## A really dumb DNS lookup for my internal network

The new Hitron cable modem in the house cowardly refuses to let me cache mac and ip address pairs, which is really annoying because my ip addresses now change on me over a couple days. The old router (also a Hitron) allowed that, so putting it on a UPS was generally enough to let me have a static IP table, provided I didn’t have to reboot it.

Here’s a hack using nmap that I just cobbled together to fill in the /etc/hosts entries on the couple machines that I want to talk to each other (mac and Linux machines, so all are unix like).

my %hostnameByMacAddr = (
'B8:4E:3F:C4:04:02' => 'router',
'E4:5C:89:C2:0F:4B' => 'macbookw # wireless',
'10:C2:C6:A0:20:58' => 'nuc2w',
'10:C2:C6:CA:93:6A' => 'nuc1w',
'A8:AE:ED:EB:39:86' => 'nuc1',
'A8:AE:ED:7D:CE:5A' => 'nuc2',
'28:C9:86:46:A8:15' => 'macbookt # thunderbolt monitor connected',
'10:24:2B:A1:7B:F7' => 'brother # printer',
'BC:87:A3:34:1A:FF' => 'macbooke # ethernet cable connected',
);

open my $h, "sudo nmap -n -p 22 192.168.0.1/24 2>&1 | grep -e '192' -e '^MAC' |" or die; my$ip;
while ( <$h> ) { if ( /scan report for.*(192\.\d+\.\d+\.\d+)/ ) {$ip = $1 ; } if ( /MAC Address: (.*) / ) { my$mac = $1; if ( defined$hostnameByMacAddr{$mac} ) { print "$ip $hostnameByMacAddr{$mac} # $mac\n" ; } else { print "#$ip $mac # unknown\n" ; } } } close$h or die;


If anybody knows how to set up an actual DNS server for internal networks, I’d be interested to see what is involved, since it looked very hard when I googled it.

## Some notes on copying and moving text in vim

April 13, 2015 perl and general scripting hackery No comments , ,

Emad just asked me a vim question (how to use a search expression instead of a line number), and I ended up learning a new vim commmand from him as a side effect.

I’d done stuff like the following before to move text to a new file

:,/Done/-1 !cat > /tmp/newfile.txt


This assumes you’d like to delete everything from the current position to the line just before the /Done/ search expression, and write it into /tmp/newfile.txt.

The mechanism here, is that the selection is filtered through a script, where the output of the script is empty, so the lines are deleted. This particular script has the side effect of creating a file with the selected range of lines. The end effect is that the text is moved.

If you’d like to keep it and copy it to the new file, you can tee instead of cat it:

:,/Done/-1 !tee /tmp/newfile.txt


This is faster than selecting a range, switching buffers copying into the buffer, saving, and switching buffers back.

Emad taught me that this can also be done with the w command, like so:

:,/Done/-1 w /tmp/newfile.txt


It doesn’t surprise me that there’s a faster way to copy text from one file to another than using tee, but since I knew one way, I never went looking for it.

## parallelized xargs

March 12, 2015 perl and general scripting hackery No comments

I’ve used xargs before to execute commands long enough that attempting them with backquotes fails. For example, if the list of files ‘c’ is too long, a checkin attempt like:

    cleartool checkin -c 'blah blah blah' cat c


could generate an “environment too long” error in the shell. Something like:

    cat c | xargs cleartool checkin -c 'blah blah blah'


is equivalent. This also checks in one file at a time, using all the filenames in the file ‘c’, but allows xargs to farm out the commands one at a time without building one giant execv argument. What I didn’t know was xargs has a parallelization option:

    cat c | xargs -P 8 -n 1 cleartool checkin -c 'blah blah blah'


This does the same job, with -n 1 restricting xargs to passing one file at a time (which may not be necessary in this case), but starting 8 different processes for the work!

## Why does touch include a utimensat() syscall?

I’m seeing odd time sequencing of files when using clearcase version 8 dynamic views, which makes me wonder about an aspect of the (gnu) touch command. Running:

> cat u
rm -f touchedEarlier touchedLater

perl -e "open (F, '> touchedEarlier') || die"
touch touchedLater

ls --full-time touchedEarlier touchedLater


produces:

> ./u
-rw-r--r-- 1 peeterj pdxdb2 0 2015-02-25 11:42:05.833044000 -0500 touchedEarlier
-rw-r--r-- 1 peeterj pdxdb2 0 2015-02-25 11:42:05.000000000 -0500 touchedLater


Notice that the file that is touched by doing a perl “open” ends up with a later time, despite the fact that it was done logically earlier than the touch.

Running this command outside of a clearcase dynamic view shows zeros only in the subsecond times (also the behaviour of clearcase V7). Needless to say, this difference in file times from their creation sequence wreaks havoc on make.

I was curious how the two touch methods differed, and stracing them shows that the touch differs by including a utimesat() syscall. The perl touch is:

open(“touchedEarlier”, O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fff29cd29f0) = -1 ENOTTY (Inappropriate ioctl for device)
lseek(3, 0, SEEK_CUR) = 0
fstat(3, {st_mode=S_IFREG|0644, st_size=0, …}) = 0
fcntl(3, F_SETFD, FD_CLOEXEC) = 0
close(3) = 0
exit_group(0) = ?

whereas the touch command has:

open(“touchedLater”, O_WRONLY|O_CREAT|O_NOCTTY|O_NONBLOCK, 0666) = 3
dup2(3, 0) = 0
close(3) = 0
dup2(0, 0) = 0
utimensat(0, NULL, NULL, 0) = 0
close(0) = 0
close(1) = 0
close(2) = 0
exit_group(0) = ?

It appears that the touch command explicitly zeros the subsecond portion of the files’ timestamp.

I also see that perl’s File::Touch module does the same thing, but uses a different mechanism. I see the following in a strace of such a Touch() call:

stat(“xxyyzz”, 0x656060)                = -1 ENOENT (No such file or directory)
open(“xxyyzz”, O_WRONLY|O_CREAT|O_NOCTTY|O_NONBLOCK, 0666) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fff16bb3c60) = -1 ENOTTY (Inappropriate ioctl for device)
lseek(3, 0, SEEK_CUR)                   = 0
fstat(3, {st_mode=S_IFREG|0644, st_size=0, …}) = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
close(3)                                = 0
utimes(“xxyyzz”, {{1424884553, 0}, {1424884553, 0}}) = 0

I am very curious why touch and perl’s File::Touch() both explicitly zero the subsecond modification time for the file (using utimensat() or utimes() syscalls)?

## Some Unix command line one liners

Here’s a couple one-liner shell commands collected over the last couple months when it occurred to me to record them.  Each of these I thought were somewhat notable at the time I did so.

## Nested “backquotes”

I often have to run commands where it is convenient to have the parameters of the commands in a file.  A simple example is to edit all the files in a list of files, say:

vim cat c
# or:
vim $(cat c)  A useful variation of this is to do the same using the output of a command that also takes its input from a file. Here’s one to edit all the “ancestor” files in the version control system, assuming a command vcsancestor that produces such filenames vim vcsancestor$(cat c)
# or
vim $(vcsancestor$(cat c))


Observe how two different methods of embedding shell commands can be combined into one command. In the past I often used for loops for something like this, say:

for i in cat c ; do vcsancestor $i ; done > f vim cat f  (because backquotes can’t be nested). It only recently occurred to me that this isn’t a limitation if$() style subshells are used.

## Batching commands with xargs

When working in a version control system, it’s often useful to do a batch checkout of all the files that have compilation errors.  Suppose that you made changes that produced the following compilation error output:

## Unix to Windows path separator switching

Suppose we have some unix filenames

</pre>
$head -5 f /vbs/bin/AEDefines.pm /vbs/bin/AEMacro.pm /vbs/bin/bld_shared_lib_Darwin /vbs/bin/chglibpaths /vbs/bin/chglibpaths_Darwin  and want the Windows paths for the same </pre>$ head -5 f | tr / '\\'
bin\AEDefines.pm
bin\AEMacro.pm
bin\bld_shared_lib_Darwin
bin\chglibpaths
bin\chglibpaths_Darwin
<pre>

The tr command above looks a bit like ascii barf, and will translate forward slashes to backward slashes (perhaps for input that’s a list of files).

I didn’t understand the requirement to both single quote the backslash as well as escaping it, but Darin explained it for me:

Quotes allow the backslashes to go through the shell to tr.  And tr has its own backslash escape mechanism (so you can do things like transform \n into \r or something – where you’d then specify ‘\n’ or just \\n and ‘\r’ or \\r).

## Vim: replace search results with contents from a file

Probably related to merging conflicting changes, I wanted to completely replace the implementation of a particular function:

void foo() {
...
}


This was an easy way one liner method to do that replacement, deleting the implementation of foo, and replacing it with the one that was found in the file ‘foo’

:,/^}/ !cat foo


## file:line: delimited output for a single file

The grep -n command is very handy for producing file:line:content delimited output.  In particular, you can iterate over such output with vim -q.  When you want to do this for a single file, grep -n doesn’t include the filename, defeating a subsequent vim -q (since vim then doesn’t know what file to open).  Here’s an example

$cat my_file_to_search blah patternOfInterest hi foo goo patternOfInterest bye blah patternOfInterest hi blah patternOfInterest hi foo goo patternOfInterest bye foo goo patternOfInterest bye$ grep -n patternOfInterest my_file_to_search | tee v
1:blah patternOfInterest hi
3:patternOfInterest bye
4:blah patternOfInterest hi
5:blah patternOfInterest hi
7:patternOfInterest bye
9:patternOfInterest bye



To get vim -q’able output, just include a second non-existent dummy file in the search

grep -n patternOfInterest my_file_to_search a_file_that_doesnt_exist | tee v
vim -q v


I usually use a very-short filename for the “does not exist file”, say, .u (which presumes I also don’t create little hidden files .u in my day-to-day work).

## vim hacking: move set of lines into new file without buffer switching

Yesterday I was faced with a task where I wanted to repeatedly edit the first N lines of a file, then when they were in the form I wanted insert them into another file to be consumed by another script.  A braindead way to do this would be something like:

:0, d
:w
:e t
:\$
:p
:0, d
:w
:e#

This is:

• delete the lines from the beginning of the file to the current line.
• save my file with these lines deleted
• switch to my destination file to be overwritten and go to the end
• paste in the new content
• delete the old content
• save this new destination file
• return to my original file

If you only need to do this once or twice, it’s not that many keystrokes, and can be done really quickly.  However, it occurred to me that this can all be done with a one liner instead, cool enough to share:

:0,!cat > t

Here’s what happens:

• Filter the lines from the beginning of the file to the current line through a script
• This script has no stdout, so those lines are deleted from the current file.
• That script uses cat to read from stdin and write the lines to a file, overwriting the destination file as desired

This is a pretty slick technique, but involves a few subtleties.  As it happens, my consumer of the moved lines was also a vim editing session, so I’m sure this was probably also possible with a split screen vim.  However, I like my screen real estate, and didn’t opt to go that route (and am not actually sure of the keystrokes required to do the same task in a split screen vim session).