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Linux

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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Extracting Audio/Video

by on Dec.01, 2007, under How To ..., Linux, Software

It’s really easy to extract either audio or video from a multimedia file using ‘ffmpeg‘. To extract audio only:


$ ffmpeg -i inputfile -vn -acodec copy outputfile

And for video only, replace ‘-vn‘ with ‘-an‘ and ‘-acodec‘ with ‘-vcodec‘.

ffmpeg is also commonly used as a transcoding tool.

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SSH Blacklisting

by on Nov.29, 2007, under How To ..., Linux, Software

After getting around 1500 failed ssh login attempts a day for a while on a server I manage, I decided to look into tools that automatically blacklist offending IPs.

Sshblack fits the bill perfectly. A HOWTO (including an init-script) for REDHAT-like systems is available from the OSS Watch Wiki.

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User installation of additional TeX/LaTeX classes and styles

by on Aug.09, 2007, under How To ..., LaTeX, Linux

When you are the sysadmin, you can simply drop the new class/style files under the system TeX path (e.g. /usr/share/texmf/tex/) and run ‘texhash’ to have them automatically picked up. But what do you do when you are just a regular user?

TeX/LaTeX looks at the TEXINPUTS environment variable to look for additional locations to search for included/referenced files. Therefore, new classes/styles can be easily added as follows:

  1. Create a directory for the files:
    $ mkdir -p $HOME/tex/latex
  2. Place the new class files into that folder (each class can be in its own directory and contain subdirectories):
    $ cp -a fancy-class $HOME/tex/latex/
  3. Export the TEXINPUTS variable and also add it to your $HOME/.bash_profile:
    $ export TEXINPUTS=.:$HOME/tex/latex//:$TEXINPUTS

The ‘.’ ensures that the working directory is included in the search path. The double-‘//’ tells bash to also include files in subdirectories of ‘$HOME/tex’ recursively.

New BibTeX files can also be added locally in a similar fashion. The variables to set are then BSTINPUTS and BIBINPUTS.

The environment variable to set for MakeIndex styles is: INDEXSTYLE.

Source: AstroNat – Installation at The Smithsonian/NASA Astrophysics Data System

UPDATE [16 July 2009] The Kpathsea manual provides a wealth of information about usable environment variables.

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Joining PDF Documents

by on Aug.03, 2007, under How To ..., Linux, Software

A quick search on the web reveals that the simplest (and most available) command to do so is:


$ gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=finished.pdf file1.pdf file2.pdf

Source: Putting together PDF files by Scott Nesbitt on NewsForge

[Update: Feb 1, 2011] jpdftweak is probably a better option with many useful features.

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