Parameter expansion flags question

Parameter expansion flags question

[John Cooper]

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I've been writing a function to run a command, extract the first word of
each line of the command's output and use that as a parameter to another
command. The command's output looks like this:

		1:/Citrix/Pres  WI
http://RA.eng.citrite.net/Citrix/Pres   4.5.5.1159
c:\inetpub\wwwroot\Citrix\Pres  
		1:/Citrix/Pres2 WI
http://RA.eng.citrite.net/Citrix/Pres2  4.5.5.1159
c:\inetpub\wwwroot\Citrix\Pres2 

After some trial-and-error I finally have the following function:

		SITEMGR=c:/Program\ Files/Citrix/Web\
Interface/4.5/sitemgr.exe
		delsites() {
		    for site in "${(f)$($SITEMGR -i)}" ; {
			sitepath=${${=site}[1]}
			[[ -n "$sitepath" ]] && $SITEMGR -r
"WICurrent=$sitepath"
		    }
		}

I'm new to the zsh parameter expansion flags, but I've gathered the
${(f) will take the command's output a line at a time, and the ${=site}
will then split each line into words, allowing me to grab the first word
of each line.

Is there a better (or simpler!) way to do this?  When the command has no
output, the `for' loop is still executed once (seemingly because the
command is within double-quotes) and is the reason for checking that the
length of $sitepath is non-zero. Is there a way to avoid the loop being
entered when the command has no output and avoid the need for this
check?

(btw, is there a good set of examples of using parameter expansion
flags? The zsh guide seemed a bit sparse in this area)

Thanks,

    --- John


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Re: Parameter expansion flags question

[Bart Schaefer]

On May 10, 10:22am, John Cooper wrote:
}
}	delsites() {
} 	    for site in "${(f)$($SITEMGR -i)}" ; {
} 		sitepath=${${=site}[1]}
} 		[[ -n "$sitepath" ]] && $SITEMGR -r "WICurrent=$sitepath"
} 	    }
} 	}
} 
} I'm new to the zsh parameter expansion flags, but I've gathered the
} ${(f) will take the command's output a line at a time, and the ${=site}
} will then split each line into words, allowing me to grab the first word
} of each line.

Right so far.

} Is there a better (or simpler!) way to do this?

You might consider using the "read" builtin:

    $SITEMGR -i | while read -A site; {
      $SITEMGR -r "WICurrent=${site[1]}"
    }

Or:

    $SITEMGR -i | while read sitepath stuff_to_discard; {
      $SITEMGR -r "WICurrent=$sitepath"
    }

If you still want to use expansion, you can combine the expansions:

    for sitepath in ${${=${(f)"$($SITEMGR -i)"}}[1]}

Be warned, however, that ${=var} is not guaranteed to produce an array;
if any line in the $SITEMGR output has only one word in it, the result
of the above will be the first *character* of the line, not the first
*word*.  The solutions with "read" do not have this problem.

} When the command has no output, the `for' loop is still executed once
} (seemingly because the command is within double-quotes) and is the
} reason for checking that the length of $sitepath is non-zero. Is there
} a way to avoid the loop being entered when the command has no output
} and avoid the need for this check?

Change the expansion to have it discard all empty array elements by
using the ":#pattern" operator with an empty pattern:

    for sitepath in ${${=${(f)"$($SITEMGR -i)":#}}[1]}

} (btw, is there a good set of examples of using parameter expansion
} flags? The zsh guide seemed a bit sparse in this area)

Other than in the archives of this list, not that I'm aware of.  Did
you look around on zshwiki.org?

RE: Parameter expansion flags question

[John Cooper]

Ah, I see. Thanks again for all the help.

    --- John

-----Original Message-----
From: Bart Schaefer [mailto:schaefer@brasslantern.com] 
Sent: 12 May 2006 15:45
To: zsh-users@sunsite.dk
Subject: Re: Parameter expansion flags question

On May 12, 12:02pm, John Cooper wrote:
} Subject: RE: Parameter expansion flags question
}
}   > Placement of the quotes is important:
}   >    for site in ${(f)"$($SITEMGR -i)":#}
} 
} OK, changing the placement of the quotes to the above fixes it.
There's
} just one thing I'm a little puzzled about - in the case where $SITEMGR
} produces no output, won't "$($SITEMGR -i)" be the empty string? If so,
} why is :# needed? It seems it will just replace an empty string with
} another empty string?

It's the same as the difference between

string=""
array=( word "$string" word )
print $#array

and

string=""
array=( word $string word )
print $#array

An unquoted empty string is discarded; a quoted empty string is
retained.
Effectively the :# construct discards the quoted empty string, leaving
an
unquoted one in its place, which the shell in turn discards when
building
the final array.

Re: Parameter expansion flags question

[Bart Schaefer]

On May 12, 12:02pm, John Cooper wrote:
} Subject: RE: Parameter expansion flags question
}
}   > Placement of the quotes is important:
}   >    for site in ${(f)"$($SITEMGR -i)":#}
} 
} OK, changing the placement of the quotes to the above fixes it. There's
} just one thing I'm a little puzzled about - in the case where $SITEMGR
} produces no output, won't "$($SITEMGR -i)" be the empty string? If so,
} why is :# needed? It seems it will just replace an empty string with
} another empty string?

It's the same as the difference between

string=""
array=( word "$string" word )
print $#array

and

string=""
array=( word $string word )
print $#array

An unquoted empty string is discarded; a quoted empty string is retained.
Effectively the :# construct discards the quoted empty string, leaving an
unquoted one in its place, which the shell in turn discards when building
the final array.

RE: Parameter expansion flags question

[John Cooper]

  > Placement of the quotes is important:
  >    for site in ${(f)"$($SITEMGR -i)":#}

OK, changing the placement of the quotes to the above fixes it. There's
just one thing I'm a little puzzled about - in the case where $SITEMGR
produces no output, won't "$($SITEMGR -i)" be the empty string? If so,
why is :# needed? It seems it will just replace an empty string with
another empty string?

Thanks,

    --- John

-----Original Message-----
From: Bart Schaefer [mailto:schaefer@brasslantern.com] 
Sent: 11 May 2006 17:39
To: zsh-users@sunsite.dk
Subject: Re: Parameter expansion flags question

[skipping around a bit]

On May 11, 10:59am, John Cooper wrote:
}
} One final thing - when I entered the "read" example into the shell as
} stated I got syntax errors. These were resolved by using "do" and
"done"
} instead of braces - is this to be expected?

Yeah, I messed up.  I never use the form with the braces, but you did
in your original "for" sample, so I tried to keep it with "while".  I
forgot that for "while" loops it only works when the first condition
is [[ ... ]] or (( ... )).

} Thanks for the detailed reply - it seems using "read" is the most
} straightforward approach. However, in the interests of learning more
} about expansions, I've been trying your suggestions and they don't
seem
} to work as expected.

Oh, duh.  I spaced that there were multiple lines of output from the
$SITEMGR program.  Obviously you need to process each line separately
before applying the subscript.

(This cold I'm coming down with must be affecting me worse than I
thought.)

} If I add the ":#pattern" operator to the original function, the "for"
} loop is still executed once in the case where $SITEMGR produces no
} output:

} delsites4 () {
}         for site in "${(f)$($SITEMGR -i):#}"

Placement of the quotes is important:

    for site in ${(f)"$($SITEMGR -i)":#}

If it still happens, I think it's because of Cygwin line termination.
Instead of an empty string when splitting with (f), you're getting a
string having a single carriage-return character.

Re: Parameter expansion flags question

[Bart Schaefer]

[skipping around a bit]

On May 11, 10:59am, John Cooper wrote:
}
} One final thing - when I entered the "read" example into the shell as
} stated I got syntax errors. These were resolved by using "do" and "done"
} instead of braces - is this to be expected?

Yeah, I messed up.  I never use the form with the braces, but you did
in your original "for" sample, so I tried to keep it with "while".  I
forgot that for "while" loops it only works when the first condition
is [[ ... ]] or (( ... )).

} Thanks for the detailed reply - it seems using "read" is the most
} straightforward approach. However, in the interests of learning more
} about expansions, I've been trying your suggestions and they don't seem
} to work as expected.

Oh, duh.  I spaced that there were multiple lines of output from the
$SITEMGR program.  Obviously you need to process each line separately
before applying the subscript.

(This cold I'm coming down with must be affecting me worse than I
thought.)

} If I add the ":#pattern" operator to the original function, the "for"
} loop is still executed once in the case where $SITEMGR produces no
} output:

} delsites4 () {
}         for site in "${(f)$($SITEMGR -i):#}"

Placement of the quotes is important:

    for site in ${(f)"$($SITEMGR -i)":#}

If it still happens, I think it's because of Cygwin line termination.
Instead of an empty string when splitting with (f), you're getting a
string having a single carriage-return character.

RE: Parameter expansion flags question

[John Cooper]

Bart,

Thanks for the detailed reply - it seems using "read" is the most
straightforward approach. However, in the interests of learning more
about expansions, I've been trying your suggestions and they don't seem
to work as expected.

$ $SITEMGR -i
1:/Citrix/Pres  WI      http://RA.eng.citrite.net/Citrix/Pres
4.5.5.1159      c:\inetpub\wwwroot\Citrix\Pres  
1:/Citrix/Pres2 WI      http://RA.eng.citrite.net/Citrix/Pres2
4.5.5.1159      c:\inetpub\wwwroot\Citrix\Pres2 

(not sure if Outlook will wrap the above, but the output is actually 2
lines, each starting "1:/Citrix")

My original function correctly finds the first word of both lines:

$ type delsites2
delsites2 () {
        for site in "${(f)$($SITEMGR -i)}"
        do
                sitepath=${${=site}[1]} 
                echo "sitepath is $sitepath"
        done
}
$ delsites2
sitepath is 1:/Citrix/Pres
sitepath is 1:/Citrix/Pres2
$

However, when the expansions are combined, it only finds the first word
of the first line:

$ type delsites3
delsites3 () {
        for sitepath in ${${=${(f)"$($SITEMGR -i)"}}[1]}
        do
                echo "sitepath is $sitepath"
        done
}
$ delsites3
sitepath is 1:/Citrix/Pres
$

If I add the ":#pattern" operator to the original function, the "for"
loop is still executed once in the case where $SITEMGR produces no
output:

$ type delsites4
delsites4 () {
        for site in "${(f)$($SITEMGR -i):#}"
        do
                sitepath=${${=site}[1]} 
                echo sitepath is "$sitepath"
        done
}

$ delsites
$ $SITEMGR -i
$
$ delsites4
sitepath is 
$                

One final thing - when I entered the "read" example into the shell as
stated I got syntax errors. These were resolved by using "do" and "done"
instead of braces - is this to be expected?

$ $SITEMGR -i
1:/Citrix/Pres  WI      http://RA.eng.citrite.net/Citrix/Pres
4.5.5.1159      c:\inetpub\wwwroot\Citrix\Pres  
$ $SITEMGR -i | while read -A site; {
pipe while cursh> echo $site   
pipe while cursh> }
pipe while> }
zsh: parse error near `}'
$
$ $SITEMGR -i | while read -A site; do
pipe while> echo $site
pipe while> done
1:/Citrix/Pres WI http://RA.eng.citrite.net/Citrix/Pres 4.5.5.1159
c:inetpubwwwrootCitrixPres 
$

I'm using zsh 4.2.6 on cygwin/WinXP.

    --- John

-----Original Message-----
From: Bart Schaefer [mailto:schaefer@brasslantern.com] 
Sent: 10 May 2006 18:28
To: zsh-users@sunsite.dk
Subject: Re: Parameter expansion flags question

On May 10, 10:22am, John Cooper wrote:
}
}	delsites() {
} 	    for site in "${(f)$($SITEMGR -i)}" ; {
} 		sitepath=${${=site}[1]}
} 		[[ -n "$sitepath" ]] && $SITEMGR -r
"WICurrent=$sitepath"
} 	    }
} 	}
} 
} I'm new to the zsh parameter expansion flags, but I've gathered the
} ${(f) will take the command's output a line at a time, and the
${=site}
} will then split each line into words, allowing me to grab the first
word
} of each line.

Right so far.

} Is there a better (or simpler!) way to do this?

You might consider using the "read" builtin:

    $SITEMGR -i | while read -A site; {
      $SITEMGR -r "WICurrent=${site[1]}"
    }

Or:

    $SITEMGR -i | while read sitepath stuff_to_discard; {
      $SITEMGR -r "WICurrent=$sitepath"
    }

If you still want to use expansion, you can combine the expansions:

    for sitepath in ${${=${(f)"$($SITEMGR -i)"}}[1]}

Be warned, however, that ${=var} is not guaranteed to produce an array;
if any line in the $SITEMGR output has only one word in it, the result
of the above will be the first *character* of the line, not the first
*word*.  The solutions with "read" do not have this problem.

} When the command has no output, the `for' loop is still executed once
} (seemingly because the command is within double-quotes) and is the
} reason for checking that the length of $sitepath is non-zero. Is there
} a way to avoid the loop being entered when the command has no output
} and avoid the need for this check?

Change the expansion to have it discard all empty array elements by
using the ":#pattern" operator with an empty pattern:

    for sitepath in ${${=${(f)"$($SITEMGR -i)":#}}[1]}

} (btw, is there a good set of examples of using parameter expansion
} flags? The zsh guide seemed a bit sparse in this area)

Other than in the archives of this list, not that I'm aware of.  Did
you look around on zshwiki.org?