Sort command in Linux

To do a descending numeric sort on the 4th column of a tab-separated file qwe.txt, do
sort -t$'\t' -rn -k4,4 qwe.txt > sorted.qwe.txt

Note the grouping: sort [separator] [options] [fields to sort on]
Here I have used the options: -r (descending order) and -n (numeric sort).

The trick for specifying tab as a field separator is borrowed from here. Also see this.

Using Perl modules

• Let the first line in the .pm file be package Red::Green::Blue;.
• The .pm file should be named Blue.pm.
• The .pm file must be stored at dir/Red/Green/Blue.pm.
• In the perl program where you want to use this module, include use lib "dir";. This tells Perl to also look in dir when searching for modules. (this basically adds an entry to @INC).
  
• In the same program, after the previous line, include use Red::Green::Blue;. This tells Perl to find path/Red/Green/Blue.pm for all paths in @INC, including dir.

When this will not work -

• If you get the error Can't locate loadable object for module qwe, then the module is a non-pure Perl module and uses external code (e.g. C code), which should come in a .so file.
  
• The easy way is to do sudo cpan Red::Green::Blue and install the module.
• If you do this, sure to remove the file dir/Red/Green/Blue.pm so that Perl does not use it instead of the installed module.

Perl tips - Unicode

First, use Encode;

Reading/Writing

• $string = Encode::decode('UTF-8',$text); (assuming the input file (or STDIN) is encoded in UTF-8).
• You can now handle $string as you would normal strings (e.g. split(//) will split it at character boundaries)
• Do $text = Encode::encode('UTF-8', $string); before writing it out to file (assuming you want the output file (or STDOUT) in that encoding).

Regex

• \p{L} - full glyph (e.g. the letter 'A')
  
• \p{M} - partial glyph (e.g. the accent ` on the letter 'A', giving 'À')
  
• \p{N} - digit
• \p{P} - punctuation
• \p{kannada} - any Kannada character
• \P{} - invert the condition

E.g. to match a line if it contains no numerals and no punctuation do
$line ~= m/\p{N}|\p{P}/

Backup and restore using SimpleBackup

I am using SimpleBackup for backing up my home directory. Download the utility and extract into some directory. Then do the following -

Backup

• First, cd to that the SimpleBackup directory.
• Run simplebackupconfig. You get a dialog where you can configure some stuff - mainly, the directories to be backed up, and the directories to be excluded from backup, and the directory where the backup files (which will be in .tar.gz form) should be stored.
• Change the script bkup at line if [ $intdayofwk = 1 -o ! -e "$timestampfile" -o $lastfullbkage -gt 7 ]; to e.g. if [ ! -e "$timestampfile" -o $lastfullbkage -gt 30 ]; to do incremental backup unless it has been 30 days since the last full backup.

Restore

• Decide on the directory or file to restore. cd to /. Run tar -xzf bkupfile.tar.gz --anchored "home/gtholpadi/Desktop/Files/some/dir".
  
• This will create the directories home, gtholpadi etc. till dir, and also the directories in dir, if they don't already exist.
• If extracted files already exist, they are overwritten.
• Other files are kept intact.
• Replace dir with file to extract file.

Update: I have now moved to rsync.

Perl tips - array of records, command line params, STDIN/STDOUT handles

set of records or 2D array

• To store a set of records, use  my @data; $data[i]->[j] = $someval; (ith record, jth field). This basically works similar to a 2-D array except that each row can have different size.
• To sort the records based on column j, do @sorteddata = sort {$a->[j] <=> $b->[j]} @data;.

Command line parameters

• The format for specifying parameters is cmd OPT1=sth OPT2=sth ...
• Use while(@ARGV){ if(m{OPT1=}){ use $'; }elsif(m{OPT2}){...}}.
  

Others

• Use $opstream = *STDOUT; (note the '*') to pass (both read and write) file handles around and use them via variables (e.g. print $opstream "qwe";).
• To get floor of a number, do use POSIX; $val = POSIX::floor($num);.

Perl tips - creating your own utility library using classes

- Creating your own library of utility functions

• Create a file MyUtils.pm
• For each new class in it, start with package MyUtils::MyClass.
• Create static class attributes using our $attr.
• Create methods using sub meth1{ my @args = @_; ... }. If this will be used as an object method (not static), remember to treat first entry of @_ as reference to object.
• Object is nothing but a normal data type (like hash or list) and is allocated in the new method (that you must define) and returned.
  
• The new method should be like sub new {my ($class, $otherargs) = @_; my ;...; my $ref_to_somedata; bless , $class; return $ref_to_somedata;}.
• Start with package MyUtils::MyClass::MySubClass to create a subclass.
  

Accessing your library MyUtils.pm

• First include use lib "path/to/dir/containing/MyUtils.pm"; use MyUtils; in the beginning of the .pl file.
• Use MyUtils::MyClass::meth1 to access static methods.
• Use $MyUtils::MyClass::attr to access static attribute $attr of the class (note the $).
• Use MyUtils::MyClass->new(...) to create an object.
• Use MyUtils::MyClass::MySubClass wherever you are using MyUtils::MyClass for subclasses.
  

Other useful stuff

• Use $ENV{env_variable} to access the value of environment variable $env_variable in Perl code, (useful for specifying "path/to/dir/" of .pm file).
  

Some LaTeX tips

- Useful commands

• \footnote{} for adding footnotes in text (numbered)
  
• \titlenote{} for adding footnotes to titles, subtitles etc. (*, dagger etc.)
• \bibliographystyle{abbrv} to get short author names in the References
• \bibliography{<.bib file>}
• \fbox{} for enclosing text within a box 
• To get a new line/line break after the paragraph title, use \paragraph{}\<space>\\
• To hyperlink text, use \href{http://my.link.org}{link text} (uses thepackage hyperref)

- About \begin{figure}

• \begin{figure/table}[h] - the [h] will try to force the figure/table to appear at that place in the text
• \epsfig{file=sth.eps, scale=0.25} - quick way to include .eps image.
• Ensure that label comes immediately after caption, i.e. stick to the order: \caption{}\label{}\end{figure} when including figure. [Updated]

- Numbering parts of figures

• Add \usepackage{subfig}
• Put \subfloat[]{ \label{} \epsfig{}} \subfloat{...} ... within \begin{figure} ... \end{figure}

- About table/tabular

• Use \begin{center}\begin{tabular}... if u dont need full-fledged table
• If u have \begin{tabular}{|c|c||c|c|c|}, use \multicolumn{2}{|c||}{text} & \multicolumn{3}{|c|}to merge the first 2 and the next 3 columns.
• Use longtable instead of tabular to allow the table to be split over two pages.
  

- For multiple authors, use this -

\numberofauthors{4}
\author{
auth1 \and auth2 \and auth3 \and auth4 \and \affaddr{institutename}\\
\affaddr{address}\\
\email{\texttt{\{auth1,auth2, auth3, auth4\}@institute.edu}}
}

-To spell check TeX files, use command ispell <.tex file>.