Posts Tagged ‘thompson shell’

Thompson Shell

Saturday, May 8th, 2010

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.

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 consisting of the < symbol followed by a filename for input or the > symbol for output to the command line, 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 and from files. By Version 4, the syntax had changed to use the | symbol for pipes:

command1 | command2
The Thompson shell syntax for redirection and piping has proven durable, having been adopted by most other Unix shells and several other operating systems, most notably MS-DOS.

Decline and replacements
The shell’s design was intentionally minimalistic; even the if and goto statements, essential for control of program flow, were implemented as separate commands. As a result, by the 1975 release of Unix Version 6, it was becoming clear that the Thompson shell was inadequate for most serious programming tasks.

At this time, the developers of the Programmer’s Workbench UNIX distribution, most notably John Mashey, began modifying the Thompson shell to make it more suitable for programming. The result, known as the PWB shell or the Mashey shell, included more advanced flow-control mechanisms and introduced shell variables, but remained limited by the necessity to remain compatible with the Thompson shell.

Finally, the Thompson shell was replaced as the main Unix shell by the Bourne shell in Unix Version 7 and the C shell in 2BSD, both released in 1979. Since virtually all modern Unix and Unix-like systems are descended from V7 and 2BSD, the Thompson shell is generally no longer used. It is, however, available as open source as part of several Ancient Unix source distributions, and has been ported to modern Unices as a historical exhibit.