CODEWITHSUNDEEP
regexperlshell
teodozjan
teodozjan

Reputation: 913

Why this single and double quote make so much difference in output

Why outputs of these two commands differ?

cat config.xml|perl -ne 'print $1,"\n" if /([0-9\.]+):161/'

cat config.xml|perl -ne "print $1,"\n" if /([0-9\.]+):161/"

First works as expected printing out matched group while seconds prints whole line.

Upvotes: 0

Views: 323

Answers (3)

TLP
TLP

Reputation: 67900

I see two main things wrong with your command.

First off, double quotes allow shell interpolation, and $1 will be taken for a shell variable and replaced. Since it unlikely exists, it will be replaced with an empty string. So instead of print $1, you get print, which is shorthand for print $_, and is probably why the entire line prints.

Second, you have unescaped double quotes inside your command, so you are in fact passing three strings to Perl:

print ,
\n
 if /(....)/

As for why or how this works with your shell, I don't know, since I do not have access to your OS, nor know which one it is. In Windows, I get a Perl bareword warning for n (Unquoted string "n" may clash with future reserved word at -e line 1.) which means that the \n is interpreted as a string. Now, here's the tricky part. What we get is this:

print , \n if /.../

Which means that \n is no longer an argument to print, it is a statement that comes after print and it is in void context, so it gets ignored. We can see this by this warning (which I had to fake in my shell):

Useless use of single ref constructor in void context at -e line 1.

(Note that you do not get these warnings as you do not use warnings -- the -w switch)

So what we are left with is

print if /.../

Which is exactly the code for the behaviour you described: It prints the whole line when a match is found.

What you can do to visualize the problem in your shell is add the -MO=Deparse switch to your one-liner, as shown here:

C:\perl>perl -MO=Deparse -ne"print ,"\n" if /a/"
LINE: while (defined($_ = <ARGV>)) {
    print($_), \'n' if /a/;
}
-e syntax OK

Now we can clearly see that the print statement is separated from the newline, and that the newline is a reference to a string.

Solution:

However, your code has other problems, and if done right you can avoid all the shell difficulties. First, you have a UUOC (Useless Use of Cat). A file argument can be given to perl when using the -n switch on the command line. Secondly, you do not need to use variables for this, you can simply print the return value of your regex:

perl -nlwe 'print for /(...)/' config.xml

The -l switch will handle newlines for you, and in this case add newline to the print. The for is necessary to avoid printing empty matches.

Upvotes: 5

falsetru
falsetru

Reputation: 369284

Inside double quote, some stuffs are substituted ($variable, `command`, ..). While inside single quote, they are remained as is.

$ echo "$HOME"
/home/falsetru
$ echo '$HOME'
$HOME

$ echo "`echo 1`"
1
$ echo '`echo 1`'
`echo 1`

Nested quotes:

$ echo ""hello""
hello
$ echo '"hello"'
"hello"
$ echo "\"hello\""
"hello"

Escape double quotes, $ to get same result:

cat config.xml | perl -ne "print \$1,\"\n\" if /([0-9\.]+):161/"

Upvotes: 3

ams
ams

Reputation: 25599

Two things:

  1. Nested quotes.
  2. Variables expand differently.

The first command has one string that happens to contain some double quotes. The variable is not expanded.

The second command has two strings with an unquoted \n in between. The variable is expanded.

Let's say $1 contains "blah"

The first passes this string to perl:

print $1,"\n" if /([0-9\.]+):161/

the second, this:

print blah,\n if /([0-9\.]+):161/

Upvotes: 2

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