How I can make the following external command within ticks work with variables instead?
Or something similar?
sed -i.bak -e '10,16d;17d' $docname; (this works)
I.e., sed -i.bak -e '$line_number,$line_end_number;$last_line' $docname;
my $result =
qx/sed -i.bak -e "$line_number,${line_end_number}d;${last_line}d" $docname/;
Where the line split avoid the horizontal scroll-bar on SO; otherwise, it would be on one line.
Or, since it is not clear that there's any output to capture:
system "sed -i.back '$line_number,${line_end_number}d;${last_line}d' $docname";
Or you could split that up into arguments yourself:
system "sed", "-i.back", "$line_number,${line_end_number}d;${last_line}d", "$docname";
This tends to be safer since the shell doesn't get a chance to interfere with the interpretation of the arguments.
#args = ("command", "arg1", "arg2");
system(#args) == 0 or die "system #args failed: $?"
Furthermore on the manual:
perldoc -f system
I think you should read up on using qq for strings.
You probably want something like this:
use strict;
use warnings;
my $line_number = qq|10|;
my $line_end_number = qq|16d|;
my $last_line = qq|17d|;
my $doc_name = qq|somefile.bak|;
my $sed_command = qq|sed -i.bak -e '$line_number,$line_end_number;$last_line' $doc_name;|;
print $sed_command;
qx|$sed_command|;
Related
My syntax is
my $pstree = `pstree -p $pid|wc`;
but i am getting an error.
sh: -c: line 1: syntax error near unexpected token `|'
any thoughts?
Your variable $pid isn't just a number; it probably has a trailing newline character.
See it with:
use Data::Dumper;
print Data::Dumper->new([$pid])->Terse(1)->Useqq(1)->Dump;
It's valid perl, your shell is what is complaining. Did you put the #!/bin/perl at the top of the script? It's probably being interpreted by bash, not perl.
host:/var/tmp root# ./try.pl
5992 zsched
6875 /usr/local/sbin/sshd -f /usr/local/etc/sshd_config
3691 /usr/local/sbin/sshd -f /usr/local/etc/sshd_config -R
3711 -tcsh
6084 top 60
===
5 16 175
host:/var/tmp root# cat try.pl
#!/bin/perl
my $pstree = `ptree 3691`;
my $wc = `ptree 3691 | wc`;
print STDOUT $pstree;
print STDOUT "===\n";
print STDOUT $wc;
Instead of using the shell to do your counting, you can use Perl, which saves you a process and some complexity in your shell command:
my $count = () = qx(pstree -p $pid);
qx() does the same thing as backticks. The empty parentheses puts the qx() in list context, which makes it return a list, which then in scalar context is the size. It is a shortcut for:
my #list = qx(pstree -p $pid);
my $count = #list;
Ok I guess I need something that will do the following:
search for this line of code in /var/lib/asterisk/bin/retrieve_conf:
$engineinfo = engine_getinfo();
insert these two lines immediately following:
$engineinfo['engine']="asterisk";
$engineinfo['version']="1.6.2.11";
Thanks in advance,
Joe
You could do it like this
sed -ne '/$engineinfo = engine_getinfo();/a\'$'\n''$engineinfo['engine']="asterisk";\'$'\n''$engineinfo['version']="1.6.2.11";'$'\n'';p' /var/lib/asterisk/bin/retrieve_conf
Add -i for modification in place once you confirm that it works.
What does it do and how does it work?
First we tell sed to match a line containing your string. On that matched line we then will perform an a command, which is "append text".
The syntax of a sed a command is
a\
line of text\
another line
;
Note that the literal newlines are part of this syntax. To make it all one line (and preserve copy-paste ability) in place of literal newlines I used $'\n' which will tell bash or zsh to insert a real newline in place. The quoting necessary to make this work is a little complex: You have to exit single-quotes so that you can have the $'\n' be interpreted by bash, then you have to re-enter a single-quoted string to prevent bash from interpreting the rest of your input.
EDIT: Updated to append both lines in one append command.
You can use Perl and Tie::File (included in the Perl distribution):
use Tie::File;
tie my #array, 'Tie::File', "/var/lib/asterisk/bin/retrieve_conf" or die $!;
for (0..$#array) {
if ($array[$_] =~ /\$engineinfo = engine_getinfo\(\);/) {
splice #array, $_+1, 0, q{$engineinfo['engine']="asterisk"; $engineinfo['version']="1.6.2.11";};
last;
}
}
Just for the sake of symmetry here's an answer using awk.
awk '{ if(/\$engineinfo = engine_getinfo\(\);/) print $0"\n$engineinfo['\''engine'\'']=\"asterisk\";\n$engineinfo['\''version'\'']=\"1.6.2.11\"" ; else print $0 }' in.txt
You may also use ed:
# cf. http://wiki.bash-hackers.org/howto/edit-ed
cat <<-'EOF' | ed -s /var/lib/asterisk/bin/retrieve_conf
H
/\$engineinfo = engine_getinfo();/a
$engineinfo['engine']="asterisk";
$engineinfo['version']="1.6.2.11";
.
wq
EOF
A Perl one-liner:
perl -pE 's|(\$engineinfo) = engine_getinfo\(\);.*\K|\n${1}['\''engine'\'']="asterisk";\n${1}['\''version'\'']="1.6.2.11";|' file
sed -i 's/$engineinfo = engine_getinfo();/$engineinfo = engine_getinfo();<CTRL V><CNTRL M>$engineinfo['engine']="asterisk"; $engineinfo['version']="1.6.2.11";/' /var/lib/asterisk/bin/retrieve_conf
In awk I can write: awk -F: 'BEGIN {OFS = FS} ...'
In Perl, what's the equivalent of FS? I'd like to write
perl -F: -lane 'BEGIN {$, = [what?]} ...'
update with an example:
echo a:b:c:d | awk -F: 'BEGIN {OFS = FS} {$2 = 42; print}'
echo a:b:c:d | perl -F: -ane 'BEGIN {$, = ":"} $F[1] = 42; print #F'
Both output a:42:c:d
I would prefer not to hard-code the : in the Perl BEGIN block, but refer to wherever the -F option saves its argument.
To sum up, what I'm looking for does not exist:
there's no variable that holds the argument for -F, and more importantly
Perl's "FS" is fundamentally a different data type (regular expression) than the "OFS" (string) -- it does not make sense to join a list of strings using a regex.
Note that the same holds true in awk: FS is a string but acts as regex:
echo a:b,c:d | awk -F'[:,]' 'BEGIN {OFS=FS} {$2=42; print}'
outputs "a[:,]42[:,]c[:,]d"
Thanks for the insight and workarounds though.
You can use perl's -s (similar to awk's -v) to pass a "FS" variable, but the split becomes manual:
echo a:b:c:d | perl -sne '
BEGIN {$, = $FS}
#F = split $FS;
$F[1] = 42;
print #F;
' -- -FS=":"
If you know the exact length of input, you could do this:
echo a:b:c:d | perl -F'(:)' -ane '$, = $F[1]; #F = #F[0,2,4,6]; $F[1] = 42; print #F'
If the input is of variable lengths, you'll need something more sophisticated than #f[0,2,4,6].
EDIT: -F seems to simply provide input to an automatic split() call, which takes a complete RE as an expression. You may be able to find something more suitable by reading the perldoc entries for split, perlre, and perlvar.
You can sort of cheat it, because perl is actually using the split function with your -F argument, and you can tell split to preserve what it splits on by including capturing parens in the regex:
$ echo a:b:c:d | perl -F'(:)' -ane 'print join("/", #F);'
a/:/b/:/c/:/d
You can see what perl's doing with some of these "magic" command-line arguments by using -MO=Deparse, like this:
$ perl -MO=Deparse -F'(:)' -ane 'print join("/", #F);'
LINE: while (defined($_ = <ARGV>)) {
our(#F) = split(/(:)/, $_, 0);
print join('/', #F);
}
-e syntax OK
You'd have to change your #F subscripts to double what they'd normally be ($F[2] = 42).
Darnit...
The best I can do is:
echo a:b:c:d | perl -ne '$v=":";#F = split("$v"); $F[1] = 42; print join("$v", #F) . "\n";'
You don't need the -F: this way, and you're only stating the colon once. I was hoping there was someway of setting variables on the command line like you can with Awk's -v switch.
For one liners, Perl is usually not as clean as Awk, but I remember using Awk before I knew of Perl and writing 1000+ line Awk scripts.
Trying things like this made people think Awk was either named after the sound someone made when they tried to decipher such a script, or stood for AWKward.
There is no input record separator in Perl. You're basically emulating awk by using the -a and -F flags. If you really don't want to hard code the value, then why not just use an environmental variable?
$ export SPLIT=":"
$ perl -F$SPLIT -lane 'BEGIN { $, = $ENV{SPLIT}; } ...'
I noticed that when I use backticks in perl the commands are executed using sh, not bash, giving me some problems.
How can I change that behavior so perl will use bash?
PS. The command that I'm trying to run is:
paste filename <(cut -d \" \" -f 2 filename2 | grep -v mean) >> filename3
The "system shell" is not generally mutable. See perldoc -f exec:
If there is more than one argument in LIST, or if LIST is an array with more than one value, calls execvp(3) with the arguments in LIST. If
there is only one scalar argument or an array with one element in it, 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 you really need bash to perform a particular task, consider calling it explicitly:
my $result = `/usr/bin/bash command arguments`;
or even:
open my $bash_handle, '| /usr/bin/bash' or die "Cannot open bash: $!";
print $bash_handle 'command arguments';
You could also put your bash commands into a .sh file and invoke that directly:
my $result = `/usr/bin/bash script.pl`;
Try
`bash -c \"your command with args\"`
I am fairly sure the argument of -c is interpreted the way bash interprets its command line. The trick is to protect it from sh - that's what quotes are for.
This example works for me:
$ perl -e 'print `/bin/bash -c "echo <(pwd)"`'
/dev/fd/63
To deal with running bash and nested quotes, this article provides the best solution: How can I use bash syntax in Perl's system()?
my #args = ( "bash", "-c", "diff <(ls -l) <(ls -al)" );
system(#args);
I thought perl would honor the $SHELL variable, but then it occurred to me that its behavior might actually depend on your system's exec implementation. In mine, it seems that exec
will execute the shell
(/bin/sh) with the path of the
file as its first argument.
You can always do qw/bash your-command/, no?
Create a perl subroutine:
sub bash { return `cat << 'EOF' | /bin/bash\n$_[0]\nEOF\n`; }
And use it like below:
my $bash_cmd = 'paste filename <(cut -d " " -f 2 filename2 | grep -v mean) >> filename3';
print &bash($bash_cmd);
Or use perl here-doc for multi-line commands:
$bash_cmd = <<'EOF';
for (( i = 0; i < 10; i++ )); do
echo "${i}"
done
EOF
print &bash($bash_cmd);
I like to make some function btck (which integrates error checking) and bash_btck (which uses bash):
use Carp;
sub btck ($)
{
# Like backticks but the error check and chomp() are integrated
my $cmd = shift;
my $result = `$cmd`;
$? == 0 or confess "backtick command '$cmd' returned non-zero";
chomp($result);
return $result;
}
sub bash_btck ($)
{
# Like backticks but use bash and the error check and chomp() are
# integrated
my $cmd = shift;
my $sqpc = $cmd; # Single-Quote-Protected Command
$sqpc =~ s/'/'"'"'/g;
my $bc = "bash -c '$sqpc'";
return btck($bc);
}
One of the reasons I like to use bash is for safe pipe behavior:
sub safe_btck ($)
{
return bash_btck('set -o pipefail && '.shift);
}
So at the command line I can conveniently do something like this:
perl -pne 's/from/to/' in > out
And if I need to repeat this and/or I have several other perl -pne transformations, I can put them in, say, a .bat file in Windows. That's a rather roundabout way of doing it, of course. I should just write one perl script that has all those regex transformations.
So how do you write it? If I have a shell script containing these lines:
perl -pne 's/from1/to1/' in > temp
perl -pne 's/from2/to2/' -i temp
perl -pne 's/from3/to3/' -i temp
perl -pne 's/from4/to4/' -i temp
perl -pne 's/from5/to5/' temp > out
How can I just put these all into one perl script?
-e accepts arbitrary complex program. So just join your substitution operations.
perl -pe 's/from1/to1/; s/from2/to2/; s/from3/to3/; s/from4/to4/; s/from5/to5/' in > out
If you really want a Perl program that handles input and looping explicitely, deparse the one-liner to see the generated code and work from here.
> perl -MO=Deparse -pe 's/from1/to1/; s/from2/to2/; s/from3/to3/; s/from4/to4/; s/from5/to5/'
LINE: while (defined($_ = <ARGV>)) {
s/from1/to1/;
s/from2/to2/;
s/from3/to3/;
s/from4/to4/;
s/from5/to5/;
}
continue {
print $_;
}
-e syntax OK
Related answer to the question you didn't quite ask: the perl special variable $^I, used together with #ARGV, gives the in-place editing behavior of -i. As with the -p option, Deparse will show the generated code:
perl -MO=Deparse -pi.bak -le 's/foo/bar/'
BEGIN { $^I = ".bak"; }
BEGIN { $/ = "\n"; $\ = "\n"; }
LINE: while (defined($_ = <ARGV>)) {
chomp $_;
s/foo/bar/;
}
continue {
print $_;
}