my text file contents
vi /root/text.conf
node.session.auth.authmethod = hello
so my one liner perl command replaces the above file contents by commenting with #. Works fine when run as single command
perl -pi -e 's/^(node.session.auth.authmethod\s*=\s*).*$/#\1hello/g' /root/text.conf
when the perl one liner code is executed inside perl script, it does not comment out the text.conf file contents.
$cmd ="perl -pi -e 's/^(node.session.auth.authmethod\s*=\s*).*$/#\1hello/g' /root/text.conf"
$line = `$cmd 2>&1`;$ret = $?;
Am I missing something while executing Perl one liner inside a Perl script.
Yes, use current perl process instead of forking new one. This is equivalent of your
perl -pi -e 's/^(node.session.auth.authmethod\s*=\s*).*$/#\1hello/g' /root/text.conf
one-liner,
use strict;
use warnings;
local $^I = "";
local #ARGV = "/root/text.conf";
while (<>) {
s/^(node.session.auth.authmethod\s*=\s*).*$/#\1hello/g;
print;
}
Defining your $cmd the Variable $/ will be replaced by it's value (the line separator). Then the regex will not match your line.
Try using single quotes (along with proper escaping):
$cmd ='perl -pi -e \'s/^(node.session.auth.authmethod\s*=\s*).*$/#\1hello/g\' /root/text.conf'
$line = `$cmd 2>&1`;$ret = $?;
This will prevent perl from expanding variables.
Nevertheless mpapec's answer is right, why forking a new process?
Related
I am trying to execute below awk command inside a perl script, but it is failing.
#!/usr/bin/perl
print `awk -F, '{print $NF}' f1.txt > f2.txt`
This is the error:
syntax error at ./MO.pl line 3, near "print"
Execution of ./MO.pl aborted due to compilation errors.
Can anyone please help what I am doing wrong here?
This is a Perl error and has nothing to do with your awk script itself. The error is usually seen when the previous statement doesn't have a semicolon at the end.
Here's a very simple program (which should include use strict; and use warnings;, but I wanted to emulate what you have).
#! /usr/bin/env perl
#
print "Hello, World\n" # Line 4
print "Hello, Back\n"; # Line 5
And the error message is:
syntax error at test.pl line 5, near "print"
Execution of test.pl aborted due to compilation errors.
Note the error is near the print in Line #5, but the error is actually at the end of Line #4 where I'm missing a semicolon.
Running your exact program works on my system (although doesn't quite produce the results you want). I am assuming this isn't your exact program, but instead a simplification of your program. Is there a statement before that print?
Several other things:
You're redirecting your awk output, so there's nothing to print.
Use strict and warnings.
Better to use qx(....) than backticks (grave accent). It's more readable and allows you to do quoted executable in quoted executable.
Watch for Perlisms in your code. The $NF is interpreted by Perl, and without the use strict;, doesn't give you an error. Instead, the print in your Awk statement is a null print which prints the entire line.
Why do you use print if nothing is printing out? You're better off in this position to use system which allows you to put single quotes around your entire statement:
system q(awk -F, '{print $NF}' f1.txt > f2.txt);
This way, $NF doesn't have to be quoted.
Why are you doing Awk in a Perl program? Perl will do anything Awk will do and do it better:
Here's a version of your program using plain ol' Perl:
#! /usr/bin/env perl
use strict;
use warnings;
use autodie;
while ( my $line = <> ) {
my #array = split /\s*,\s*/, $line;
print $array[-1];
}
To run this program:
$ test.pl f1.txt > f2.txt
Yes, it's longer, but half of the program is taken up by pragmas that all Perl programs should use.
I'm sure people smarter than me can turn this into a Perl one-liner.
Since you're redirecting the awk output, there's nothing for perl to print. You might as well use system and the quoting operator q():
system q(awk -F, '{print $NF}' f1.txt > f2.txt)
Or, of course, do it in perl, which saves you from having to spawn a shell and then spawn awk:
open my $in, '<', 'f1.txt';
open my $out, '>', 'f1.txt';
while (<$in>) {
print $out (split " ")[-1], "\n";
}
close $in;
close $out;
If there are more lines in the script, you need a semi-colon at the end of the print statement.
When i am using "sed" in command line it is working but not when included in perl script.
An example is sed 's/\s+//g' aaa > bbb
but say when i am trying to call the same command through perl script
$gf = `sed 's/\s\+//g' aaa > bbb` ;
the output file remains same as the input file!!!! Please suggest.
In Perl, backticks have the same escape and interpolation rules as double quoted strings: A backslash forming an unknown escape code forgets the backslash, e.g. "\." eq ".".
Therefore, the Perl code
print `echo \"1\"`;
print `echo \\"1\\"`;
outputs
1
"1"
If you want to embed that sed command into Perl, you have to escape the backslashes so that they even reach the shell:
$gf = `sed 's/\\s\\+//g' aaa > bbb`;
Actually, you won't get any output into $gf as you redirect the output to a file. We could just do
use autodie;
system "sed 's/\\s\\+//g' aaa > bbb";
or with single quotes:
use autodie;
system q{ sed 's/\s\+//g' aaa > bbb };
which keeps the backslashes.
Still, this is quite unneccessary as Perl could apply the substitution itself.
use autodie; # automatic error handling
open my $out, ">", "bbb";
open my $in, "<", "aaa";
while (<$in>) {
s/\s\+//g; # remove all spaces followed by a plus
print {$out} $_;
}
In these weird situations, I ensure that I'm running the right command. I'll construct it, store it, and output the command so I can see exactly what I created:
my $command = '....';
print "Command is [$command]\n";
my $output = `$command`;
If you're running sed from Perl, you might be doing it wrong since Perl can already do all that.
have you got
use strict;
use warnings;
at the top of your file?
you could need backticks to execute the command
$gf = `sed 's/\s\+//g' aaa > bbb`;
Can anyone show me, how to use shell command result in Perl script ?
#!/usr/bin/perl
$whoami=`whoami`;
system ('cd /var/home/'.$whoami.'/htdocs');
print $whoami;
Script output
[user1#srv _1]$ ./sys.pl
sh: line 1: /htdocs: No such file or directory
user1
I want to change dir to /var/home/user1/htdocs
$whoami contains the endline character \n, which causes your command string to look like:
cd /var/home/user1
/htdocs
You should use chomp to delete the trailing newline from $whoami:
my $whoami = `whoami`;
chomp $whoami;
This can be accomplished w/o shelling out:
#!perl
my $dir = '/home/'.getlogin().'/htdocs';
chdir $dir;
#!/usr/bin/perl
$whoiami=`whoami`;
print "$whoiami";
chomp $whoiami;
system ("cd /home/$whoiami/reports");
print $whoiami;
I find a lot of Perl one-liners online. Sometimes I want to convert these one-liners into a script, because otherwise I'll forget the syntax of the one-liner.
For example, I'm using the following command (from nagios.com):
tail -f /var/log/nagios/nagios.log | perl -pe 's/(\d+)/localtime($1)/e'
I'd to replace it with something like this:
tail -f /var/log/nagios/nagios.log | ~/bin/nagiostime.pl
However, I can't figure out the best way to quickly throw this stuff into a script. Does anyone have a quick way to throw these one-liners into a Bash or Perl script?
You can convert any Perl one-liner into a full script by passing it through the B::Deparse compiler backend that generates Perl source code:
perl -MO=Deparse -pe 's/(\d+)/localtime($1)/e'
outputs:
LINE: while (defined($_ = <ARGV>)) {
s/(\d+)/localtime($1);/e;
}
continue {
print $_;
}
The advantage of this approach over decoding the command line flags manually is that this is exactly the way Perl interprets your script, so there is no guesswork. B::Deparse is a core module, so there is nothing to install.
Take a look at perlrun:
-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";
}
If a file named by an argument cannot be opened for some reason, Perl warns you about it, and moves on to the next file. Note that the lines are printed automatically. An error occurring during printing is treated as fatal. To suppress printing use the -n switch. A -p overrides a -n switch.
BEGIN and END blocks may be used to capture control before or after the implicit loop, just as in awk.
So, simply take this chunk of code, insertyour code at the "# your program goes here" line, and viola, your script is ready!
Thus, it would be:
#!/usr/bin/perl -w
use strict; # or use 5.012 if you've got newer perls
while (<>) {
s/(\d+)/localtime($1)/e
} continue {
print or die "-p destination: $!\n";
}
That one's really easy to store in a script!
#! /usr/bin/perl -p
s/(\d+)/localtime($1)/e
The -e option introduces Perl code to be executed—which you might think of as a script on the command line—so drop it and stick the code in the body. Leave -p in the shebang (#!) line.
In general, it's safest to stick to at most one "clump" of options in the shebang line. If you need more, you could always throw their equivalents inside a BEGIN {} block.
Don't forget chmod +x ~/bin/nagiostime.pl
You could get a little fancier and embed the tail part too:
#! /usr/bin/perl -p
BEGIN {
die "Usage: $0 [ nagios-log ]\n" if #ARGV > 1;
my $log = #ARGV ? shift : "/var/log/nagios/nagios.log";
#ARGV = ("tail -f '$log' |");
}
s/(\d+)/localtime($1)/e
This works because the code written for you by -p uses Perl's "magic" (2-argument) open that processes pipes specially.
With no arguments, it transforms nagios.log, but you can also specify a different log file, e.g.,
$ ~/bin/nagiostime.pl /tmp/other-nagios.log
Robert has the "real" answer above, but it's not very practical. The -p switch does a bit of magic, and other options have even more magic (e.g. check out the logic behind the -i flag). In practice, I'd simply just make a bash alias/function to wrap around the oneliner, rather than convert it to a script.
Alternatively, here's your oneliner as a script: :)
#!/usr/bin/bash
# takes any number of arguments: the filenames to pipe to the perl filter
tail -f $# | perl -pe 's/(\d+)/localtime($1)/e'
There are some good answers here if you want to keep the one-liner-turned-script around and possibly even expand upon it, but the simplest thing that could possibly work is just:
#!/usr/bin/perl -p
s/(\d+)/localtime($1)/e
Perl will recognize parameters on the hashbang line of the script, so instead of writing out the loop in full, you can just continue to do the implicit loop with -p.
But writing the loop explicitly and using -w and "use strict;" are good if plan to use it as a starting point for writing a longer script.
#!/usr/bin/env perl
while(<>) {
s/(\d+)/localtime($1)/e;
print;
}
The while loop and the print is what -p does automatically for you.
Suppose I have a Perl script, namely mytest.pl. Can I run it by something like cat mytest.pl | perl -e?
The reason I want to do this is that I have a encrypted perl script and I can decrypt it in my c program and I want to run it in my c program. I don't want to write the decrypted script back to harddisk due to secruity concerns, so I need to run this perl script on-the-fly, all in memory.
This question has nothing to do with the cat command, I just want to know how to feed perl script to stdin, and let perl interpreter to run it.
perl < mytest.pl
should do the trick in any shell. It invokes perl and feeds the script in via the shell redirection operator <.
As pointed out, though, it seems a little unnecessary. Why not start the script with
#!/usr/bin/perl
or perhaps
#!/usr/bin/env perl
? (modified to reflect your Perl and/or env path)
Note the Useless Use of Cat Award. Whenever I use cat I stop and think whether the shell can provide this functionality for me instead.
Sometimes one needs to execute a perl script and pass it an argument. The STDIN construction perl input_file.txt < script.pl won't work. Using the tip from How to assign a heredoc value to a variable in Bash we overcome this by using a "here-script":
#!/bin/bash
read -r -d '' SCRIPT <<'EOS'
$total = 0;
while (<>) {
chomp;
#line = split "\t";
$total++;
}
print "Total: $total\n";
EOS
perl -e "$SCRIPT" input_file.txt
perl mytest.pl
should be the correct way. Why are you doing the unnecessary?
cat mytest.pl | perl
…is all you need. The -e switch expects the script as a command line argument.
perl will read the program from STDIN if you don't give it any arguments.
So you could theoretically read an encrypted file, decrypt it, and run it, without saving the file anywhere.
Here is a sample program:
#! /usr/bin/perl
use strict;
use warnings;
use 5.10.1;
use Crypt::CBC;
my $encrypted = do {
open my $encrypted_file, '<', 'perl_program.encrypted';
local $/ = undef;
<$encrypted_file>;
};
my $key = pack("H16", "0123456789ABCDEF");
my $cipher = Crypt::CBC->new(
'-key' => $key,
'-cipher' => 'Blowfish'
);
my $plaintext = $cipher->decrypt($encrypted);
use IPC::Run qw'run';
run [$^X], \$plaintext;
To test this program, I first ran this:
perl -MCrypt::CBC -e'
my $a = qq[print "Hello World\n"];
my $key = pack("H16", "0123456789ABCDEF");
my $cipher = Crypt::CBC->new(-key=>$key,-cipher=>"Blowfish");
my $encrypted = $cipher->encrypt($a);
print $encrypted;
' > perl_program.encrypted
This still won't stop dedicated hackers, but it will prevent most users from looking at the unencrypted program.