in reply to Open Filehandle once

Hello Dunkellbusch,

Welcome to the Monastery. The monks have already provided you with solutions to your problem, but I would like to add one more. You can use the module File::Slurp/append_file, so you can minimize your code to simply one call in every time you need it, no closing file handles etc.

But to be honest I would combine the module with the nice solution of fellow monk Corion, executable sample bellow below (change grep from /enable/ to /smb\.service/):

#!/usr/bin/perl
use strict;
use warnings;
use File::Slurp;

my $filePath = 'test_1.txt';
my @services = `systemctl list-unit-files`;

if( my @smb = grep { /enabled/ } @services ) {
    map { s/\s+\z// } @smb; # remover white space
    append_file( $filePath, "chkconfig smb: OK\n");
    append_file( $filePath, map { "$_\n" } @smb);
    # append_file( $filePath, "\n"); # if you want to append multiple 
+times
}

[download]

Update: Based on comments try to avoid File::Slurp, for simple implementations (read, write, etc) you can use File::Slurper, unfortunately there the author has not added the append proposal. I was looking online and I found the IO::All. Never used it before but I read that it is really really good.

Sample of updated solution, based on commends:

#!/usr/bin/perl
use strict;
use warnings;
use IO::All -utf8;  # Turn on utf8 for all io

my $path = 'file.txt';
my @services = `systemctl list-unit-files`;

if( my @smb = grep { /enabled/ } @services ) {
    s{ ^\s* | \s*$ }{}gx for @smb;
    io($path)->appendln(@smb);
}

[download]

Output:

$ cat test_1.txt
chkconfig smb: OK
acpid.path                                 enabled
cups.path                                  enabled
accounts-daemon.service                    enabled
anacron-resume.service                     enabled
anacron.service                            enabled
apparmor.service                           enabled
autovt@.service                            enabled
avahi-daemon.service                       enabled
bluetooth.service                          enabled
console-setup.service                      enabled
cron.service                               enabled
cups-browsed.service                       enabled
cups.service                               enabled
dbus-org.bluez.service                     enabled
dbus-org.freedesktop.Avahi.service         enabled
dbus-org.freedesktop.ModemManager1.service enabled
dbus-org.freedesktop.nm-dispatcher.service enabled
dbus-org.freedesktop.resolve1.service      enabled
dbus-org.freedesktop.thermald.service      enabled
denyhosts.service                          enabled
display-manager.service                    enabled
dns-clean.service                          enabled
friendly-recovery.service                  enabled
getty@.service                             enabled
gpu-manager.service                        enabled
keyboard-setup.service                     enabled
lightdm.service                            enabled
ModemManager.service                       enabled
network-manager.service                    enabled
networking.service                         enabled
NetworkManager-dispatcher.service          enabled
NetworkManager-wait-online.service         enabled
NetworkManager.service                     enabled
ondemand.service                           enabled
openvpn.service                            enabled
pppd-dns.service                           enabled
repowerd.service                           enabled
resolvconf.service                         enabled
rsync.service                              enabled
rsyslog.service                            enabled
setvtrgb.service                           enabled
snapd.autoimport.service                   enabled
snapd.core-fixup.service                   enabled
snapd.service                              enabled
snapd.system-shutdown.service              enabled
ssh.service                                enabled
sshd.service                               enabled
syslog.service                             enabled
systemd-resolved.service                   enabled
systemd-timesyncd.service                  enabled
thermald.service                           enabled
ufw.service                                enabled
unattended-upgrades.service                enabled
ureadahead.service                         enabled
vboxautostart-service.service              enabled
vboxballoonctrl-service.service            enabled
vboxdrv.service                            enabled
vboxweb-service.service                    enabled
acpid.socket                               enabled
apport-forward.socket                      enabled
avahi-daemon.socket                        enabled
cups.socket                                enabled
snapd.socket                               enabled
uuidd.socket                               enabled
remote-fs.target                           enabled
apt-daily.timer                            enabled
motd-news.timer                            enabled
snapd.refresh.timer                        enabled

[download]

Hope this helps, BR.

Seeking for Perl wisdom...on the process of learning...not there...yet!

Replies are listed 'Best First'.
Re^2: Open Filehandle once
by zentara (Cardinal) on Aug 24, 2017 at 17:51 UTC
[reply]

      Hello zentara,

      Thanks for the commend, I remember that I read about it somewhere but I forgot. See my updated answer, BR.

      Seeking for Perl wisdom...on the process of learning...not there...yet!
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Re^2: Open Filehandle once
by 1nickt (Canon) on Aug 24, 2017 at 22:00 UTC

    Hello thanos1983,

    Please do not recommend File::Slurp. It's broken and wrong.

    Also please do not recommend using map in void context. This is considered bad practice by many programmers since you are only making the call for its side-effects whilst throwing away its output.


    The way forward always starts with a minimal test.
[reply]
      "Also please do not recommend using map in void context."

      In general, I agree with this statement.

      "This is considered bad practice by many programmers since you are only making the call for its side-effects whilst throwing away its output."

      However, I disagree with this reason.

      The issue with "throwing away its output" was resolved, about 14 years ago[perlhist], in Perl 5.8.1. From "perl581delta: Miscellaneous Enhancements":

      "map in void context is no longer expensive. map is now context aware, and will not construct a list if called in void context."

      The reason I would give for not using map in void context is based on efficiency. I've had reason to benchmark map vs. for on a number of occasions over the years: I've always found for to be measurably faster than map; and, unsurprisingly, map EXPR is faster than map BLOCK.

      Here's a very quick and dirty benchmark I just wrote to show this: 

      #!/usr/bin/env perl

use strict;
use warnings;

use Benchmark 'cmpthese';

my @x = 0 .. 99;

cmpthese 1e6 => {
    map_expr => sub { my @y = @x; map ++$_, @y; return },
    map_block => sub { my @y = @x; map { ++$_ } @y; return },
    for => sub { my @y = @x; ++$_ for @y; return },
};

      [download]

      I ran this five times. All results were much the same. This one was roughly in the middle: 

                    Rate map_block  map_expr       for
map_block  75019/s        --      -44%      -47%
map_expr  134953/s       80%        --       -5%
for       141844/s       89%        5%        --

      [download]

      Benchmark run using:

      $ perl -v | head -2 | tail -1
This is perl 5, version 26, subversion 0 (v5.26.0) built for darwin-th
+read-multi-2level

      [download]

      — Ken

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        "map in void context is no longer expensive. map is now context aware, and will not construct a list if called in void context."

        Hi Ken, I wasn't referring to "throwing away the output" in terms of wasted resources, but rather in terms of unclear code.

        map is a function that returns a list -- when it is used in void context it smells bad, because it forces the reader to double-check that the author didn't make a mistake, falling into the same general category as any other distracting syntax or idiom.

        It's also a bad habit in general to use functions for their side effects, as those may change.


        The way forward always starts with a minimal test.
[reply]
[d/l]

        Timings could change from one version of perl to the next, so this kind of micro-optimization is meh. I would tend to use for over map myself in this situation, but it doesn't seem to matter all that much at this point. Whatever you do, don't write this:

        @smb = map { s/\s+\z//; $_ } @smb;

        Yes, I've seen people who habitually write this kind of thing. I don't know where they learned it.

[reply]

      Hello 1nickt,

      You are right I completely forgot about it. Thanks for the observation of map also. See my updated answer, BR.

      Seeking for Perl wisdom...on the process of learning...not there...yet!
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Re^2: Open Filehandle once (updated)
by soonix (Chancellor) on Aug 25, 2017 at 08:49 UTC
    regarding File::Slurp alternatives, I like Path::Tiny, which has an append method, too.

    syntax would be e.g.

    use Path::Tiny (); # otherwise 'path' will be imported
...
Path::Tiny::path($filePath)->append(map { "$_\n" } 'chkconfig smb: OK'
+, @smb))

    [download]
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In Section Seekers of Perl Wisdom