In perl you can use both qq(<text>) and "<text>" to create double quoted text. Is there any difference between the two?
(Same question for q(<text>) and '<text>')
No.
"<text>" is short for qq"<text>".
'<text>' is short for q'<text>'.
Quote-Like Operators
Deparse Perl programs to see how Perl interpreted you wrote. When you use the generalized quotes, you get back the single and double quotes:
$ perl -MO=Deparse -e '$s = q(abc)'
$s = 'abc';
-e syntax OK
$ perl -MO=Deparse -e '$s = qq/ab "c" d/'
$s = 'ab "c" d';
-e syntax OK
$ perl -MO=Deparse -e '$s = qq(abc$$)'
$s = "abc${$}";
-e syntax OK
Besides using these to allow the ' and " to appear in the string, I find them very handy for one liners so that the ' and " don't trigger shell problems:
% perl -le "print qq(PID is $$)"
In addition to them being identical, there is the point that you can use q() qq() with different delimiters (like you can with many Perl operators) to make quoting simpler. For example qq#The movie "Foo"#, q{Some other 'Foo'}. Compare
"Bob said, \"What's that?\"" 'Bob said, "What\'s that?"' qq(Bob said, "What's that?")
"Kim whispers, \"(...)\"" qq|Kim whispers, "(...)"|
s/http:\/\/www.example.com\//http:\/\/www.example.net\// # "leaning toothpick syndrome" (1)
s#http://www.example.com/#http://www.example.net/# # fixed
(1): leaning toothpick syndrome
Related
It's needed to build a string foobar is not foo and not bar.
In printf format %$2s, "2" means a particular argument position.
But it doesn't work in perl:
$ perl -e "printf('%$1s$2s is not %$1s and not %$2s', 'foo', 'bar');"
%2 is not %1 and not %2
My env:
$ perl --version
This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-linux-thread-multi
(with 29 registered patches, see perl -V for more detail)
Your quoting is off.
perl -E 'say sprintf(q{%1$s%2$s is not %1$s and not %2$s}, "foo", "bar");'
foobar is not foo and not bar
You cannot use double quotes "" for the -e because your shell gets confused. You need single quotes there. But if you use double quotes for the printf pattern with the %1$s syntax, Perl will try to interpolate the $s, which doesn't work. So use a non-quoting q{} or escape the single quotes '' with \'. Or escape the $s.
If you turn on use strict and use warnings you'll see:
$ perl -E 'use strict; use warnings; say sprintf("%1$s%2$s is not %1$s and not %2$s", "foo", "bar");'
Global symbol "$s" requires explicit package name at -e line 1.
Global symbol "$s" requires explicit package name at -e line 1.
Global symbol "$s" requires explicit package name at -e line 1.
Global symbol "$s" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.
That's with single quotes '' for -e and double quotes "" for the pattern.
$ perl -E "use strict; use warnings; say sprintf('%1$s%2$s is not %1$s and not %2$s', 'foo', 'bar');"
Invalid conversion in sprintf: "%1 " at -e line 1.
Invalid conversion in sprintf: "%2" at -e line 1.
%2 is not %1 and not %2
Now the shell tried to interpolate $s because of the double quotes "". So Perl never sees it. It sees the pattern as "%1 %2 is not %1 and not %2", which it cannot understand. (Note that the % will not get interpolated in double quoted strings in Perl).
This works for me on *nix:
perl -e "printf('%s%s is not %1\$s and not %2\$s', 'foo', 'bar');"
See the sprintf documentation, in particular the examples at the very end:
Here are some more examples; be aware that when using an explicit index, the $ may need escaping:
printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
printf "%*1\$.*f\n", 4, 5, 10; # will print "5.0000\n"
Let's have a look at the program you pass to perl:
$ printf '%s' "printf('%$1s$2s is not %$1s and not %$2s', 'foo', 'bar');"
printf('%ss is not %s and not %s', 'foo', 'bar');
As you can see, there is no $1 or $2 in your program because you improperly built your shell command. Just like Perl interpolates in double-quotes, so do sh and related shells. You should be using single quotes!
perl -e'printf("%\$1s\$2s is not %\$1s and not %\$2s\n", "foo", "bar");'
(I would have suggested switching from '' to q{} inside the Perl program so you wouldn't have to escape the dollar signs, but you need double-quotes for the \n you were missing anyway.)
I want to feed input to a C program with a perl script like this
./cprogram $(perl -e 'print "\xab\xcd\xef";').
However, the string must be read from a file. So I get something like this:
./cprogram $(perl -e 'open FILE, "<myfile.txt"; $file_contents = do { local $/; <FILE> }; print $file_contents'. However, now perl interprets the string as the string "\xab\xcd\xef", and I want it to interpret it as the byte sequence as in the first example.
How can this be achieved? It has to be ran on a server without File::Slurp.
In the first case, you pass the three bytes AB CD EF (produced by the string literal "\xAB\xCD\xEF") to print.
In the second case, you must be passing something other than those three bytes to print. I suspect you are passing the twelve character string \xAB\xCD\xEF to print.
So your question becomes: How does one convert the twelve-character string \xAB\xCD\xEF into the three bytes AB CD EF. Well, you'd require some kind of parser such as
s/\\x([0-9a-fA-F][0-9a-fA-F])|\\([^x])|([^\\]+)/
$1 ? chr(hex($1)) : $2 ? $2 : $3
/eg
And here it is at work:
$ perl -e'print "\\xAB\\xCD\\xEF";' >file
$ echo -n "$( perl -0777pe'
s{\\x([0-9a-fA-F][0-9a-fA-F])|\\([^x])|([^\\]+)}{
$1 ? chr(hex($1)) : $2 // $3
}eg;
' file )" | od -t x1
0000000 ab cd ef
0000003
Is Perl's eval too evil? If not, end in print eval("\"$file_contents\"");
Or can you prepare the file in advance using Perl? EG print FILE "\xAB\xCD\xED"; then read the resulting file with your existing code.
using a bash trick:
perl -e "$(echo "print \"$(cat input)"\")"
which for your example becomes:
./cprogram "$(perl -e "$(echo "print \"$(cat myfile.txt)"\")")"
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|;
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}; } ...'