Random Wisdom

Status report

by on Oct.23, 2009, under Life

Looks like it’s been more than a year since the last time I’ve published something here. I’ve just been really busy, now more than ever. I have been periodically saving some blurbs as drafts but I never quite seem to have the time to polish them into real posts.

I really should be working on my thesis and a number of other papers but it does get a bit tiresome. Whenever that happens, I’ll work a bit on the drafts as a distraction and hopefully manage to push a few of them out over the coming weeks…

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Moving from Blogger to WordPress

by on Oct.18, 2009, under How To ...

The initial move is easy enough using the Blogger importer plugin of WordPress. Setting up redirects from the old blog to the new one however takes a bit more work. I followed the instructions at:

http://underscorebleach.net/jotsheet/2006/05/move-blogger-to-wordpress

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Printing multi-page duplex documents

by on Oct.18, 2009, under How To ..., Linux, Software

The psnup tool can be used to place multiple pages on each sheet of a document. E.g., the following command places two pages from the input file into each sheet of the output:

$ psnup -l -2 input.ps output.ps

While psnup is excellent for quick “N-up” conversion jobs, it doesn’t provide much control over the layout. The pstops utility on the other hand allows for fine grained scale, rotation and placement settings for each page that goes into a sheet of the output. The command syntax is a bit more complicated on account of the page specification strings that must now be provided. The following example shows a typical command needed to prepare a document for duplex printing with two pages on each side of a sheet:

$ pstops -pa4 \
  '4:0L@0.8(21cm,-1cm)+1L@0.8(21cm,12.55cm),2R@0.8(0,29.85cm)+3R@0.8(0,16.25cm)' \
  input.ps output.ps

The command is best understood by referring to the relevant section from the manpage:

       Pstops rearranges pages from a  PostScript  document,  creating  a  new
       PostScript  file.   The  input  PostScript file should follow the Adobe
       Document Structuring Conventions.  Pstops can  be  used  to  perform  a
       large  number  of  arbitrary  re-arrangements  of  Documents, including
       arranging for printing 2-up, 4-up, booklets, reversing, selecting front
       or back sides of documents, scaling, etc.

       pagespecs follow the syntax:

              pagespecs   = [modulo:]specs

              specs       = spec[+specs][,specs]

              spec        = [-]pageno[L][R][U][@scale][(xoff,yoff)]

       modulo is the number of pages in each block. The value of modulo should
       be greater than 0; the default value is 1.  specs are the page specifi-
       cations  for  the  pages in each block. The value of the pageno in each
       spec should be between 0 (for the first page in the block) and modulo-1
       (for  the  last page in each block) inclusive.  The optional dimensions
       xoff and yoff shift the page by the specified amount.   xoff  and  yoff
       are  in  PostScript’s points, but may be followed by the units cm or in
       to convert to centimetres or inches, or the flag w or h to specify as a
       multiple  of  the width or height.  The optional parameters L, R, and U
       rotate the page left, right, or upside-down.  The optional scale param-
       eter  scales the page by the fraction specified.  If the optional minus
       sign is specified, the page is relative to the  end  of  the  document,
       instead of the start.

       If  page  specs  are  separated  by + the pages will be merged into one
       page; if they are separated by  they will be  on  separate  pages.   If
       there  is only one page specification, with pageno zero, the pageno may
       be omitted.

       The shift, rotation, and scaling are performed in that order regardless
       of which order they appear on the command line.
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Restricting access to SFTP / SCP

by on Oct.18, 2009, under How To ..., Linux, Software

rssh is a tool that allows SFTP/SCP for file transfers over SSH but denies shell access — useful for preventing users from running commands on the system. More details are available on the tool’s homepage.

I first came across it on this page.

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Realtime collaborative text editing

by on Nov.21, 2008, under Linux, Software

A while ago, I came across Etherpad. It a web based platform that allows multiple users to simultaneously edit a single text file. Since it doesn’t seem to support any kind of mark-up at the moment, it would seem that it’s not terribly useful for word processing tasks. Perhaps it’s good for real-time collaborative coding and the creation of agenda type lists …

The software equivalent of Etherpad is Gobby. It’s a multi-platform tool that claims to run on Microsoft Windows, Mac OS X, Linux and other Unix-like platforms — making it almost as flexible as a web-based service. There are a number of other advantages:

  • Flexibility and security that comes from having absolute control over the sessions.
  • Syntax highlighting!
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Bash process substitution

by on Oct.03, 2008, under Linux, Software

From the Advanced Bash-Scripting Guide:

“Piping the stdout of a command into the stdin of another is a powerful technique. But, what if you need to pipe the stdout of multiple commands? This is where process substitution comes in.

Process substitution feeds the output of a process (or processes) into the stdin of another process.”

The syntax is:

 >(cmd_list)
 <(cmd_list)

Example: comparing the head of two files using diff

$ diff -u <(head -n3 /var/log/dmesg) <(head -n3 /tmp/dmesg)
--- /proc/self/fd/63 2009-05-26 19:52:45.144544140 +0100
+++ /proc/self/fd/62 2009-05-26 19:52:45.149544007 +0100
@@ -1,3 +1,3 @@
-Initializing cgroup subsys cpuset
-Initializing cgroup subsys cpu
-Linux version 2.6.27.21-170.2.56.fc10.i686 (mockbuild@xenbuilder2.fedora.redhat.com)
 (gcc version 4.3.2 20081105 (Red Hat 4.3.2-7) (GCC) )
 #1 SMP Mon Mar 23 23:37:54 EDT 2009
+Linux version 2.6.22.9-61.fc6 (brewbuilder@hs20-bc2-4.build.redhat.com)
 (gcc version 4.1.2 20070626 (Red Hat 4.1.2-13))
 #1 SMP Thu Sep 27 18:48:03 EDT 2007
+BIOS-provided physical RAM map:
+ BIOS-e820: 0000000000000000 - 000000000009f000 (usable)

The diff header clearly shows that file descriptors are used as the underlying mechanism.

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Embedding fonts in a PDF document

by on Oct.03, 2008, under How To ..., LaTeX, Linux, Software

It is often a good idea (or a requirement) to embed the used font faces in a PDF document. This is easily accomplished using ps2pdf during the final stage of conversion of a document from PS to PDF:

$ ps2pdf -sPAPERSIZE=a4 -dPDFSETTINGS=/printer -dCompatibilityLevel=1.3 \
         -dMaxSubsetPct=100 -dSubsetFonts=true -dEmbedAllFonts=true \
         'input_file.ps' 'output_file.pdf'

An explanation of the command options can be found in the Ps2pdf.htm file in the Ghostscript documentations (or here).

[Source]

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Re-encoding MP3 files using LAME

by on Jun.06, 2008, under How To ..., Linux, Software

I have some MP3 files encoded at a constant bitrate of 320kbps that my phone seems to have trouble playing smoothly. So, I looked into LAME.

The files I had were named using the following scheme:

01 - Title of track 01.mp3
02 - Title of track 02.mp3
...

I used the BASH for-loop construct to process the files:

$ for A in *.mp3;\              # Process one mp3 at a time
  do B=${A%.mp3};\              # Extract track number and title
     C=${B#?? -};\              # Extract the title
     D=${B%% - *};\             # Extract the track number
     lame --vbr-new -V0 -q0\    # Variable-bitrate, high-quality
          --mp3input\           # Inputs are MP3 files
          --tt "$C"\            # ID3v2 tags: title
          --ta 'Artist Name'\   # ID3v2 tags: artist
          --tl 'Album Title'\   # ID3v2 tags: album
          --ty 2007\            # ID3v2 tags: year
          --tn "$D"\            # ID3v2 tags: track no.
          --tg 'GENRE'\         # ID3v2 tags: genre
          "$A" processed/"$A";\ # Keep filename and save in ./processed/
  done

Since no bit-rate bounds are explicitly provided, the re-encoded files can contain anything between 32kbps and 320kbps. The LAME man-page provides an extensive list of options and their meanings.

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Processing files using ‘find’

by on Mar.26, 2008, under How To ..., Linux, Software

In its most basic form, find is often used to locate files that are subsequently piped through a complex set of commands for processing. However, this particular method is easily broken by files that contain spaces in their names.

This is where the ‘exec’ option provided by find comes in handy. From the man-page:

-exec command ;
       Execute  command;  true  if 0 status is returned.  All following
       arguments to find are taken to be arguments to the command until
       an  argument  consisting of ‘;’ is encountered.  The string ‘{}’
       is replaced by the current file name being processed  everywhere
       it occurs in the arguments to the command, not just in arguments
       where it is alone, as in some versions of find.  Both  of  these
       constructions might need to be escaped (with a ‘\’) or quoted to
       protect them from expansion by the shell.  See the EXAMPLES sec-
       tion  for examples of the use of the ‘-exec’ option.  The speci-
       fied command is run once for each matched file.  The command  is
       executed  in  the  starting  directory.    There are unavoidable
       security problems surrounding  use  of  the  -exec  option;  you
       should use the -execdir option instead.

An example that recursively touches all *.log files from the current directory would be:

$ find . -name \*.log -exec touch {} \;
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Saving power with Linux

by on Dec.03, 2007, under Linux

An interesting site with numerous tips and tricks on power efficient computing using Linux:

LessWatts

It is also home to the rather useful “PowerTOP” tool. If the testimonials are anything to go by, everyone running a recent release of Linux should give this a try.

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