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;
Related
I'm trying to run a tail command from within a perl script using the usual backticks.
The section in my perl script is as follows:
$nexusTime += nexusUploadTime(`tail $log -n 5`);
So I'm trying to get the last 5 lines of this file but I'm getting the following error when the perl script finishes:
sh: line 1: -n: command not found
Even though when I run the command on the command line it is indeed successful and I can see the 5 lines from that particular.
Not sure what is going on here. Why it works from command line but through perl it won't recognize the -n option.
Anybody have any suggestions?
$log has an extraneous trailing newline, so you are executing
tail file.log
-n 5 # Tries to execute a program named "-n"
Fix:
chomp($log);
Note that you will run into problems if log $log contains shell meta characters (such as spaces). Fix:
use String::ShellQuote qw( shell_quote );
my $tail_cmd = shell_quote('tail', '-n', '5', '--', $log);
$nexusTime += nexusUploadTime(`$tail_cmd`);
ikegami pointed out your error, but I would recommend avoiding external commands whenever possible. They aren't portable and debugging them can be a pain, among other things. You can simulate tail with pure Perl code like this:
use strict;
use warnings;
use File::ReadBackwards;
sub tail {
my ($file, $num_lines) = #_;
my $bw = File::ReadBackwards->new($file) or die "Can't read '$file': $!";
my ($lines, $count);
while (defined(my $line = $bw->readline) && $num_lines > $count++) {
$lines .= $line;
}
$bw->close;
return $lines;
}
print tail('/usr/share/dict/words', 5);
Output
ZZZ
zZt
Zz
ZZ
zyzzyvas
Note that if you pass a file name containing a newline, this will fail with
Can't read 'foo
': No such file or directory at tail.pl line 10.
instead of the more cryptic
sh: line 1: -n: command not found
that you got from running the tail utility in backticks.
The answer to this question is to place the option -n 5 before the target file
my text file contents
vi /root/text.conf
node.session.auth.authmethod = hello
so my one liner perl command replaces the above file contents by commenting with #. Works fine when run as single command
perl -pi -e 's/^(node.session.auth.authmethod\s*=\s*).*$/#\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?
I want to search for a string and get the full line from a text file through Perl scripting.
So the text file will be like the following.
data-key-1,col-1.1,col-1.2
data-key-2,col-2.1,col-2.2
data-key-3,col-3.1,col-3.2
Here I want to apply data-key-1 as the search string and get the full line into a Perl variable.
Here I want the exact replacement of grep "data-key-1" data.csv in the shell.
Some syntax like the following worked while running in the console.
perl -wln -e 'print if /\bAPPLE\b/' your_file
But how can I place it in a script? With the perl keyword we can't put it into a script. Is there a way to avoid the loops?
If you'd know the command line options you are giving for your one-liner, you'd know exactly what to write inside your perl script. When you read a file, you need a loop. Choice of loop can yield different results performance wise. Using for loop to read a while is more expensive than using a while loop to read a file.
Your one-liner:
perl -wln -e 'print if /\bAPPLE\b/' your_file
is basically saying:
-w : Use warnings
-l : Chomp the newline character from each line before processing and place it back during printing.
-n : Create an implicit while(<>) { ... } loop to perform an action on each line
-e : Tell perl interpreter to execute the code that follows it.
print if /\bAPPLE\b/ to print entire line if line contains the word APPLE.
So to use the above inside a perl script, you'd do:
#!usr/bin/perl
use strict;
use warnings;
open my $fh, '<', 'your_file' or die "Cannot open file: $!\n";
while(<$fh>) {
my $line = $_ if /\bAPPLE\b/;
# do something with $line
}
chomp is not really required here because you are not doing anything with the line other then checking for an existence of a word.
open($file, "<filename");
while(<$file>) {
print $_ if ($_ =~ /^data-key-3,/);
}
use strict;
use warnings;
# the file name of your .csv file
my $file = 'data.csv';
# open the file for reading
open(FILE, "<$file") or
die("Could not open log file. $!\n");
#process line by line:
while(<FILE>) {
my($line) = $_;
# remove any trail space (the newline)
# not necessary, but again, good habit
chomp($line);
my #result = grep (/data-key-1/, $line);
push (#final, #result);
}
print #final;
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`;
I found this perl script here which seems will work for my purposes. It opens a Unicode text file and reads each line so that a command can be run. But I cannot figure out how to run a certain ICU command on each line. Can someone help me out? The error I get is (largefile is the script name):
syntax error at ./largefile line 11, near "/ ."
Search pattern not terminated at ./largefile line 11.
#!/usr/bin/perl
use strict;
use warnings;
my $file = 'test.txt';
open my $info, $file or die "Could not open $file: $!";
while( my $line = <$info>) {
do
LD_LIBRARY_PATH=icu/source/lib/ ./a.out "$line" >> newtext.txt
done
}
close $info;
Basically I want to open a large text file and run the command (which normally runs from the command line...I think how I call this in the perl script is the problem, but I don't know how to fix it) "LD_LIBRARY_PATH=icu/source/lib/ ./a.out "$line" >> newtext.txt" on each line so that "newtext.txt" is then populated with all the lines after they have been processed by the script. The ICU part is breaking words for Khmer.
Any help would be much appreciated! I'm not much of a programmer... Thanks!
For executing terminal commands, the command needs to be in system(), hence change to
system("LD_LIBRARY_PATH=icu/source/lib/ ./a.out $line >> newtext.txt");
Have you tried backticks:
while (my $line = <$info>) {
`LD_LIBRARY_PATH=icu/source/lib/ ./a.out "$line" >> newtext.txt`
last if $. == 2;
}