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Top Forums Shell Programming and Scripting Difference between "set $<var>" and "set -- $<var>" Post 302251287 by Perderabo on Sunday 26th of October 2008 09:10:59 AM
Old 10-26-2008
It took you 6 months to post a followup question? Oh well...

Code:
$ set one two
 $ set -vx
$ echo hello
echo hello
+ echo hello
hello
$ echo 1 = $1 and 2 = $2
echo 1 = $1 and 2 = $2
+ echo 1 = one and 2 = two
1 = one and 2 = two
$ set -- -o emacs
set -- -o emacs
+ set -- -o emacs
$ echo 1 = $1 and 2 = $2
echo 1 = $1 and 2 = $2
+ echo 1 = -o and 2 = emacs
1 = -o and 2 = emacs
$ echo hello
echo hello
+ echo hello
hello

So, ok I set the x and v option and I'm getting a bunch of output. The "set -- -o emacs" had the usual effect that we discussed above. Now I'll try a "set - -o vi" and see and happens...
Code:
$ set - -o vi
set - -o vi
+ set - -o vi
$ echo 1 = $1 and 2 = $2
1 = -o and 2 = vi
$ echo hello
hello
$

The "set -" had the additional effect of turning off both -v and -x. The "set -" was invented first and at the time, x and v were the only options available and so "set -" turned off all options. Modern thinking frowns on syntax like "set -" and now "set --" is preferred. Modern thinking also frowns on a single item like "set -" having that many effects. Now we have "set +v" to turn off an option. And while "set -v" and "set +v" may seem a little backwards, at least it's now one effect per syntax item and that's progress.

So "set -" is a historical oddity and should be avoided.

I'll check back next April in case you have a further question. Smilie
 

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shift(1)							   User Commands							  shift(1)

NAME
shift - shell built-in function to traverse either a shell's argument list or a list of field-separated words SYNOPSIS
sh shift [n] csh shift [variable] ksh * shift [n] DESCRIPTION
sh The positional parameters from $n+1 ... are renamed $1 ... . If n is not given, it is assumed to be 1. csh The components of argv, or variable, if supplied, are shifted to the left, discarding the first component. It is an error for the variable not to be set or to have a null value. ksh The positional parameters from $n+1 $n+1 ... are renamed $1 ..., default n is 1. The parameter n can be any arithmetic expression that evaluates to a non-negative number less than or equal to $#. 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
csh(1), ksh(1), sh(1), attributes(5) SunOS 5.10 15 Apr 1994 shift(1)
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