I want the output of the shell command captured in variable of a perl script, only the first section of the command before the pipe "|" is getting executed, and there is no error while executing the script
File.txt
Error input.txt got an error while parsing
Info output.txt has no error while parsing
my $var = `grep Error ./File.txt | awk '{print $2}'`;
print "Errored file $var";
Errored file Error input.txt got an error while parsing
I want just the input.txt which gets filtered by awk command but not happening. Please help
The $ in $2 is interpolated by Perl, so the command that the shell receives looks like:
grep Error ./File.txt | awk '{print }'
(or something else if you have recently matched a regular expression with capture groups). The workaround is to escape the dollar sign:
my $var = `grep Error ./File.txt | awk '{print \$2}'`
Always include use strict; and use warnings; in EVERY perl script.
If you had, you'd have gotten the following warning:
Use of uninitialized value $2 in concatenation (.) or string at scratch.pl line 4.
This would've alerted you to the problem in your command, namely that the $2 variable is being interpolated instead of being treated like a literal.
There are three ways to avoid this.
1) You can do what mob suggested and just escape the $2
my $var = `grep Error ./File.txt | awk '{print \$2}'`
2) You can use the qx form of backticks with single quotes so that it doesn't interpolate, although that is less ideal because you are using single quotes inside your command:
my $var = qx'grep Error ./File.txt | awk \'{print $2}\''
3) You can just use a pure perl solution.
use strict;
use warnings;
my ($var) = do {
local #ARGV = 'File.txt';
map {(split ' ')[1]} grep /Error/, <>;
};
Related
I am trying to use following command in perl but it giving me error
system("zcat myfile.gz | wc > abc.txt");
But when i run this I am getting error
syntax error near unexpected token `|'
Even if I remove >abc.txt I am still getting error.
Can we use pipe with system command?
Here are error details:
sh: -c: line 1: syntax error near unexpected token `|'
sh: -c: line 1: ` | wc '
Next time, test your demo program to make sure it actually exhibits the behaviour you said it does. You actually ran something closer to
while (my $file_name = <>) {
system("zcat $file_name | wc > abc.txt");
}
There are two errors in that:
You didn't remove the trailing newline, so the shell was trying to execute
zcat def.gz
| wc >abc.txt
instead of
zcat def.gz | wc >abc.txt
You didn't transform the file name into a shell literal before emdedding it your command.
Consider what would happen if the file name contained a space. You would be executing
zcat def ghi.gz | wc >abc.txt
instead of
zcat 'def ghi.gz' | wc >abc.txt
Solution:
use String::ShellQuote qw( shell_quote );
while (my $file_name = <>) {
chomp($file_name);
system("zcat -- ".shell_quote($file_name)." | wc > abc.txt");
}
It is working as expected:
perl -lne 'system("cat *.java|wc");'
Something odd with your filename, maybe.
You could check the interpolation of your shell like this:
my #file = `ls -1 myfile*.gz`;chomp(#files);
print join("\n",#files);
There are other possibilites to execute in perl, like backtick, open with |, qx.
If you are trouble with filenames, you could get the filenames by yourself and call the system in a specific way to avoid executing shell: http://docstore.mik.ua/orelly/perl/cookbook/ch19_07.htm
If there is only one scalar argument, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is /bin/sh -c on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly to execvp , which is more efficient.
I got this error message when trying to use backticks to execute a pipeline that included a cut -d \| command. Turns out I had to double escape the pipe character eg cut -d \\|
I want to redirect this awk output to the file handle but no luck.
Code:
open INPUT,"awk -F: '{print $1}'/etc/passwd| xargs -n 1 passwd -s | grep user";
while (my $input=<INPUT>)
{
...rest of the code
}
Error:
Use of uninitialized value in concatenation (.) or string at ./test line 12.
readline() on closed filehandle INPUT at ./test line 13.
The error message shown is not directly related to the question in the subject.
In order to open a pipe and retrieve the result in Perl you have to add "|" at the very end of the open call.
The error message comes from the fact that Perl interprets the $1 you use in that double-quoted string. However, your intention was to pass that verbatim to awk. Therefore you have to escape the $ on the Perl side with \$.
There's a space missing in front of the /etc/passwd argument.
Summary: this should work better:
open INPUT,"awk -F: '{print \$1}' /etc/passwd| xargs -n 1 passwd -s | grep user|";
However, you should also check for errors etc.
It looks like $1 in the string you've passed is making Perl look for a variable $1 which you've not defined. Try escaping the $ in the string by putting a \ in front of it.
Because the string is not valid it doesn't do the open which then produces your second error.
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 $_.scan(/((?:\d{1,3}\.){3}\d{1,3})/)' file
I've been trying to grep an exact shell 'variable' using word boundaries,
grep "\<$variable\>" file.txt
but haven't managed to; I've tried everything else but haven't succeeded.
Actually I'm invoking grep from a Perl script:
$attrval=`/usr/bin/grep "\<$_[0]\>" $upgradetmpdir/fullConfiguration.txt`
$_[0] and $upgradetmpdir/fullConfiguration.txt contains some matching "text".
But $attrval is empty after the operation.
#OP, you should do that 'grepping' in Perl. don't call system commands unnecessarily unless there is no choice.
$mysearch="pattern";
while (<>){
chomp;
#s = split /\s+/;
foreach my $line (#s){
if ($line eq $mysearch){
print "found: $line\n";
}
}
}
I'm not seeing the problem here:
file.txt:
hello
hi
anotherline
Now,
mala#human ~ $ export GREPVAR="hi"
mala#human ~ $ echo $GREPVAR
hi
mala#human ~ $ grep "\<$GREPVAR\>" file.txt
hi
What exactly isn't working for you?
Not every grep supports the ex(1) / vi(1) word boundary syntax.
I think I would just do:
grep -w "$variable" ...
Using single quotes works for me in tcsh:
grep '<$variable>' file.txt
I am assuming your input file contains the literal string: <$variable>
If variable=foo are you trying to grep for "foo"? If so, it works for me. If you're trying to grep for the variable named "$variable", then change the quotes to single quotes.
On a recent linux it works as expected. Do could try egrep instead
Say you have
$ cat file.txt
This line has $variable
DO NOT PRINT ME! $variableNope
$variable also
Then with the following program
#! /usr/bin/perl -l
use warnings;
use strict;
system("grep", "-P", '\$variable\b', "file.txt") == 0
or warn "$0: grep exited " . ($? >> 8);
you'd get output of
This line has $variable
$variable also
It uses the -P switch to GNU grep that matches Perl regular expressions. The feature is still experimental, so proceed with care.
Also note the use of system LIST that bypasses shell quoting, allowing the program to specify arguments with Perl's quoting rules rather than the shell's.
You could use the -w (or --word-regexp) switch, as in
system("grep", "-w", '\$variable', "file.txt") == 0
or warn "$0: grep exited " . ($? >> 8);
to get the same result.
Using single quote it wont work. You should go for double quote
For example:
this wont work
--------------
for i in 1
do
grep '$i' file
done
this will work
--------------
for i in 1
do
grep "$i" file
done