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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 10:59:35 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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exit(1) 							   User Commands							   exit(1)

NAME
exit, return, goto - shell built-in functions to enable the execution of the shell to advance beyond its sequence of steps SYNOPSIS
sh exit [n] return [n] csh exit [ ( expr )] goto label ksh *exit [n] *return [n] DESCRIPTION
sh exit will cause the calling shell or shell script to exit with the exit status specified by n. If n is omitted the exit status is that of the last command executed (an EOF will also cause the shell to exit.) return causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command exe- cuted. csh exit will cause the calling shell or shell script to exit, either with the value of the status variable or with the value specified by the expression expr. The goto built-in uses a specified label as a search string amongst commands. The shell rewinds its input as much as possible and searches for a line of the form label: possibly preceded by space or tab characters. Execution continues after the indicated line. It is an error to jump to a label that occurs between a while or for built-in command and its corresponding end. ksh exit will cause the calling shell or shell script to exit with the exit status specified by n. The value will be the least significant 8 bits of the specified status. If n is omitted then the exit status is that of the last command executed. When exit occurs when executing a trap, the last command refers to the command that executed before the trap was invoked. An end-of-file will also cause the shell to exit except for a shell which has the ignoreeof option (See set below) turned on. return causes a shell function or '.' script to return to the invoking script with the return status specified by n. The value will be the least significant 8 bits of the specified status. If n is omitted then the return status is that of the last command executed. If return is invoked while not in a function or a '.' script, then it is the same as an exit. On this man page, ksh(1) commands that are preceded by one or two * (asterisks) are treated specially in the following ways: 1. Variable assignment lists preceding the command remain in effect when the command completes. 2. I/O redirections are processed after variable assignments. 3. Errors cause a script that contains them to abort. 4. Words, following a command preceded by ** that are in the format of a variable assignment, are expanded with the same rules as a vari- able assignment. This means that tilde substitution is performed after the = sign and word splitting and file name generation are not performed. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | |Availability |SUNWcsu | +-----------------------------+-----------------------------+ SEE ALSO
break(1), csh(1), ksh(1), sh(1), attributes(5) SunOS 5.10 15 Apr 1994 exit(1)
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