[TUHS] Which years saw the introduction of (1) #! and (2) symbolic links

Jeremy C. Reed reed at reedmedia.net
Tue Dec 29 11:41:24 AEST 2020

(Just first question here ...)

Early BSD shells could look at first bytes (0404) to indicate was a 
pascal object to run the Berkeley pascal interpreter.
When csh was introduced, the csh would run the other shell /bin/sh if 
the first character was not a # pound.  So csh used the single # as 
magic to run the script with csh. The # was a comment for csh, make, and 
awk (but not for standard sh which could use a : colon for the comment).

Have a look at top of 4BSD's /usr/src/sys/newsys/sys1.c below. This code 
is not in v7 nor 32V.

>From uucp Thu Jan 10 01:37:58 1980
>From dmr Thu Jan 10 04:25:49 1980 remote from research
The system has been changed so that if a file being executed
begins with the magic characters #! , the rest of the line is understood
to be the name of an interpreter for the executed file.
Previously (and in fact still) the shell did much of this job;
it automatically executed itself on a text file with executable mode
when the text file's name was typed as a command.
Putting the facility into the system gives the following

1) It makes shell scripts more like real executable files,
because they can be the subject of 'exec.'

2) If you do a 'ps' while such a command is running, its real
name appears instead of 'sh'.
Likewise, accounting is done on the basis of the real name.

3) Shell scripts can be set-user-ID.

4) It is simpler to have alternate shells available;
e.g. if you like the Berkeley csh there is no question about
which shell is to interpret a file.

5) It will allow other interpreters to fit in more smoothly.

To take advantage of this wonderful opportunity,

	#! /bin/sh

at the left margin of the first line of your shell scripts.
Blanks after ! are OK.  Use a complete pathname (no search is done).
At the moment the whole line is restricted to 16 characters but
this limit will be raised.

>From uucp Thu Jan 10 01:37:49 1980
>From dmr Thu Jan 10 04:23:53 1980 remote from research

More information about the TUHS mailing list