I have a system command like this :
unix_command "#output_file path_to_file"
Now when I try exec or system commands in a perl script I get this error :
Getting a string when expecting an operator.
Can you please help me how to do it in Perl.
Appreciate your help.
Thanks a ton!
Rakesh
system is really two different functions.
You can use it to launch a program.
The following syntax are used to launch a program:
system($prog, #one_or_more_args)
system({ $prog }, $arg0, #args)
Using one of these syntax, all strings passed as arguments are passed untouched to the child program.
Example usage:
system('perl', '-e', 'my #a = "foo"; print "#a\n";');
You can use it to execute a shell command.
The following syntax are used to execute a shell command:
system($shell_cmd)
The above is short for
system('/bin/sh', '-c', $shell_cmd)
You must provide a valid shell command. It you are building the command, you will need to take care to properly escape anything that needs escaping.
Example usage:
use String::ShellQuote qw( shell_quote );
my $cmd = shell_quote('perl', '-e', 'my #a = "foo"; print "#a\n";');
system($cmd);
A bit more specifically to your case, the shell command
program #file1 file2
can be executed as follows:
system('program', '#'.$file1, $file2);
If you actually need to construct a shell command (e.g. because you want to redirect output), you can use the following:
use String::ShellQuote qw( shell_quote );
my $cmd = shell_quote('program', '#'.$file1, $file2) . ' >output.txt 2>&1';
system($cmd);
If you don't need interpolation, use single quotes.
system 'echo #a';
If you do, use backslash.
system "echo \#a";
Related
Here's my code
#!/usr/bin/perl
use strict;
use warnings;
use diagnostics;
my $file = $ARGV[0];
system('wc -l $file');
print "\nprinting alpha \n";
sleep 1;
exit;
After I run (in tcsh shell) perl script.pl /path/to/file I don't see printing alpha until I press Ctrl+C. Even when I add another statement $|=1 either before or after system call, the behaviour remains the same.
What is happening?
You are executing the shell command
wc -l $file
The shell has no variable $file defined, so that's the same as
wc -l
This causes the shell to execute wc with the lone arg -l. With no file name provided, wc in turn reads from STDIN until you kill it with SIGINT from Ctrl-C.
You were perhaps aiming for
system("wc -l $file"); # XXX
but that's wrong too. That doesn't pass the args -l and the value of $file to wc. Consider what would happen if a file name with a space in it was provided.
To build a shell literal that results in the correct file name, you could use
use String::ShellQuote qw( shell_quote );
system(shell_quote('wc', '-l', $file));
But a better option is to avoid the shell and execute wc directly, passing to it the values you want without having to build a shell command.
system('wc', '-l', $file);
Because the single quotes prevent interpolation of $file. Change to double quotes.
What is happening is that the string is being executed without substituting a value for $file. When the shell gets this it looks for a shell variable $file which does not exist, so it executes the command with no file. This causes wc to read from stdin, resulting in the behavior you see.
Perl Code
`. /home/chronicles/logon.sh `;
print "DATA : $ENV{ID}\n";
In logon.sh , we are exporting the variable "ID" (sourcing of shell script).
Manual run
$> . /home/chronicles/logon.sh
$> echo $LOG
While I am running in terminal manually (not from script). I am getting the output. (But not working from the script)
I followed this post :
How to export a shell variable within a Perl script?
But didnt solve the problem.
Note
I am not allowed to change "logon.sh" script.
The script inside the backticks is executed in a child process. While environment variables are inherited from parent processes, the parent can't access the environment of child processes.
However, you could return the contents of the child environment variable and put it into a Perl variable like
use strict; use warnings; use feature 'say';
my $var = `ID=42; echo \$ID`;
chomp $var;
say "DATA: $var";
output:
DATA: 42
Here an example shell session:
$ cat test_script
echo foo
export test_var=42
$ perl -E'my $cmd = q(test_var=0; . test_script >/dev/null; echo $test_var); my $var = qx($cmd); chomp $var; say "DATA: $var"'
DATA: 42
The normal output is redirected into /dev/null, so only the echo $test_var shows.
It won't work.
An environment variable can't be inherited from a child process.
The environment variable can be updated in your "manual run" is because it's in the same "bash" process.
Source command is just to run every command in login.sh under current shell.
More info you can refer to: can we source a shell script in perl script
You could do something like:
#/usr/bin/perl
use strict;
use warnings;
chomp(my #values = `. myscript.sh; env`);
foreach my $value (#values) {
my ($k, $v) = split /=/, $value;
$ENV{$k} = $v;
}
foreach my $key (keys %ENV) {
print "$key => $ENV{$key}\n";
}
Well, I've find a solution, that sound nice for me: This seem robust, as this use widely tested mechanism to bind shell environment to perl (running perl) and robust library to export them in a perl variable syntax for re-injecting in root perl session.
The line export COLOR tty was usefull to ask my bash to export newer variables... This seem work fine.
#!/usr/bin/perl -w
my $perldumpenv='perl -MData::Dumper -e '."'".
'\$Data::Dumper::Terse=1;print Dumper(\%ENV);'."'";
eval '%ENV=('.$1.')' if `bash -c "
. ./home/chronicles/logon.sh;
export COLOR tty ID;
$perldumpenv"`
=~ /^\s*\{(.*)\}\s*$/mxs;
# map { printf "%-30s::%s\n",$_,$ENV{$_} } keys %ENV;
printf "%s\n", $ENV{'ID'};
Anyway, if you don't have access to logon.sh, you have to trust it before running such a solution.
Old...
There is my first post... for history purpose, don't look further.
The only way is to parse result command, while asking command to dump environ:
my #lines=split("\n",`. /home/chronicles/logon.sh;set`);
map { $ENV{$1}=$2 if /^([^ =])=(.*)$/; } #lines;
This can now be done with the Env::Modify module
use Env::Modify 'source';
source("/home/chronicles/logon.sh");
... environment setup in logon.sh is now available to Perl ...
Your Perl process is the parent of the shell process, so it won't inherit environment variables from it. Inheritance works the other way, from parent to child.
But when you run the script with backticks, as shown, the standard output of the script is returned to the Perl script. So, either modify the shell script to end with the echo $LOG statement you show, or create a new shell script that runs the login.sh and then has echo $LOG. Your Perl script would then be:
my $value = `./myscript.sh`;
print $value;
I want to invoke a shell command from a Perl script. The arguments to the command are present in a Perl array.
What's the simplest way to do this?
Thanks for replies
You probably need to call system, and it is most efficient to pass the parameters as a list, which avoids using the shell to parse the command line. A call like
my $status = system 'command', #arguments;
should do what you need.
There are usually two forms accepted by most functions to launch programs:
Accepts a path and a list of arguments
Accepts a shell command.
The first is safer, and require fewer resources.
system($prog, #args); # #args > 0
system({ $prog } $prog, #args); # #args >= 0
But if you must use the shell or if you must provide a shell command, there are String::ShellQuote (if you're on a unix system) or Win32::ShellQuote (if on a Win32 system) to help you.
use String::ShellQuote qw( shell_quote );
my $shell_cmd = shell_quote($prog, #args);
system($shell_cmd);
Let's say I want to run an external program from my script with backticks and at the same time I want to capture both STDOUT and STDERR but in two different variables. How can I do that? For istance if I run this script...
my $cmd = `snmpwalk -v $version -c $community $hostname $oid`;
...if there is no error everything works just fine BUT if the command raise an error this error will be printed on the command line and I don't want that to happen. I want to capture the error as well. Nothing has to be printed on the screen. Any ideas?
You needn't go all the way to open3, which IIRC is only for when you need to read and write to an external command, and even then there are other methods.
For your problem I suggest using Capture::Tiny, which can capture (or even tee) the STDOUT and STDERR from anything run inside its block. For example, per your question:
#!/usr/bin/env perl
use strict;
use warnings;
use Capture::Tiny qw/capture/;
...
my ($stdout, $stderr) = capture {
system ( "snmpwalk -v $version -c $community $hostname $oid" );
};
For another example consider this functioning code:
#!/usr/bin/env perl
use strict;
use warnings;
use Capture::Tiny qw/capture/;
my ($stdout, $stderr) = capture {
system ( "echo 'hello'" );
system ( "date" );
warn "Arg1!";
};
print "STDOUT:\n$stdout";
print "STDERR:\n$stderr";
which just gave me:
STDOUT:
hello
Mon Dec 19 23:59:06 CST 2011
STDERR:
Arg1! at ./test.pl line 11.
The only way to do this with backticks is to redirect to a file inside the shell command:
my $cmd = `snmpwalk -v $version -c $community $hostname $oid 2>error.dat`;
If you want to capture the STDERR inside your script, you need IPC::Open3 instead of backticks
In the Perl FAQ you have different options depending how do you want to proceed:
http://perldoc.perl.org/perlfaq8.html#How-can-I-capture-STDERR-from-an-external-command%3f
IO::CaptureOutput
is a very convenient wrapper for what you want to do.
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.