On Sunday 06 August 2006 14:31, Paul Howarth wrote:
> On Sun, 2006-08-06 at 13:27 -0400, Tom Diehl wrote:
> > Any idea why the authors feel the need to break 20+ years of
> > history? I fail to see why foo=`command` is better or worse than
> > foo=$(command).
[snip]
> It allows constructs like this:
>
> $ var=$(foo $(bar))
You mean like this?
var=`foo \`bar\``
There are restrictions, but that isn't one of them.
$ sh -c 'var=`echo \`ls .zshrc\``;echo $var'
.zshrc
The Korn shell manual page does document the restrictions:
Additionally, the backquoted syntax has historical restric-
tions on the contents of the embedded command. While the new
$() form can process any kind of valid embedded script, the
backquoted form cannot handle some valid scripts that
include backquotes. For example, these otherwise valid
embedded scripts do not work in the left column, but do work
on the right:
echo ` | echo $(
cat <<eeof | cat <<eeof
a here-doc with ` | a here-doc with )
eof | eof
` | )
echo ` | echo $(
echo abc # a comment with ` | echo abc # a comment with )
` | )
echo ` | echo $(
echo '`' | echo ')'
` | )
Because of these inconsistent behaviors, the backquoted
variety of command substitution is not recommended for new
applications that nest command substitutions or attempt to
embed complex scripts.
--
Garry T. Williams --- +1 678 656-4579
