Editing Thompson shell (section)

==History==
The name "shell" for a [[Command-line interface#Command-line interpreter|command-line interpreter]] and the concept of making the shell a user program outside of the operating system [[Kernel (operating system)|kernel]] were introduced in Unix's precursor [[Multics]].

An early feature of the Thompson shell was a compact syntax for input/output [[Redirection (computing)|redirection]].  In Multics, redirecting the  input or output of a command required separate commands to start and stop redirection; in Unix, one could simply add an argument to the command line consisting of the &lt; symbol followed by a filename for input or the &gt; symbol for output, and the shell would redirect I/O for the duration of the command.  This syntax was already present by the release of the first version of Unix in 1971.

A later addition was the concept of [[pipeline (Unix)|pipes]].  At the suggestion of [[Douglas McIlroy]], the redirection syntax was expanded so that the output of one command could be passed to the input of another command.  The original pipe syntax, as described in the Version 3 manual, was:

 command1 >command2>

This syntax proved too ambiguous and was easily confused with redirection to files—the system cannot tell whether "command2" is a command or a file.{{Citation needed|reason=this statement makes no sense.  Ken Thompson was no novice programmer.  The syntax must have been sufficiently clear to the interpreter.  The syntax shown *does* look unambiguous (but hard to read for humans).  More likely, it was confusing for users.  A citation will settle this.|date=November 2024}} By Version 4, the syntax had changed to use both the | and ^ symbols to denote pipes:

 command1 | command2

This produces exactly the same result as:

 command1 ^ command2

The > symbol changed into:

 command1 > file1

This would put the output of command1 into file1.

The Thompson shell syntax for redirection with &lt; and &gt;, and piping with |, has proven durable and has been adopted by most other Unix shells and command shells of several other operating systems, most notably on [[DOS]], [[OS/2]] and [[Microsoft Windows]].