Thompson shell

The Thompson shell was the first Unix shell, introduced in the first version of Unix in 1971, and was written by Ken Thompson.

It was a simple command interpreter, not designed for scripting, but nonetheless introduced several innovative features to the command-line interface and led to the development of the later Unix shells.

History

The name "shell" for a command-line interpreter and the concept of making the shell a user program outside of the 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. 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 < symbol followed by a filename for input or the > 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 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. 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.