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Top Forums Shell Programming and Scripting Goto - UNIX v6 Manual Questions Post 302929699 by Perderabo on Wednesday 24th of December 2014 09:59:35 AM
Old 12-24-2014
The Thompson goto command was bizarre because it was not a shell built-in. It was actually an external command. Decisions like this were made to get the thing to run in a 16 bit machine.

First of all the label concept was piggy-backed onto the colon command. It is a command that does nothing. It still exists today in bash and ksh. I hope you have heard of it. You can give the colon command arguments which the shell "processes" but the command itself ignores. This was used to make the colon command a form of comment. The leading number-sign comment had not yet appeared. A comment like this:
Code:
: we should not do stuff like > /etc/passwd

was harmful because the shell would try to send the colon command's output to /etc/passwd. You had to be sure that your comments had no side effects. You could put the : command anywhere you want. The shell would know it should do nothing.

But a colon command used as label had additional restrictions becuase it had to be processed by /bin/goto. There were various versions of /bin/goto. Your label had to match whatever rules your version of /bin/goto had. The goto command used some magic to reposition the shell's input file. This means the input file has to be seekable. A file on paper tape is an example of a non-seekable file. A file on a deck of cards is another example.

The alternate syntax you have a seen "label:" is what csh uses.

Warning: I don't use goto's so maybe some of this is wrong.
This User Gave Thanks to Perderabo For This Post:
 
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