I just dont understand why the escape of double-quotes would cause this, because in my logic:
Lets say, in the file.dat is only this:
my name is ratna
Then, it will be:
Code:
eval echo ""my name is ratna""
this is so wierd right? I dont want to just know that the one version causes a, the other one causes b. I really want to know how eval works.. Is it really that difficult?
Regards,
Ratna
No it isn't difficult. With:
Code:
eval echo "`cat $var`"
the eval processes the command line yielding:
Code:
echo echo "this is step 1"
date
and then passes those commands to the shell yielding:
Code:
echo this is step 1
<<<output from date command>>>
but with:
Code:
eval echo "\"`cat $var`\""
the eval processes the command line yielding:
Code:
echo "echo "this is step 1"
date"
and then passes this single echo command to the shell yielding:
Code:
echo this is step 1
date
The escaped quotes from this eval and the quotes inside file were paired together by the shell just as it would if you typed the command:
All of the descriptions I've seen of eval are vague (including in the formal standards) and just say something like:
Quote:
The eval utility shall construct a command by concatenating arguments together, separating each with a <space> character. The constructed command shall be read and executed by the shell.
From years of experience, I believe that what eval does when concatenating arguments is to perform alias substitution, tilde expansion, parameter expansion, command substitution, arithmetic expansion, field splitting, pathname expansion, quote removal, and (maybe) I/O redirections. (I've never had a case when using eval where it mattered whether redirections happened on the first command line evaluation or were only evaluated when eval passed the resulting command to the shell for its second evaluation, so I've never needed to try to construct a case to determine when redirections are performed.)
The easy way to see what eval does is to issue the command:
Code:
set -xv
and type some eval commands into your shell. (When you're done with this experiment, enter the command:
thanks for the hint. I tried to "log" the steps. For the line:
Code:
eval echo "`cat $var`"
, I got this steps:
Code:
1. eval echo "`cat $var`"
2. cat $var
3. ++ cat datei.dat
4. + eval echo 'echo "this is step 1"
date'
5. echo echo "this is step 1"
6. ++ echo echo 'this is step 1'
7. echo this is step 1
date
8. ++ date
9. Friday, April 26, 2013 1:14:54 PM CEST
I dont understand, what shell does from step 4 to step 5..
It seems like, the single quotes will just disappered.
Do you have any clue?
Thank you.
Regards,
Ratna
After the command substitution performed by the eval of the contents of the file, the quotes that resulted from the command substitution remain. When eval gives that quoted string to the shell to reevaluate and execute, the quotes will be removed during the 2nd evaluation preceding the execution of the reevaluated commands.
I have repeatedly answered your questions. You have repeatedly avoided answering our questions:
Why do you need to echo an echo command?
What pipe are you talking about?
What file redirection are you talking about?
What are you hoping to achieve by evaluating the contents of a file that can't be accomplished by sourcing the file?
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04-25-2013
