Vertical bar : "|"
The vertical bar (|) is a character with various uses in mathematics, computing, and typography. It may be called by various other names including the polon, pipe (by the Unix community, referring to the I/O pipeline construct), Sheffer stroke (by computer or mathematical logicians), verti-bar, vbar, stick, vertical line, straight vertical line, vertical slash, or bar, glidus, think colon, poley, or divider line.
— symbols.com
有 Linux 基础的人会知道，在类 Unix 操作系统中，我们用“|”来做管道命令(pipeline)的符号，它意味着信息在不同进程间的流动：
$$process1 \rightarrow stdout \Rightarrow stdin \rightarrow process2 $$
| |
我想，用流动的信息串联隔离的进程是 Pipeline 独特的设计哲学，抑或美学：
◎ from Wikipedia
Pipeline 的概念由 Douglas Mcllroy 首次提出，并在1973年开始实施，他称那是一个“feverish night”。“|”符号的使用被 Mcllroy 归功于 Ken Thompson ，这个符号简化了管道命令的表达。
Mcllory 曾这样描述他们工作的场面：
The next day saw an unforgettable orgy of one-liners as everybody joined in the excitement of plumbing.
— Douglas Mcllroy
我一直认为，用“|”这样一个意味着隔离的符号表达分隔的进程间的信息传递是天才之举。