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Re: Replace line in file based on pattern

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On 1/2/22 8:46 PM, Greg Wooledge wrote:
> I'm rearranging the order of the quoted sections.
> 
> On Sun, Jan 02, 2022 at 08:14:47PM -0500, The Wanderer wrote:
>> That's very interesting, although not all that accessible to the
>> relative newcomer to the field. It does leave me sad about the apparent
>> conclusion that there is no safe way to edit a file programmatically
>> (i.e., not via an interactive editor, and possibly not at all) in a way
>> which preserves the inode; the reasons why I thought of that as
>> desirable seem less clearly so on further analysis, but it's still
>> unfortunate for a thing to not be possible to do.
> 
> There are several things going on here.  I'll address two of them.
> 
> First: in order to "preserve the inode", you would need to open the
> file in either read+write mode, or clobber+write mode, and then start
> overwriting the file once you have the content in memory.  This is
> inherently dangerous, because if the writing is interrupted before
> completion, the file is now corrupted (in the read+write case), or
> truncated (in the clobber+write case).
> 
> The writing can be interrupted if the process is terminated, or if the
> system crashes.  Even with uninterruptible power supplies, there's no
> 100% sure way to prevent a system crash, so there is always a risk.
> 
> Second: the idea that a program should be altering (overwriting) the
> contents of configuration files *at all* is sketchy.  It's a really
> bad design.  The types of files that people usually want to alter with
> sed are designed to be edited by humans.  They are *not* amenable to
> programmatic alteration, which is why the answers that people come up
> with are so contorted.

Altering a config file programmatically is exactly what ansible does, 
and the functionality desired in the original post is one of the things 
ansible has a module for.

I have no idea how ansible handles interruptions.

> 
> If you really need to alter a textual configuration file on a bunch of
> servers, I suggest editing the file by hand on one machine, making a
> diff out of that, and then using patch to apply that diff on all the
> other machines.  Or use ansible, chef, puppet, etc.  This is not a new
> or unique problem.  Stop reinventing wheels.

I'm not interested in hacking a bunch of servers. Just one. The whole 
purpose here is to restore a system to more or less its original 
configuration when a reinstall must occur. A reinstall must occur when 
there is a severe hardware failure, or when Debian (or any other 
operating system) comes up with a major revision (in Debian's case, 
every couple of years). The problem I've seen with just copying an old 
config file over the top of the one installed by default is that in some 
cases, syntax changes or options are added or deleted in the config 
files from one version to another. Thus, my idea of "surgical" 
alteration of the config files.

Ansible supposedly will do this, but in certain circumstances, I've been 
unable to make it work on a remote machine. I can't say why. However, it 
occurred to me that ssh, rsync and bash/basic tools can accomplish the 
same goals.

You mentioned diff and patch earlier, and these might well be good 
tools. The problem is that I'm relatively unfamiliar with patch and 
diff. I know how they work, but I'm inexpert in their use.

> 
> A much better approach is what people have started doing with ".d"
> directories.  Instead of a single monolithic text configuration file
> that requires a human brain to comprehend, you put individual pieces
> of configuration in separate files.  If you want to change the phlogiston
> level, you just put the new value in the foobar.d/phlogiston file.  No
> need for a parser to find the phlogiston variable in a text file.

I absolutely abhor the *.d solution. Most software that I'm aware of is 
built to read a single config file. For example, exim. Personally, I 
find it vastly easier to load a single config file in an editor and 
search for a value, than to grep through a bunch of files and load the 
one which has the value I seek. My personal preference.

> 

> Of course, the program being configured has to be written to support
> such a thing.
> 
>>> unicorn:~$ printf %s\\n foo foo foo | awk '!done && /foo/{$0="bar"; done=1} 1'
>>> bar
>>> foo
>>> foo
>>
>> I keep forgetting that printf is even a thing in shell, and while awk
>> did occur to my mind, I don't know it well enough to do anything useful
>> in it. I can only kind-of parse the syntax you gave there, and that only
>> because I just saw very similar-looking syntax given for sed in the
>> Stack Overflow answer linked above.
> 
> An awk program consists of a series of alternating conditions and actions.
> Each line of input is matched against the conditions, in order.  If any
> of the conditions matches the input line, then the corresponding action
> block is executed.

Awk is another tool I'm only vaguely familiar with. I'm much more 
familiar with sed and grep. If I had your expertise, I might look at it 
differently.

Paul

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Thread

Replace line in file based on pattern "Paul M. Foster" <paulf@quillandmouse.com> - 2022-01-03 00:10 +0100
  Re: Replace line in file based on pattern Andy Smith <andy@strugglers.net> - 2022-01-03 00:20 +0100
    Re: Replace line in file based on pattern "Paul M. Foster" <paulf@quillandmouse.com> - 2022-01-03 03:00 +0100
  Re: Replace line in file based on pattern The Wanderer <wanderer@fastmail.fm> - 2022-01-03 00:30 +0100
    Re: Replace line in file based on pattern Greg Wooledge <greg@wooledge.org> - 2022-01-03 00:50 +0100
      Re: Replace line in file based on pattern The Wanderer <wanderer@fastmail.fm> - 2022-01-03 02:20 +0100
        Re: Replace line in file based on pattern Greg Wooledge <greg@wooledge.org> - 2022-01-03 02:50 +0100
          Re: Replace line in file based on pattern "Paul M. Foster" <paulf@quillandmouse.com> - 2022-01-03 03:30 +0100
            Re: Replace line in file based on pattern Andrei POPESCU <andreimpopescu@gmail.com> - 2022-01-15 13:40 +0100
      Re: Replace line in file based on pattern "Paul M. Foster" <paulf@quillandmouse.com> - 2022-01-03 04:00 +0100
        Re: Replace line in file based on pattern Greg Wooledge <greg@wooledge.org> - 2022-01-03 05:10 +0100
          Re: Replace line in file based on pattern "Paul M. Foster" <paulf@quillandmouse.com> - 2022-01-03 14:40 +0100
            Re: Replace line in file based on pattern David Wright <deblis@lionunicorn.co.uk> - 2022-01-04 20:50 +0100
    Re: Replace line in file based on pattern "Paul M. Foster" <paulf@quillandmouse.com> - 2022-01-03 03:00 +0100
      Re: Replace line in file based on pattern john doe <johndoe65534@mail.com> - 2022-01-03 07:50 +0100

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