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Unix example questions

Posted on 2006-06-12
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Last Modified: 2013-12-26
Hi Experts,

Thought i'd have a bash at learning UNIX as I mostly know scripting languages or higher level OO based languages. Got to admin its all a little confusing and I am attempting to example questions I found from various sites. First one is to write a script that takes two arguments a file and a string and inputs the string at top of file and in middle line of file.

I know how to get the number of lines from a file i.e.

wc -w File

But I am unsure how to use that as a variable in my script. All that I have read about variables is simple assignment i.e.

myVariable=This

I dont know how to assign the return of a function to the variable i.e.

myVariable=wc -w File

doesnt seem to work. And I dont think its possible to redirect to variables is it? i.e.

wc -w File > $LineCount

Once I have the line numbers I can determine where to add the string but I wouldnt know how to do this either.  I would guess that to add the string in the middle line of the file I would first assign the head lines of a variable then append the var to add the sting then append it to add the tail values. But again I dont know how to assign the results of the Tail and Head returns to a file i.e.

$First=$LineCount/2
$Text= Tail -$First

Am I completely off the mark here with what I am wanting to do? I think i probably am and the other questions are alot harder still.

Thanks in advance,
Chris.
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Question by:icb01co1
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Expert Comment

by:ahoffmann
ID: 16889172
> I dont know how to assign the return of a function to the variable i.e.

myVariable="`wc -w File`"

> Once I have the line numbers I can determine where to add the string but I wouldnt know how to do this either.
sed -e "1,$myVariable p" file > head
sed -e "$myVariable,"'$ p' file > tail
( cat head; echo your line; cat tail) > new file

a bit cumbersome in shell, but something you asked for ;-)
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Author Comment

by:icb01co1
ID: 16889210
Ok now I know that I should be using the sed command and have figured out how to add a new line to a file i.e:

sed '/1/G' myFile

But am still stumped
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Expert Comment

by:ahoffmann
ID: 16889231
stumped at which part?
What's wronh with my suggestion?
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Author Comment

by:icb01co1
ID: 16889264
Thanks ahoofmann, just one problem here, it says dont use external files and your redirecting to the head and tail files. Is there not a simpler way to add a string to file using sed?

Seems as if I can add a new line to a file with sed i.e.

sed '/3/G' myFile > myFile

I must be able to add a string. I am not sure about the sed syntax though, theres all this talk of

[address[,address]][!]{
command1
command2
}

Which may as well be greek to me.
Thanks, Chris.
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Author Comment

by:icb01co1
ID: 16889480
ahoffmann, Ive a problems actually.

"wc -w File" doesnt just return an integer.

It is returning something like this: "      6 File"

Should I use sed on this to remove the spaces and the occurance of the filename?

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Expert Comment

by:ahoffmann
ID: 16889593
> .. it says dont use external files  ..
who says? your teacher?

> Is there not a simpler way to add a string to file using sed?
I already sad: cumbersome.
If you stick on sed, then
  man sed
it's possible, but more cumbersome as I suggested
otherwise
  man awk

> "wc -w File" doesnt just return an integer.
myVariable="`wc File|awk '{print $2}'`"

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Author Comment

by:icb01co1
ID: 16896684
My teacher?  

Ok just read up about awk, thats quite a usefull language. I am still having trouble with the first part though, here:

sed -e "1,$myVariable p" file > head

I dont nessacarily want an answer, just a link to better explain sed and maybe awk too. I cant find anything thats making any sense.

Thanks, Chris.



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Accepted Solution

by:
ahoffmann earned 125 total points
ID: 16897210
> sed -ne "1,$myVariable p" file > head
prints from line 1 to line in $myVariable

Bugfix: just seen that I missed the -n option for the sed commands, please use instead
sed -ne "1,$myVariable p" file > head
sed -ne "$myVariable,"'$ p' file > tail


http://www.cs.bell-labs.com/cm/cs/awkbook/index.html

>  I cant find anything thats making any sense.
use perl, it can do it all in one
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