[TriLUG] HERE document question

[Joseph Mack NA3T]

On Sun, 25 Jul 2010, Kevin Hunter wrote:

> At 5:16pm -0600 Sun, 25 Jul 2010, Joseph Mack NA3T wrote:
>> partition=/dev/sda1
>> 
>> cat > restore.sh << EOF
>> count=`df | grep -c $partition`
>> EOF
>
>> I want the HERE document to be
>> 
>> count=`df | grep -c /dev/sda1`
>> 
>> so that the mount state is determined at the time the HERE document is
>> run, not at the time the HERE document is generated
>
> I may not be clear on exactly what you're getting at, but I'll hazard a 
> guess.
>
> 1. Don't use backticks for subshell work.

Thanks for your help.

I didn't know about this, but then it's been a while since I 
looked for anything new in bash. I did find an example of 
nested backticks in case anyone needs to extend the idiom 
:-)

word_count=` wc -w \`echo * | awk '{print $8}'\`

(at the bottom of

http://tldp.org/LDP/abs/html/commandsub.html)

I didn't know you could nest backticks either. I've wanted 
to do it occasionally.

I'm using bash v2.05, in case that makes any difference. I 
couldn't find any tutorials on v3 or v4 just in case your 
examples only worked in the newer versions.

> Suggest you don't use the backticks for subshell 
> evaluation.  They're deprecated for a number of reasons, 
> but the easiest reason is for forward compatibility and 
> nestability.  Consider the $( ) construct instead:
>
> count=$(some set | of subshell | work)
> count=$(some set $(of subshell) | work)

OK

count=$(df | grep -c /dev/sda1)

is the same as

count=`df | grep -c /dev/sda1)`

> 2. I think your issue is when the command gets evaluated 
> and when the string gets interpreted.  Specifically, you 
> want the string interpreted, but you want the command 
> evaluated later.  Try this:
>
> command="df | grep -c $partition"
> count=\$($command)

I've split the problem into two parts.

o I have to write a HERE document where some variables are 
substituted before the HERE document is written AND some 
variables are not substituted till the HERE document is run.
Yesterday, when I first had a go at this, I expected to find
examples explained in the Advanced BASH scriping guide. I 
thought this would be routine (if not a little tricky).


o The HERE document has to run

I'm tackling the 2nd part first, since this turns out to be 
a harder problem than I thought. I'm writing the HERE 
document by hand at the moment. When I get something that 
works and I can generate from another script, I'll tackle 
the generating script.

Here's the code inside the HERE document (/dev/sda1 was the 
result of substitution in the generating script)

command="df | grep -c /dev/sda1"

I now want to execute command (I run the code with sh -x )

count=\$($command)	#syntax error

count=$(command)	#$(command) is blank

count=$($command)	#the -c in the command is interpreted as an option to df
 			#using \| doesn't help

Any more ideas?

Thanks Joe


>
> The backslash says "interpret this following character literally, not as a 
> special operator".  So, the $command token will get interpreted into your 
> restore.sh, but the \$( ... ) will get put in as $( ... ), and not an 
> evaluation.  Note that if you were to try count=$($command) (no backslash), 
> the line would be interpreted with automatic escaping, which is probably not 
> what you want.
>
> count=$(df \| grep -c $partition),
>
> HTH,
>
> Kevin
>

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!