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From: Jilles Tjoelker <jilles@FreeBSD.org>
Date: Sun, 24 Mar 2013 22:48:45 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r248692 - head/bin/sh
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Author: jilles
Date: Sun Mar 24 22:48:45 2013
New Revision: 248692
URL: http://svnweb.freebsd.org/changeset/base/248692

Log:
  sh(1): Mention possible ambiguities with $(( and ((.
  
  In some other shells, things like $((a);(b)) are command substitutions.
  
  Also, there are shells that have an extension ((ARITH)) that evaluates an
  arithmetic expression and returns status 1 if the result is zero, 0
  otherwise. This extension may lead to ambiguity with two subshells starting
  in sequence.

Modified:
  head/bin/sh/sh.1

Modified: head/bin/sh/sh.1
==============================================================================
--- head/bin/sh/sh.1	Sun Mar 24 22:37:10 2013	(r248691)
+++ head/bin/sh/sh.1	Sun Mar 24 22:48:45 2013	(r248692)
@@ -32,7 +32,7 @@
 .\"	from: @(#)sh.1	8.6 (Berkeley) 5/4/95
 .\" $FreeBSD$
 .\"
-.Dd January 22, 2013
+.Dd March 24, 2013
 .Dt SH 1
 .Os
 .Sh NAME
@@ -1061,6 +1061,9 @@ A subshell environment may be implemente
 If job control is enabled in an interactive shell,
 commands grouped in parentheses can be suspended and continued as a unit.
 .Pp
+For compatibility with other shells,
+two open parentheses in sequence should be separated by whitespace.
+.Pp
 The second form never forks another shell,
 so it is slightly more efficient.
 Grouping commands together this way allows the user to
@@ -1618,6 +1621,16 @@ and
 .Ic times
 returns information about the same process
 if they are the only command in a command substitution.
+.Pp
+If a command substitution of the
+.Li $(
+form begins with a subshell,
+the
+.Li $(
+and
+.Li (
+must be separated by whitespace
+to avoid ambiguity with arithmetic expansion.
 .Ss Arithmetic Expansion
 Arithmetic expansion provides a mechanism for evaluating an arithmetic
 expression and substituting its value.