The somewhat apocryphal history of comments in the Bourne shell
(This is the story as I heard it and learned it in the past. It may or may not be actually accurate, but then a lot of Unix history is passed around as folklore, and in some ways the folklore is more important and more influential than the truth.)
In the beginning, in V7, the Bourne shell didn't actually have
comments. Instead, all it had was
:, which is not quite a comment (you can see this for yourself in the V7 sh
part of V7's minimalism, the kernel had no mechanism to execute shell
scripts; instead, the shell did that itself when the kernel's
refused to run an executable file.
When people at Berkeley created
csh they gave it actual comments,
# characters. Having two shells created a dilemma,
csh went to run a shell script, how would it know which
shell the script was written for? The solution was simple. Since the
Bourne shell didn't have
csh could look at the start of
the file; if it had a
#, it was a
csh script, and otherwise it was a
Bourne shell script.
(Hence you at least used to be able to find some very, very old Bourne
shell scripts that started '
: this is a sh script' or the like.)
Shortly after that the people at UCB came to their senses and invented
#!', which makes the kernel
exec() mechanisms directly handle shell
scripts. In order to make Bourne shell scripts compatible with this
new mechanism, BSD added support for
# comments to the Bourne shell.
For backwards compatibility, they left support for the old 'executed
directly by the shell when
exec() fails' form of shell scripts in both
sh, complete with
csh's special peeking for '
:' (where it lingers on even today).
I am not sure when
# made it into the System V version of the Bourne
shell, although I think that it didn't take too long, possibly because
it was seen as a generally sensible idea given
:'s defects as a
general comment mechanism.
(I believe that kernel support for '
#!' took much longer and thus you
couldn't use a shell script as a user login shell on System V machines
for quite a while.)
Sidebar: an amusing experiment
Put the following in an executable file and try to run it from
within various shells (with eg '
sh -c /tmp/exper') on various
# for i in 1; do echo bourne-like shell done
Now, change the first line from '
#' to '
:' and try the experiment
(This is not the only example of Unix fossilization.)