What is the difference between the perl -n and perl -p options?
What is a simple example to demonstrate the difference?
How do you decide which one to use?
How do you decide which one to use?
You use -p if you want to automatically print the contents of $_ at the end of each iteration of the implied while loop. You use -n if you don't want to print $_ automatically.
An example of -p. Adding line numbers to a file:
$ perl -pe '$_ = "$.: $_"' your_file.txt
An example of -n. A basic grep replacement.
$ perl -ne 'print if /some search text/' your_file.txt
-p is short for -np, and it causes $_ to be printed for each pass of the loop created by -n.
perl -ne'...'
executes the following program:
LINE: while (<>) {
...
}
while
perl -pe'...'
executes the following program:
LINE: while (<>) {
...
}
continue {
die "-p destination: $!\n" unless print $_;
}
See perlrun for documentation about perl's command-line options.
What is the difference between the perl -n and perl -p options?
-p causes each line to be printed; equivalent to:
while (<>) { ... } continue { print }
-n does not automatically print each line; equivalent to:
while(<>) {...}
What is a simple example to demonstrate the difference?
e.g., replace foo with FOO:
$ echo 'foo bar' | perl -pe 's/foo/FOO/'
FOO bar
$ echo 'foo bar' | perl -ne 's/foo/FOO/'
$
How do you decide which one to use?
One example where -n is useful is when you don't want every line printed, and there is a conditional print in the code, e.g., only show lines containing foo:
$ echo -e 'foo\nbar\nanother foo' | perl -ne 'print if /foo/;'
foo
another foo
$
The command-line options are documented in perlrun documentation
perl -n is equivalent to while(<>){...}
perl -p is equivalent to while(<>){...;print;}
Related
I am not familiar with perl. I am reading an installation guide atm and the following Linux command has come up:
perl -p -i -e "s/enforcing/disabled/" /etc/selinux/config
Now, I am trying to understand this. Here is my understanding so far:
-e simply allows for executing whatever follows
-p puts my commands that follow -e in a loop. Now this is strange to me, as to me this command seems to be trying to say: Write "s/enforcing/disabled/" into /etc/selinux/config. Then again, where is the "write" command? And what is this -i (inline) good for?
-p changes
s/enforcing/disabled/
to something equivalent to
while (<>) {
s/enforcing/disabled/;
print;
}
which is short for
while (defined( $_ = <ARGV> )) {
$_ =~ s/enforcing/disabled/;
print($_);
}
What this does:
It reads a line from ARGV into $_. ARGV is a special file handle that reads from the each of the files specified as arguments (or STDIN if no files are provided).
If EOF has been reached, the loop and therefore the program exits.
It replaces the first occurrence of enforcing with disabled.
It prints out the modified line to the default output handle. Because of -i, this is a handle to a new file with the same name as the one from which the program is currently reading.*
Repeat.
For example,
$ cat a
foo
bar enforcing the law
baz
enforcing enforcing
$ perl -pe's/enforcing/disabled/' -i a
$ cat a
foo
bar disabled the law
baz
disabled enforcing
* — In old versions of Perl, the old file has already been deleted at this point, but it's still accessible as long as there's an open file handle to it. In very new versions of Perl, this writes to temporary file that will later overwrite the file from which the program is reading.
To find out exactly what Perl is going to do, you can use the O module
perl -MO=Deparse -p -i -e "s/enforcing/disabled/" file
outputs
BEGIN { $^I = ""; }
LINE: while (defined($_ = readline ARGV)) {
s/enforcing/disabled/;
}
continue {
die "-p destination: $!\n" unless print $_;
}
-e syntax OK
This question already has answers here:
How can I process options using Perl in -n or -p mode?
(2 answers)
Closed last year.
I often need to run some Perl one-liners for fast data manipulations, like
some_command | perl -lne 'print if /abc/'
Reading from a pipe, I don't need a loop around the command arg filenames. How can I achieve the next?
some_command | perl -lne 'print if /$ARGV[0]/' abc
This gives the error:
Can't open abc: No such file or directory.
I understand that the '-n' does the
while(<>) {.... }
around my program, and the <> takes args as filenames, but doing the next every time is a bit impractical
#/bin/sh
while read line
do
some_command | perl -lne 'BEGIN{$val=shift #ARGV} print if /$val/' "$line"
done
Is there some better way to get "inside" the Perl ONE-LINER command line arguments without getting them interpreted as filenames?
Some solutions:
perl -e'while (<STDIN>) { print if /$ARGV[0]/ }' pat
perl -e'$p = shift; while (<>) { print if /$p/ }' pat
perl -e'$p = shift; print grep /$p/, <>' pat
perl -ne'BEGIN { $p = shift } print if /$p/' pat
perl -sne'print if /$p/' -- -p=pat
PAT=pat perl -ne'print if /$ENV{PAT}/'
Of course, it might make more sense to create a pattern that's an ORing or all patterns rather than executing the same command for each pattern.
Also reasonably short:
... | expr=abc perl -lne 'print if /$ENV{expr}/'
Works in bash shell but maybe not other shells.
It depends on what you think will be in the lines you read, but you could play with:
#/bin/sh
while read line
do
some_command | perl -lne "print if /$line/"
done
Clearly, if $line might contain slashes, this is not going to fly. Then, AFAIK, you're stuck with the BEGIN block formulation.
I am using Perl to search and replace multiple regular expressions:
When I execute the following command, I get an error:
prompt> find "*.cpp" | xargs perl -i -pe 's/##(\W)/\1/g' -pe 's/(\W)##/\1/g'
syntax error at -e line 2, near "s/(\W)##/\1/g"
Execution of -e aborted due to compilation errors.
xargs: perl: exited with status 255; aborting
Having multiple -e is valid in Perl, then why is this not working? Is there a solution to this?
Several -e's are allowed.
You are missing the ';'
find "*.cpp" | xargs perl -i -pe 's/##(\W)/\1/g;' -pe 's/(\W)##/\1/g;'
Perl statements has to end with ;.
Final statement in a block doesn't need a terminating semicolon.
So a single -e without ; will work, but you will have to add ; when you have multiple -e statements.
Having multiple -e values are valid, but is it useful? The values from the multiple -e are merely combined into one program, and it's up to you to ensure that together they make a syntactically correct program. The B::Deparse program can show you what perl thinks the program is:
$ perl -MO=Deparse -e 'print' -e 'q(Hello' -e ')'
print "Hello\n";
-e syntax OK
A curious thing to note is that a newline snuck in there. Think about how it got there to see what else perl is doing to combine multiple -e values.
In your program, you are substituting on the current line, then taking the modified line and substituting again. That's better written as:
prompt> find "*.cpp" | xargs perl -i -pe 's/##(\W)/\1/g; s/(\W)##/\1/g'
Now, if you are building up this command line by adding more and more -e through some automated process and you don't know ahead of time what you get, maybe those -e make sense. However, you might consider that you can do the same thing to build up the string you give to -e. I don't know what might be better because you didn't explain why you are doing it that way.
But, I suspect that in some cases, people are actually thinking about having only one substitution work. They want to try one and if its pattern doesn't work, try a different one until one succeeds. In that case you don't want to separate the substitutions by semicolons. Use the short-circuiting || instead. The s/// returns the number of substitutions it made and || will stop (short circuit) when it finds a true value:
prompt> find "*.cpp" | xargs perl -i -pe 's/##(\W)/\1/g || s/(\W)##/\1/g'
And note, you only need one -p. It only does its job once. Here's the program with multiple -p deparsed:
$ perl -MO=Deparse -i -pe 's/##(\W)/\1/g;' -pe 's/(\W)##/\1/g;'
BEGIN { $^I = ""; }
LINE: while (defined($_ = readline ARGV)) {
s/##(\W)/$1/g;
s/(\W)##/$1/g;
}
continue {
die "-p destination: $!\n" unless print $_;
}
-e syntax OK
It's the same thing as having only one -p:
$ perl -MO=Deparse -pi -e 's/##(\W)/\1/g;' -e 's/(\W)##/\1/g;'
BEGIN { $^I = ""; }
LINE: while (defined($_ = readline ARGV)) {
s/##(\W)/$1/g;
s/(\W)##/$1/g;
}
continue {
die "-p destination: $!\n" unless print $_;
}
-e syntax OK
Thanks so much! You helped me reduce my ascii / decimal / 8-bit binary table printer enough to fit in a tweet:
for i in {32..126};do printf "'\x$(printf %x $i)'(%3i) = " $i; printf '%03o\n' $i | perl \
-pe 's#0#000#g;' -pe 's#1#001#g;' -pe 's#2#010#g;' -pe 's#3#011#g;' \
-pe 's#4#100#g;' -pe 's#5#101#g;' -pe 's#6#110#g;' -pe 's#7#111#g' ; done | \
perl -pe 's#= 0#= #'
I'm trying to extract all ip addresses from a file. So far, I'm just using
cat foo.txt | perl -pe 's/.*?((\d{1,3}\.){3}\d{1,3}).*/\1/'
but this also prints lines that don't contain a match. I can fix this by piping through grep, but this seems like it ought to be unnecessary, and could lead to errors if the regexes don't match up perfectly.
Is there a simpler way to accomplish this?
Try this:
cat foo.txt | perl -ne 'print if s/.*?((\d{1,3}\.){3}\d{1,3}).*/\1/'
or:
<foo.txt perl -ne 'print if s/.*?((\d{1,3}\.){3}\d{1,3}).*/\1/'
It's the shortest alternative I can think of while still using Perl.
However this way might be more correct:
<foo.txt perl -ne 'if (/((\d{1,3}\.){3}\d{1,3})/) { print $1 . "\n" }'
If you've got grep, then just call grep directly:
grep -Po "(\d{1,3}\.){3}\d{1,3}" foo.txt
You've already got a suitable answer of using grep to extract the IP addresses, but just to explain why you were seeing non-matches being printed:
perldoc perlrun will tell you about all the options you can pass Perl on the command line.
Quoting from it:
-p causes Perl to assume the following loop around your program, which makes it
iterate over filename arguments somewhat like sed:
LINE:
while (<>) {
... # your program goes here
} continue {
print or die "-p destination: $!\n";
}
You could have used the -n switch instead, which does similar, but does not automatically print, for example:
cat foo.txt | perl -ne '/((?:\d{1,3}\.){3}\d{1,3})/ and print $1'
Also, there's no need to use cat; Perl will open and read the filenames you give it, so you could say e.g.:
perl -ne '/((?:\d{1,3}\.){3}\d{1,3})/ and print $1' foo.txt
ruby -0777 -ne 'puts $_.scan(/((?:\d{1,3}\.){3}\d{1,3})/)' file
I have a file that contains a lot of these
"/watch?v=VhsnHIUMQGM"
and I would like to output the letter code using a perl one-liner. So I try
perl -nle 'm/\"\/watch\?v=(.*?)\"/g' filename.txt
but it doesn't print anything.
What am I doing wrong?
The -n option processes each line but doesn't print anything out. So you need to add an explicit print if you successfully match.
perl -ne 'while ( m/\"\/watch\?v=(.+?)\"/g ) { print "$1\n" }' filename.txt
Another approach, if you're sure every line will match, is to use the -p option which prints out the value of $_ after processing, e.g.:
perl -pe 's/\"\/watch\?v=(.+?)\"/$1//' filename.txt
Your regex is fine. You're getting no output because the -n option won't print anything. It simply wraps a while (<>) { ... } loop around your program (run perl --help for brief explanations of the Perl options).
The following uses your regex, but add some printing. In list context, regexes with the /g option return all captures. Effectively, we print each capture.
perl -nle 'print for m/\"\/watch\?v=(.*?)\"/g' data.dat
You can split the string on "=" instead of matching:
perl -paF= -e '$_= #F[1]' filename.txt