Bourne Shell

Developed by Stephen Bourne at Bell Labs and released 1979 as a replacement for the earlier Thompson Shell, Bourne Shell was the default shell of Version 7 of AT&T UNIX. It was introduced by its manual as „command language“, and it features control structures for conditional and repetitive execution. It enables the user to implement algorithms that perform non-interactive tasks such as the orchestration of batch processes or accumulating long-term reporting data. The same executable provides an interactive mode in which it reads commands from a user in realtime and replies to them with status and error messages as well as the output of executed programs.

• http://man.cat-v.org/unix_7th/1/sh

With the advent of the „Shebang“ convention, letting text files state the executable they should be interpreted with, Bourne Shell scripts have conventionally been headlined as follows:

#!/bin/sh

Over the course of many years, the software has undergone several forks and diversions.

As a case study, consider IBM AIX version 7:

• A version of Korn Shell (ksh) will by default be set up as what is called the „default shell“, set up at a location /usr/bin/sh.
• /bin is a symlink to /usr/bin, so the #!/bin/sh convention will work.
• The default shell can be changed; the documentation specifically states that a „Bourne Shell interpreter“ (bsh) can also be linked to by sh. By changing this setting, every script that uses #!/bin/sh would be interpreted in Bourne instead of Korn Shell.
• A variant refered to as „POSIX shell“ will be available as executable psh, which in turn is a link to sh.
• The version of ksh installed by default will be „88“, but a newer version „93“ is available, too.
• There is also the system-wide setting of a „default (operating system) login shell“ which is set to /usr/bin/sh, again, really a link to ksh, which can be changed as well, which is a different setting that that of the default shell: What is the default shell is decided by what the symlink /usr/bin/sh points to, while the setting of default login shell will govern what login shell will be assigned to newly created users.

For the official explanations see

• https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/osmanagement/avail_shells.html
• https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/filesreference/chapter_5.html

It is noteworthy that all interpreters mentioned above (POSIX-, both versions of Korn Shell and the Bourne Shell interpreter) claim to be „compatible to Bourne Shell“. However, the amount of variants and possible choices on a single UNIX implementation alone suggests that they are either not equivalent or unable to stand in for one another without friction. Each one of them has to be required by a significant amount of users, justifying its maintenance effort as part of the commercial product „AIX“.