Randy McMurchy wrote:
> Bruce Dubbs wrote these words on 02/28/06 19:12 CST:
>
>> You have me interested. Can you give an example?
>
> Yes I can.
>
> However, you must concede that if you agree there is difference in
> behavior (one works, one does not), then it is worth switching. Okay,
> the shell syntax is complicated, but I found a need to have to do it.
>
> I will explain as I go.
>
> First set a string variable
>
> BIGSTRING="this is the string to test for"
>
> Now I want to pick a word out of that string using sed. That word is
> "string" and I must identify it by a variable.
>
> MYSTRING="string"
>
> Notice this is one word out of $BIGSTRING
>
> We are in a loop right now with a variable being a counter of the
> loop. Each repetition through the loop increments the counter.
>
> COUNTER=1
>
> Now I echo the string, sedding each line and if it is a match, then
> I want to make a replacement based on the number of the counter.
>
> So, the command is as follows (I won't insult you by explaining the sed):
>
> echo ${BIGSTRING} | sed "s/^.*\($MYSTRING\).*$/\\${COUNTER}/"
>
> The word "string" was returned, as expected.
>
> Now, we need to make the same expression so that it is assigned to
> a variable. Here is where we get into trouble.
>
> First using `...` syntax.
>
> JUSTFORBRUCE=`echo ${BIGSTRING} | sed "s/^.*\($MYSTRING\).*$/\\${COUNTER}/"`
>
> We have assigned a variable:
>
> echo $JUSTFORBRUCE
>
> Woops, we didn't get what we wanted. We got ${COUNTER} instead
>
> Now issue the *same exact command* using $(...) syntax:
>
> JUSTFORBRUCE=$(echo ${BIGSTRING} | sed "s/^.*\($MYSTRING\).*$/\\${COUNTER}/")
>
> Notice that now the desired string was returned: "string".
>
> Need I say more? :-)
No. But the behavior is documented in the bash man page:
"When the old-style backquote form of substitution is used, backslash
retains its literal meaning except when followed by $, `, or \. The
first backquote not preceded by a backslash terminates the command
substitution. When using the $(command) form, all characters
between the parentheses make up the command; none are treated specially."
So the backslash is treated differently in the different constructs. I
like to learn new things so today is a success. :)
I still have no problems with using backquoted form if the command has
been tested. AFAIK it is not depreciated.
I also note, not that it means much, that the backquote form is easier
to type for lazy typists. :) The backquote is 2 keystrokes. The $()
for is six, counting the shift key three times.
-- Bruce
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