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Trying to understand a complex bit of code

 
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# 1  
Old 01-10-2013
Trying to understand a complex bit of code
Hi,

To re-introduce myself, I'm a router guy trying to learn some scripting from the examples in my work place...

In a ksh script, one of the script guys wrote the following and I am trying to understand it. I'm hoping someone can explain it to me.

The script flow enters a case structure.
Inside the case, one of the options is as follows:
Code:
      @RESPONSE*) ADDRESPONSE=$(print "$cmd" | sed -e   's/^[   ]*@RESPONSE //')       
  [[ "$ADDRESPONSE" ==   "^M" ]] && \       
  
  				cat $WORKDIR/ADDPROMPTS | sed -e s/ADDRESPONSE/\\\r/g" >   $WORKDIR/ADDPROMPTS.tmp || \       
  
  cat $WORKDIR/ADDPROMPTS | sed -e   "s/ADDRESPONSE/${ADDRESPONSE}\\\r/g" > $WORKDIR/ADDPROMPTS.tmp

What I "get" is:
the first line removes the string "@RESPONSE //" from $cmd putting the new value in $ADDRESPONSE

I don't understand why the second line is in double bracket but this "seems to me" to be a short hand "if" statement with an "and"

and I would appreciate anyone who can explain what's happening in the two sed commands?

Thanks!

Marc
# 2  
Old 01-10-2013
If you've ever used 'if' in a shell script, you know what [ ] or [[ ]] statments do.

[ ] and [[ ]] are not even part of if -- they work just fine without it.

&&, || are short-form if then.

Code:
true && echo "This will print"
false && echo "This will not"

true || echo "This will not print"
false || echo "But this will"

# 3  
Old 01-10-2013
First, thanks for answering!

To be honest, I've only ever explicitly written if statements.
So the "[]" and "[[]]" without the "if" included in the code is new to me.

To make sure I have read your answer correctly, you are saying the code is equal to:

If $ADDRESPONSE equals ^M, execute the first "cat / sed" command
else execute the second "cat / sed" command ?

Marc
# 4  
Old 01-10-2013
Looks like it.

Code:
$ true && echo "command succeeded" || echo "command failed"
command succeeded

$ false && echo "command succeeded" || echo "command failed"
command failed

$

This User Gave Thanks to Corona688 For This Post:
Marc G
# 5  
Old 01-10-2013
OK,

I still don't get why the "[[]]" rather than the "[]"

And then the sed commands.
It appears to me that the first one says:
Code:
   Take the contents of "$WORKDIR/ADDPROMPTS" and swap the string  
   value of $ADDRESPONSE"   for "\r" globally before saving it to 
   $WORKDIR/ADDPROMPTS.tmp

And the second one seems to say:
Code:
   Take the contents of "$WORKDIR/ADDPROMPTS" and swap the string  
   value of $ADDRESPONSE"   for "the string value of $ADDRESPONSE"
    - AND -"\r" globally before saving it to $WORKDIR/ADDPROMPTS.tmp

But I am not sure I am correct.

Marc
# 6  
Old 01-10-2013
Quote:
Originally Posted by Marc G
OK,

I still don't get why the "[[]]" rather than the "[]"
[[ ]] is an extended version of [ ]. Some shells don't have it.

I'd start by looking at what strings get input to it, and what strings come out of each half.
 
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