Release Date: June 12, 1972
Released By: Bell Labs Research
Source Code: some source code is in the Unix Archive, browsable in the Unix Tree
Documentation: 2nd Edition man pages

The second edition of Unix was developed for the PDP-11 at Bell Labs by Ken Thompson, Dennis Ritchie and others. It extended the First Edition with more system calls and more commands. This edition also saw the beginning of the C language, which was used to write some of the commands.

The code in the Unix Archive is only the source to some of the commands, some of the library functions, and the C compiler. The files in c/ come from the last1120c.tar.gz tape, and form a working C compiler for Second Edition Unix.

The files in lib/ come from the libc.sa file which is on the last1120c.tar.gz tape, and form the C library for the compiler.

The files in cmd/ are recreated from the text fragments found on the file s1-bits.gz. These were reconstructed by Doug Merritt and Warren Toomey, and form the source code to some of the commands in Second Edition Unix.

Another archive, s2-bits.tar.gz, contains what appears to be a dump of the root filesystem of a Second Edition UNIX installation. The contents include binaries of three different formats: those that explicitly call brk upon start, those with a magic number of 405(8), the First Edition a.out(V) format, and those with a magic number of 407(8), the Second Edition a.out(V) format. These format discrepancies may imply the rough linking time of the various objects present.

Changes include:

• Exit/wait now support returning a status
• Many system limits are increased (e.g. mounts, buffer sizes)
• The hog, kill, sleep, and sync syscalls are added

• Added mount command to the INIT tape
• Dropped chown, cp, ln, mv, rm, rmdir, stat commands from INIT tape

• The init system now uses getty and login
• The shell now supports [ ] character class matches, appending with >>
• The shell has expanded programming support with control flow (provided by programs) and error messages
• Many commands add diagnostics
• The stty command is added to set terminal characteristics
• The cp and mv commands no longer operate on pairs
• The ds, istat, and salv filesystem tools are added
• NROFF is allegedly added or very close, as it is in the manual TOC but the page is absent

• The assembler and linker now support relocation and segmentation
• The bc B interpreter replaces the old B system
• The cc C compiler is introduced
• The m6 macro processor is introduced
• The tmg compiler-compiler is introduced
• The fc Fortran compiler replaces the old for compiler
• The linker now supports the Explor language
• Libraries move from /etc to /usr/lib

• The floating point simulator is improved
• The atan, hypot, and sqrt math functions are added
• The salloc string library is added
• The qsort algorithm is added

• Connectivity to GECOS via the 201 data phone is added
• IBM 2741 support is added
• Magtape support is added
• The motd file is added
• Removed the binary punched paper tape format and utilities
• Removed many device-specific commands (e.g. dtf, rkf)

The following papers are mentioned in SEE ALSO sections in the manual:

• Users' Reference to B - K. Thompson
• C Reference Manual - D.M. Ritchie
• M6 Manual - A.D. Hall
• ROFF Manual - J.F. Ossanna
• The UNIX Time-sharing System - D.M. Ritchie, K. Thompson

For more information about Second Edition Unix, see The Evolution of the Unix Time-sharing System by Dennis Ritchie.