Shell – Why the True Bourne Shell is Rarely Used as /bin/sh

LINENO is a ksh feature, also present in bash and zsh. There is no such feature in the Bourne shell, in the POSIX specification or in dash. If you need the line number, make sure that your script is executed under a shell that has the feature. Most systems have either bash or ksh available.

if [ -z "$LINENO" ]; then
  if type ksh >/dev/null 2>/dev/null; then
    exec ksh "$0" "$@"
  elif type bash >/dev/null 2>/dev/null; then
    exec ksh "$0" "$@"
  else
     echo 1>&2 "$0: Fatal error: unable to find a shell that supports LINENO."
     exit 100
  fi
fi

The short answers are, yes, it was done for compatibility (lots of programs referenced /bin/sh and /bin/ed), and in the early days /bin and /usr/bin contained totally disjoint sets of files. /bin was on the root filesystem, a small disk that the computer's boot firmware had to be able to access, and held the more critical and often-used files. /usr/bin was on /usr, typically an entirely separate, larger disk. /usr, at first, also contained users' home directories. As /usr grew, we would periodically replace its drive with something larger. The system could run with no /usr mounted, even if wasn't all that useful.

/usr's disk (or disk partition) was mounted after the Unix kernel had been booted and the system was partway through the user-mode boot process (/etc/rc), so programs like sh and mount and fsck had to be in the root filesystem, generally in /bin and /etc. Sun had even rearranged / and /usr so that a shared copy of /usr could be mounted read-only across a network. /usr/tmp became a symlink to /var/tmp. /var was either on the root filesystem or, preferably, on another partition.

I believe it was Sun that decided, at one point, that it wasn't worth heroically trying to have a system be able to come up if its /usr was trashed. Most users either had / and /usr on the same physical disk - so if it died, both filesystems were toast - or had /usr mounted read-only from a server. So some critical programs used for system boot and maintenance were compiled statically and put in /sbin, but most of the programs in /bin were moved to /usr/bin and /bin became a symlink to /usr/bin.

System V prior to R4 didn't even have symlinks. Sun and AT&T worked to combine SunOS and SVR3, and that became SVR4 (and Solaris 2). It had /bin as a symlink to /usr/bin.

So when that web site says "On SysV Unix /bin traditionally has been a symlink to /usr/bin", they really should have said "On System V Release 4 and followons, ...".