understanding an rpm spec file

HI,
I have a following spec file . Please help me understand its different components :
Version: 0.4.3
Release: 1
License: Commercial
Group: System Environment/Daemons
Source0: %{name}-%{version}.tar.gz
BuildRoot: %{_tmppath}/%{name}-%{version}-%{release}-root
BuildArch: x86_64
BuildRequires: libevent-devel
Requires: libevent
Requires(pre): shadow-utils
Requires(post): chkconfig, initscripts
Requires(preun): chkconfig, initscripts


%description

%prep
%setup -q

%build
make

%install
rm -rf $RPM_BUILD_ROOT
make DESTDIR=$RPM_BUILD_ROOT install

%clean
rm -rf $RPM_BUILD_ROOT


%files
%defattr(-,root,root,-)
%doc README.md
/usr/local/bin/uecho
/etc/init.d/uecho
/etc/logrotate.d/uecho

%define groupname %{name}
%define username %{name}

%pre
getent group %{groupname} >/dev/null || groupadd -r %{groupname}
getent passwd %{username} >/dev/null || useradd -r -g %{groupname} -s /sbin/nologin %{username}
install -d /var/log/%{name} -o %{username} -g %{groupname} -m 750
exit 0

%post
/sbin/chkconfig --add uecho

%preun
if [ "$1" -eq "0" ] ; then
    /sbin/service uecho stop >/dev/null 2>&1
    /sbin/chkconfig --del uecho
fi


%changelog
- Create log path with appropriate permissions during installation.
- Add logrotate script.

- Initial build.

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Any pointers to some of the above thing would be great. Also please share any tutorial or link for the same.
Some particular things i want to understand is the code part :
%pre
getent group %{groupname} >/dev/null || groupadd -r %{groupname}
getent passwd %{username} >/dev/null || useradd -r -g %{groupname} -s /sbin/nologin %{username}
install -d /var/log/%{name} -o %{username} -g %{groupname} -m 750
exit 0

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I think this is what is performed prior to installation of the rpm.
what is this doing. i dont understand the getent part.
why a group is added and user is added ?
Thanks
Rohit BajajAsked:
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joolsSenior Systems AdministratorCommented:
most of the sections are self explanatory but the %pre and %post sections are executed prior to and after installation.

The getent is a linux command for this; http://man7.org/linux/man-pages/man1/getent.1.html but you can obviously have anything in the %pre and %post as these are basically scripts.

There is more info here https://fedoraproject.org/wiki/How_to_create_an_RPM_package
Rohit BajajAuthor Commented:
when does %files and %post gets executed.
I wrote a spec file and it looks like the file i have specified in the %files is copied after the %post gets executed.
joolsSenior Systems AdministratorCommented:
this gives more information about scriptlets https://fedoraproject.org/wiki/Packaging:ScriptletSnippets?rd=Packaging/ScriptletSnippets.

Are you able to share your spec file?
Rohit BajajAuthor Commented:
I guess some problem with my scrip file only.

I am using the following post script :
#!/bin/bash
PATH="$PATH:/bin"
ps -ef | grep '[j]etty' >/dev/null
if [ $? -ne 0 ]; then
  nohup service jetty start &
else
  service jetty stop &;
  nohup service jetty start &
fi

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Do you see any problem with it seems like the two statements  (else )are not getting executed...
The second statement needs to run only after the first statement has finished
probably thats what is causing the issue.
joolsSenior Systems AdministratorCommented:
best check this out, https://fedoraproject.org/wiki/Packaging:SysVInitScript?rd=Packaging/SysVInitScript

I only scanned the article but look for postun, there is also a note about chroot environments.

I have to admit I didnt know that myself and would have probably tried starting the script like you have above.

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