cd to directory containing a file

Many times I have run a program on a file and then immediately, wanted to change to the directory containing it, to do more operations. For example:

vim path/to/some/dir/file

then I next want to do:

cd path/to/some/dir

I used to do

cd !$

and then use the up arrow key to get the previous failed command to edit out the trailing filename and re-execute. Then I decided I should write a shell alias or function to do this. It has to be an alias or a function and not a script as cd needs to work in the current shell.

This is what I came up with:

cdf() { cd ${1%/[^/]*}; }

This uses the remove matching suffix operation of parameter substitution, see the bash manual page for details. So now I can do:

cdf !$

and I will end up in the directory containing the file I just worked on.

I picked cdf as an abbreviation for change to directory of file, but you may prefer some other name.

VirtualBox host/guest conflct over audio device

I started a VirtualBox instance on my workstation that contains a CentOS instance to develop software. A bit later on I found this error from a periodic cron job I run on the workstation to record a radio program using the sound card:

rec FAIL formats: can't open input `hw:0': snd_pcm_open error: Device or resource busy

That error message comes from the rec program, part of the sox package. The first time it happened I thought the audio hardware had locked up so I rebooted it and the error went away. The next morning it happened again. I realised that the reason it went away the first time was because the CentOS guest had been shutdown by the reboot.

I disabled the audio device in the guest OS and had no more failures recording from the sound card. In general if you do not need audio in the guest, disable the device so that the guest does not interfere with the host OS's use of it.

You can also disable by editing the XML config fie, but only when the guest OS is not running. This is the relevant line:

<AudioAdapter controller="AC97" driver="ALSA" enabled="false"/>

Filezilla, Domain OS (Apollo), and Cygwin

What do these three things have in common? Well, there's a story behind it.

At a site where I work there are a couple of ancient Apollo workstations running Domain OS, a Unix-like OS. This OS had an early networked filesystem where the super-root is called // and hosts have their filesystems underneath this. E.g. if you have two workstations named ws1 and ws2, their roots will be at //ws1 and //ws2.

Users needed to connect to the workstation filesystem using FTP. I proposed Filezilla as a replacement for an older, less friendly client. A side note,  you need to use active mode FTP when connecting to this old FTP server. We could login fine and get a directory listing, but when we tried to enter a directory by clicking on the folder icon, we would get an error like: /ws1 not found.  Looking at the command stream it was obvious what was going wrong. Filezilla was issuing CWD /ws1 when it should be CWD //ws1.

How to get Filezilla to either 1. use relative paths for CWD, or 2. understand that the root is //, not /? There was no option to use relative paths, it seems that Filezilla always converts paths to absolute ones for CWD. By trial and error I discovered that Cygwin has by coincidence the same pathname convention, // is the super-root. So by setting the Server Type to Cygwin in the Site Manager entry for this Apollo workstation, Filezilla connections worked.

So that's the connection. Hope this tip helps you if you happen to have to connect to a Domain OS FTP server with Filezilla.

SFTP is not FTP

Today I encountered another confused person who thought that to provide SFTP service, he had to install an FTP server.

I'm writing this post so that I can point people to it next time I encounter this misconception.

Yes, SFTP stands for Secure File Transfer Program and FTP stands for File Transfer Protocol but there the similarity ends. SFTP is run over a ssh connection, which normally uses the single service port 22. FTP is a different protocol using two ports, normally 20 and 21 for data and command (I will not go into the complexity of active and passive modes here). They are not related. The Wikipedia entry for FTP explains it succinctly. SFTP servers are different from FTP servers. Although there are clients that are capable of connecting to both types of servers, for example, Filezilla.

If you can ssh to a server, you can probably sftp also. I qualified that claim with "probably" because the sftp functionality has to be enabled and allowed to users.

SFTP is much much preferred over FTP due to encryption of the stream.

To complicate things there is a variant of FTP called FTPS which uses TLS to encrypt the stream.

What good is the --target option of cp?

If you look at the man page for cp(1) on operating systems where the GNU tools are used, such as Linux, you will see there is an third form that uses the -t option or alternatively the equivalent long form --target.

cp [OPTION]... -t DIRECTORY SOURCE...

So what good is this when you already can do:

cp [OPTION] SOURCE... DIRECTORY

Here's a reason. Suppose the SOURCE list is large and comes from a file or another command. So you have to use the xargs command to invoke cp as many times as necessary to consume the list, without running into command line argument limits. Assuming the source list is one per line, you could do something like this:

xargs -d '\n' cp -pr -t destdir < listofsources

The -t allows the destination directory to be put before the source arguments in the cp command. Without it,  you would have to resort to the interpolation feature of xargs, i.e. -I {}

TXT_DB error number 2 when generating openvpn client certificates

You may have followed the openvpn quick start instructions either from the online tutorial or using the README file in easy-rsa where it asks you to go through these steps:

[edit vars with your site-specific info]
source ./vars
./clean-all
./build-dh
./pkitool --initca
./pkitool --server myserver
./pkitool client1

 and then you get

failed to update database TXT_DB error number 2

at the last step. I did and a web search mostly turned up suggestions to run ./clean-all again. But this article was the key. It's about openssl, but openvpn's easy-rsa is just a front-end to openssl. The important observation is that every certificate must have a unique CN in the database. In the file vars, this is controlled by KEY_CN. You left the settings read in from vars unchanged between generating the server cert and the client cert. You could edit vars before generating the client certificate and re-source vars, or you could do this before generating each client key.

KEY_CN=someuniqueclientcn ./pkitool client1

and you will stop getting that TXT_DB error.


I'm a bit surprised that the documentation for openvpn hasn't been updated to make this clear.


NB: It is also affected by the setting unique_subject = yes in the file keys/index.txt.attr, but I prefer not to go against the default setting.

How to get a list of installed software on RPM based systems

You might want to do this to know what packages to restore, or to discover the difference between two installations.


In this article, the suggested command is rpm -qa. That is correct, but it has a problem. It will list the package names without the architecture. If you are on an x86_64 system, there may be both x86_64 and i386 packages. If you use the generated list to (re-)install the software you may end up getting both architectures. You would get extra packages or worse, there may be a conflict due to common pathnames in the two architectures.


Therefore we need to also output the architecture with the package name. For this we use the --queryformat option of rpm, or the shorter form --qf.


rpm -qa --qf '%{NAME}.%{ARCH}\n' > listofpackages