#+TITLE: Study Note
#+AUTHOR: Peng Li
#+EMAIL: seudut@gmail.com
#+DATE: 2016-08-18

* Programming Languages
** Perl 
*** One line command
**** perl help
#+BEGIN_SRC sh :exports both :results output replace
  perl --help
#+END_SRC

#+RESULTS:
#+begin_example

Usage: perl [switches] [--] [programfile] [arguments]
  -0[octal]         specify record separator (\0, if no argument)
  -a                autosplit mode with -n or -p (splits $_ into @F)
  -C[number/list]   enables the listed Unicode features
  -c                check syntax only (runs BEGIN and CHECK blocks)
  -d[:debugger]     run program under debugger
  -D[number/list]   set debugging flags (argument is a bit mask or alphabets)
  -e program        one line of program (several -e's allowed, omit programfile)
  -E program        like -e, but enables all optional features
  -f                don't do $sitelib/sitecustomize.pl at startup
  -F/pattern/       split() pattern for -a switch (//'s are optional)
  -i[extension]     edit <> files in place (makes backup if extension supplied)
  -Idirectory       specify @INC/#include directory (several -I's allowed)
  -l[octal]         enable line ending processing, specifies line terminator
  -[mM][-]module    execute "use/no module..." before executing program
  -n                assume "while (<>) { ... }" loop around program
  -p                assume loop like -n but print line also, like sed
  -s                enable rudimentary parsing for switches after programfile
  -S                look for programfile using PATH environment variable
  -t                enable tainting warnings
  -T                enable tainting checks
  -u                dump core after parsing program
  -U                allow unsafe operations
  -v                print version, patchlevel and license
  -V[:variable]     print configuration summary (or a single Config.pm variable)
  -w                enable many useful warnings
  -W                enable all warnings
  -x[directory]     ignore text before #!perl line (optionally cd to directory)
  -X                disable all warnings
  
Run 'perldoc perl' for more help with Perl.

#+end_example

**** Used as =grep=
#+BEGIN_SRC sh
  perl -ne 'print if /expression/' xxx.log;
#+END_SRC

#+BEGIN_SRC perl :results output replace
  my $tt = "hello";
  ($tt =~ /h(.*)o/ ) and print $1;
#+END_SRC

#+RESULTS:
: ell

Or 
#+BEGIN_SRC sh
  perl -wnl -e '/keyword/ and print;' xxx.log
#+END_SRC

**** Used as =awk=
#+BEGIN_SRC sh
  perl -ne 'print "$1\n" if /aaa=(\d+)\s.*/' xxx.log;
#+END_SRC

#+BEGIN_SRC fundamental :tangle ./temp/aa.txt
  aa: 1
  bb: 2
  cc: 3
  dd: 4
#+END_SRC

#+BEGIN_SRC sh :results output replace
  perl -wnl -e 'print "==$.: $_"' ./temp/aa.txt
#+END_SRC

#+RESULTS:
: ==1: 
: ==2: aa: 1
: ==3: bb: 2
: ==4: cc: 3
: ==5: dd: 4

*** Special Variable
See special variable in perl [[http://perldoc.perl.org/perlvar.html][perlvar]], or =perldoc perlvar=
- =$.= Current line number

** gnuplog

1. normal
#+BEGIN_SRC gnuplot :exports code :file ./temp/sin.png
  reset
  plot sin(x)/x
#+END_SRC

#+RESULTS:
[[file:./temp/sin.png]]

2. plot with data file
#+BEGIN_SRC fundamental :tangle ./temp/plot.dat 
  # x y
  1 11
  2 12
  3 13
  4 14
  5 15
  6 16
  7 17
  8 18
  9 19
#+END_SRC

#+BEGIN_SRC gnuplot :file ./temp/ll.png
  reset
  set xrange [0:10]
  set yrange [0:20]
  plot "./temp/plot.dat" using 1:2 with lines
#+END_SRC

#+RESULTS:
[[file:./temp/ll.png]]

* TCP/IP
** netstat
#+BEGIN_SRC sh :results output replace
  netstat -atn    # For tcp port
  netstat -aun    # For udp port
  netstat -atun   # both
#+END_SRC

** nc (netcat)
#+BEGIN_SRC sh :results output replace
  # listening on port 1234 for a connection
  nc -l 1234

  # connect to the machine and port
  nc 127.0.0.1 1234
#+END_SRC

* Emacs
** =add-hook= vs =eval-after-load=
Ref [[http://stackoverflow.com/questions/2736087/eval-after-load-vs-mode-hook][eval-after-load-vs-mode-hook]]
#+BEGIN_QUOTE
Code wrapped in =eval-after-load= will be executed only once, so it is typically used to perform
one-time setup such as setting default global values and behaviour. An example might be setting
up a default keymap for a particular mode. In =eval-after-load= code, there's no notion of the 
"current buffer".

Mode hooks exectue once for every buffer in which the mode is enabled, so they're used for per-buffer
configuration. Mode hooks are therefore run later than =eval-after-load= code; this lets them take 
actions baed upon such information as whether other modes are enabled in the current buffer.
#+END_QUOTE