并且及时存在了，否则可能会有如下问题:
1. 读 FIFO 的程序没有启动过，写 FIFO 的程序永远阻塞在 写 FIFO 操作上。
2. 读 FIFO 的程序后来启动了，写 FIFO 的程序可能丢失了大量数据当其阻塞在写 FIFO 操作之上时。
3. 假如写 FIFO 的程序是一个多线程程序，她的产生数据操作与写数据操作是分开，并且有内部数据缓存,
当读 FIFO 的程序启动较晚时，会有大量数据要读，而且写 FIFO 程序也有大量数据要写，
这至少会耗费时间。
4. 实在要考虑写 FIFO 程序与读 FIFO 程序的启动先后顺序问题，要考虑时间窗口。
5. Linux Programmer's Manual, pipe(7) 给出的设计思想：
Applications should not rely on a particular capacity: an application should be designed so that
a reading process consumes data as soon as it is available, so that a writing process does not
remain blocked.
remain blocked.
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2022年12月09日 21:01
This post shares the details regarding an issue that is related to the shell script. the t score osteoporosis standard output of the program is redirected to FIFO and the issue needs more attention. Hope that the experts will provide answers to this issue and keep sharing more updates here.
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