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Full Discussion: pipe to grep doesn't work in bash script
Top Forums Shell Programming and Scripting pipe to grep doesn't work in bash script Post 302597726 by agama on Saturday 11th of February 2012 07:52:02 PM
Old 02-11-2012
When the shell expands the variable holding the command, it recognises that the pipe symbol is special, and quotes it. If you consider this example:


Code:
cmd="echo  foo | awk '{print \"bar \"; print }'"
data=`$cmd`

you would expect that when $cmd is executed the string "bar foo" would be assigned to data. However, the string assigned to data is actually:
Code:
foo | awk '{print "bar "; print }'

If you execute it with -x you'll see in the trace the quotes that the shell adds and that the pipe symbol is being passed to echo (along with everything else past it), and not interpreted by the shell. The output when I ran it was:


Code:
+ cmd=$'echo  foo | awk \'{print "bar "; print }\''
+ echo foo '|' awk $'\'{print' '"bar' '";' print $'}\''
+ data=$'foo | awk \'{print "bar "; print }\''
+ echo $'foo | awk \'{print "bar "; print }\''
foo | awk '{print "bar "; print }'

Notice in the second line the '|' is the clue that the shell has treated the contents of the variable as a single string and has not parsed it as a command.


When you add the eval in front of the expansion, the shell expands the variable and then parses it as a command. The result is that the pipe is then recognised as you intended and both commands are executed as expected.


In your case, the whole string would have been passed to the gvfs-mount command and as that string contained -i, it would have listed details. I suspect it might have also complained about the pipe and/or grep but that would have been to stderr and not captured in your variable.

When things seem strange, I usually put a set -x before the statement that isn't working and set +x ater it to get the details of how the shell is interpreting my code. The output is usually enough to clear up the confusion.


Hope this helps
 

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LEARN ABOUT DEBIAN

devel::beginlift

Devel::BeginLift(3pm)					User Contributed Perl Documentation				     Devel::BeginLift(3pm)

NAME

       Devel::BeginLift - make selected sub calls evaluate at compile time

SYNOPSIS

	 use Devel::BeginLift qw(foo baz);

	 use vars qw($i);

	 BEGIN { $i = 0 }

	 sub foo { "foo: $_[0]
"; }

	 sub bar { "bar: $_[0]
"; }

	 for (1 .. 3) {
	   print foo($i++);
	   print bar($i++);
	 }

	 no Devel::BeginLift;

	 print foo($i++);

       outputs -

       foo: 0 bar: 1 foo: 0 bar: 2 foo: 0 bar: 3 foo: 4

DESCRIPTION

       Devel::BeginLift 'lifts' arbitrary sub calls to running at compile time - sort of a souped up version of "use constant". It does this via
       some slightly insane perlguts magic.

   import
	 use Devel::BeginLift qw(list of subs);

       Calls Devel::BeginLift->setup_for(__PACKAGE__ => @list_of_subs);

   unimport
	 no Devel::BeginLift;

       Calls Devel::BeginLift->teardown_for(__PACKAGE__);

   setup_for
	 Devel::BeginLift->setup_for($package => @subnames);

       Installs begin lifting magic (unless already installed) and registers "${package}::$name" for each member of @subnames to be executed when
       parsed and replaced with its output rather than left for runtime.

   teardown_for
	 Devel::BeginLift->teardown_for($package);

       Deregisters all subs currently registered for $package and uninstalls begin lifting magic is number of teardown_for calls matches number of
       setup_for calls.

   setup_for_cv
	 $id = Devel::BeginLift->setup_for_cv(&code);

       Same as "setup_for", but only registers begin lifting magic for one code reference. Returns an id to be used in "teardown_for_cv".

   teardown_for_cv
	 Devel::BeginLift->teardown_for_cv($id);

       Deregisters begin lifting magic referred to by $id.

AUTHOR

       Matt S Trout - <mst@shadowcatsystems.co.uk>

       Company: http://www.shadowcatsystems.co.uk/ Blog: http://chainsawblues.vox.com/

LICENSE

       This library is free software under the same terms as perl itself

perl v5.14.2							    2012-04-22						     Devel::BeginLift(3pm)
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