Benchmark CPU in SHELL

$ time echo "scale=5000; a(1)*4" | bc -l

brackets, parentheses, curly braces in BASH

In Bash, test and [ are builtins.

The double bracket enables additional functionality. For example, you can use && and || instead of-a and -o and there's a regular expression matching operator =~.

The braces, in addition to delimiting a variable name are used for parameter expansion so you can do things like:

• Truncate the contents of a variable
  $ var="abcde"; echo ${var%d*}
abc
• Make substitutions similar to sed
  $ var="abcde"; echo ${var/de/12}
abc12
• Use a default value
  $ default="hello"; unset var; echo ${var:-$default}
hello
• and several more

Vim folding, auto fold xml, fold, unfold

To allow folds based on syntax add something like the following to your .vimrc:

set foldmethod=syntax
set foldlevelstart=1

let javaScript_fold=1         " JavaScript
let perl_fold=1               " Perl
let php_folding=1             " PHP
let r_syntax_folding=1        " R
let ruby_fold=1               " Ruby
let sh_fold_enabled=1         " sh
let vimsyn_folding='af'       " Vim script
let xml_syntax_folding=1      " XML




zo -- To open a fold block

zc -- To fold a block

zr -- Fold less

zm -- Fold more

Python multiple thread processing

from multiprocessing import Pool
pool = Pool(processes=5)
pages = pool.map(visit, get_lines(file))

What every programmer should know…

What Every Computer Scientist Should Know About Floating-Point Arithmetic

What Every Programmer Should Know About Memory

What every programmer absolutely, positively needs to know about encodings and character sets to work with text

Latency Numbers Every Programmer Should Know

What every programmer should know about time

What Every Programmer Should Know About Races

What Every C Programmer Should Know About Undefined Behavior

What every data programmer needs to know about disks

Above contents are from http://wangcong.org/blog/archives/2255

List recently modified files under a directory in linux shell

In order to list files that have been modified recently, we could use the find command to retrieve the file information and sort them by modified date:

find . -type f -exec stat --format '%Y :%y %n' {} \; | sort -nr | cut -d: -f2- | head

However, this will miss some folders if the folders are symoblic links. So in this case we could specify find to follow symbolic links.

find -L . -type f -exec stat --format '%Y :%y %n' {} \; | sort -nr | cut -d: -f2- | head

Fourther more, if you just want to get files modified in last a few days, it's build in in find:

find . -mtime n

list files that modified n*24 hours ago.

The difference between soft link and hard link in linux

From: http://linuxgazette.net/105/pitcher.html

Unix files consist of two parts: the data part and the filename part.

The data part is associated with something called an 'inode'. The inode carries the map of where the data is, the file permissions, etc. for the data.

                               .---------------> ! data ! ! data ! etc
                              /                  +------+ !------+
        ! permbits, etc ! data addresses !
        +------------inode---------------+

The filename part carries a name and an associated inode number.

                         .--------------> ! permbits, etc ! addresses !
                        /                 +---------inode-------------+
        ! filename ! inode # !
        +--------------------+

More than one filename can reference the same inode number; these files are said to be 'hard linked' together.

        ! filename ! inode # !
        +--------------------+
                        \
                         >--------------> ! permbits, etc ! addresses !
                        /                 +---------inode-------------+
        ! othername ! inode # !
        +---------------------+

On the other hand, there's a special file type whose data part carries a path to another file. Since it is a special file, the OS recognizes the data as a path, and redirects opens, reads, and writes so that, instead of accessing the data within the special file, they access the data in the file named by the data in the special file. This special file is called a 'soft link' or a 'symbolic link' (aka a 'symlink').