will_paginate with Bootstrap

2019 Jan 29

will_paginate doesn’t come with Bootstrap style pagination by default. However, as its doc says, it does support customization by providing your own LinkRenderer.

to customize HTML output of will_paginate, you’ll need to subclass WillPaginate::ActionView::LinkRenderer

Below is a simple implementation to render pagination links with Bootstrap(4) css components.

# app/helpers/application_helper.rb

module ApplicationHelper
  def will_paginate(collection_or_options = nil, options = {})
    if collection_or_options.is_a? Hash
      options, collection_or_options = collection_or_options, nil
    unless options[:renderer]
      options = options.merge :renderer => BootstrapRenderer
    super *[collection_or_options, options].compact

  class BootstrapRenderer < WillPaginate::ActionView::LinkRenderer
    def html_container(html)
      tag :nav, tag(:ul, html, class: "pagination pagination-sm"), container_attributes

    def page_number(page)
      tag :li, link(page, page, rel: rel_value(page), class: 'page-link'),
        class: (page == current_page ? 'page-item active': 'page-item')

    def previous_or_next_page(page, text, classname)
      tag :li, link(text, page || '#', class: 'page-link'),
        class: ['page-item', classname, ('disabled' unless page)].join(' ')
# app/views/posts/index.html.erb

# use 'text-center' to center inline-blocks (<ul>) within <nav>
<%= will_paginate @posts, params: @will_paginate_params, class: 'text-center' %>

Daily Dev Log: "su - app" vs. "su app"

2019 Jan 24

From man su,

   -, -l, --login
      Provide an environment similar to what the user would expect had the user logged in directly.

So with su - app, after switch to the user app, you end up in the user’s HOME directory, and have the user’s ~/.bash_profile (not ~/.bashrc) executed.

Tools like RVM need a “login shell”.

RVM by default adds itself currently to ~/.bash_profile file

So if use su app, RVM will not be ready there for you after su.

Daily Dev Log: Avoid the Pitfall of Using the Same File to Redirect Input and Output

2019 Jan 15


Do Not Use the Same File to Redirect Input and Output

tr -d '\015' <DOS-file >DOS-file

The above command will delete all content in the file!

From man bash,

[n]>word, if it does exist it is truncated to zero size.

(How did I find the file back? Luckily, the working directory is managed by Dropbox, and I found it back in the Dropbox.)


Convert Line Endings from DOS/Windows Style to Unix/Linux Style

tr -d '\015' <DOS-file >UNIX-file

(For what character \015 is, see man 7 ascii or ascii '\015' if the ascii command is installed.)

More ascii Command Examples

$ ascii '\r'
ASCII 0/13 is decimal 013, hex 0d, octal 015, bits 00001101: called ^M, CR
Official name: Carriage Return
C escape: '\r'
Other names: 

Search Manuals

-k Search the short descriptions and manual page names for the keyword

$ man -k ascii
ascii (1)            - report character aliases
ascii (7)            - ASCII character set encoded in octal, decimal, and hexadecimal

Fix a Maven Dependency Conflict

2019 Jan 4

The Dependency Conflict Cannot be Resolved

A project I work on is using both “com.google.sitebricks:sitebricks”, a now inactively under development web framework, and “org.drools:drools-compiler” as its dependencies, both of which then depend on “org.mvel:mvel2”. Maven used to find a version of mvel2 to satisfy both of the two dependencies.

However, when the “drools-compiler” dependency is upgraded to a newest version (“7.13.0.Final”), something unfortunate happens. The “drools-compiler:7.13.0.Final” uses a mvel2 version, which is incompatible with the one sitebricks uses. The “drools-compiler” uses some new APIs from the newer version of mvel2, unfortunately, that version of mvel2 deletes some APIs “sitebricks” uses. In this case, Maven cannot resolve the dependency conflict easily since there is NO version of mvel2 to satisfy “drools-compiler:7.13.0.Final” and “sitebricks:*”.

A Fix

Use Maven Shade Plugin to package sitebricks and its mvel2 dependency into a “shaded jar”, and also use this plugin to “relocate” mvel2 classes inside the “shaded jar”. “Relocation” here is to move mvel2 classes from package “org.mvel2” to some other package like “org.shaded.mvel2”, and to also modify bytecode of some sitebricks classes, which refers mvel2 classes, correspondingly, so that these “mvel2 classes” do not conflict with those used by “drools-compiler”.

Create a Maven project/module for the “shaded jar”.


In the project, use the “sitebricks-shaded” as a dependency instead.


And one more step.

Miss Newline Characters When "cat" Text Files

2019 Jan 4

The cat is often used to concatenate text files into one single file. In most cases, the cat works fine like below.

$ echo line 1 > file1.txt
$ echo line 2 > file2.txt
$ cat file{1,2}.txt
line 1
line 2

However, if some of files to be concatenated don’t end with the newline character, using cat to concatenate files may not generate expected file.

# -n, let echo not add the trailing newline character
$ echo -n line 1 > file1.txt
$ echo line 2 > file2.txt
$ cat file{1,2}.txt
line 1line 2

Note that in the above example, file1.txt doesn’t end with newline, so when two files concatenated there is no newline between them. This may not be the expected result. For example, we have multiple large text files. Every line in each file is a user ID. We want to concatenate these files into one file to be fed into a processing program at once. If some of files are not ended with newline, using cat may generate ill user IDs like user-id-foouser-id-bar. If the input volume is huge, these problematic IDs usually would not be detected by human eyes.

If the newlines between files is important in your case, using awk is safer.

# -n, let echo not add the trailing newline character
$ echo -n line 1 > file1.txt
$ echo line 2 > file2.txt
$ $ awk 1 file{1,2}.txt
line 1
line 2

See this SO answer.

Also, it’s a good idea to tune text editors to always show non-printable characters like the newline. Or, use cat -e, which prints invisible characters and a $ for the newline.

$ cat -e file1.txt | tail -1

grep Command Examples

2019 Jan 1

First, grep –help lists most of its options, which is the go-to command for most grep questions.

Like most CLI tools, options of grep can be combined. For example, -io is same as -i -o, -A3 is same as -A 3. Also, the options can be anywhere in the command.

$ grep hello a.txt -i --color

Stop after first match

$ grep -m 1 search-word file

-m, –max-count=NUM stop after NUM matches

Only print the 1000th match.

$ grep -m1000 search-word file | tail -n1
$ grep -l search-word *.txt

-l, –files-with-matches print only names of FILEs containing matches

It’s useful when you grep lots of files and only care about names of matched files.

Find unmatched files

-L, –files-without-match print only names of FILEs containing no match

-L is the opposite of -l option. It outputs the files which don’t contain the word to search.

$ grep -L search-word *.txt

Show line number of matched lines

$ grep -n search-word file

-n, –line-number print line number with output lines

Don’t output filename when grep multiple files

When grep multiple files, by default filename is included in the output. Like,

$ grep hello *.txt

Use -h to not output filenames.

$ grep -h hello *.txt

-h, –no-filename suppress the file name prefix on output

Search in “binary” files

Sometimes, a text file may contains a few non-printable characters, which makes grep consider it as a “binary” file. grep doesn’t print matched lines for a “binary” file.

$ printf "hello\000" > test.txt
$ grep hello test.txt 
Binary file test.txt matches

Use -a to let grep know the file should be seen as a “text” file.

$ grep -a hello test.txt 

-a, –text equivalent to –binary-files=text

Search in directories

-r, –recursive like –directories=recurse

-R, –dereference-recursive likewise, but follow all symlinks

Without specifying a directory, grep searches in current working directory by default.

$ grep -R hello

Specify directories.

$ grep -R hello tmp/ tmp2/

–include=FILE_PATTERN search only files that match FILE_PATTERN

Use --include to tell grep the pattern of the filenames you’re interested in.

$ grep -R hello --include="*.md"

-i, –ignore-case ignore case distinctions

$ grep -i Hello a.txt 

The pattern to search begins with - (hyphen)

$ grep -- -hello a.txt

To know what -L option does.

$ grep --help | grep -- -L
  -L, --files-without-match  print only names of FILEs containing no match

Daily Dev Log: Find Lines in One File but Not in Another

2018 Dec 12

We can use comm to find lines in one file but not in another file

# fine lines only in file-a
comm -23 file-a file-b

From comm --help,

-2 suppress column 2 (lines unique to FILE2)

-3 suppress column 3 (lines that appear in both files)

So to find lines exist in both file-a and file-b.

comm -12 file-a file-b

Google keywords: “linux command two file not contain” hit link

Inject a Method Interceptor in Guice

2018 Sep 4

I recently made a mistake to new an object (MethodInterceptor) in an plain old way in a Guice configuration, which caused the object’s @Inject-annotated fields, like Logger for example, were initialized with null value.

public class FooInterceptor implements MethodInterceptor{
  private Logger logger;

  public Object invoke(MethodInvocation invocation) throws Throwable {
    // NPE if logger not inject correctly
  	logger.info("start to invoke"); 
  	return invocation.proceed();

public class BarModule extends AbstractModule {
	protected void configure() {	
      // new the interceptor in the plain old way
      new FooInterceptor()); 


Guice wiki clearly states that requestInjection() should be used for injection of a “method interceptor”.

How do I inject a method interceptor?

In order to inject dependencies in an AOP MethodInterceptor, use requestInjection() alongside the standard bindInterceptor() call.

public class NotOnWeekendsModule extends AbstractModule {
  protected void configure() {
    MethodInterceptor interceptor = new WeekendBlocker();
    // for injection of a "method interceptor"
    bindInterceptor(any(), annotatedWith(NotOnWeekends.class), interceptor);

Some Thoughts

Once you decide to use a dependency injection framework, like Guice here, please DO NOT new Java objects in the plain old way any more. Otherwise, you may very possibly fail to set up the objects’ dependencies correctly.

Secondly, use constructor-injection for mandatory dependency, like Logger here. It’s impossible to forget to inject a dependency using constructor-injection, even if that object is constructed in the plain old way. (However, too many dependencies injected via the constructor makes the constructor look a bit ugly.)

Browsers Ignore Change in Hosts File

2018 Aug 2

I modified C:\Windows\System32\drivers\etc\hosts for a local test. However, the browser did not respect to the change in hosts file. At last, I found it’s due to the proxy settings in my machine. Change in hosts took effect once after I unchecked all proxy configuration in Control Panel -> Internet Options -> Connections -> LAN settings.

If unfortunately in your network, connections are proxied by force (for example, in a corporate network), you can try to let the proxy bypass some domains by adding the domains into LAN settings -> Proxy server -> Advanced -> Exceptions.

Bug Tracking and Feature Planning

2018 Jul 27

Now OneFeed is using Trello to track bugs and plan features, though it’s a tiny application in any respect. A nice thing of having a bug tracking system is that history is logged, which can be used for future reference.

Trello is a simple tool to create/manage boards, lists, and cards. And based on these basic elements, one can build his/her own workflows.

trello usecase

Trello has a free plan, which is enough for OneFeed to track its bugs. The free plan allows users to add one Power-Up (add-on to extend Trello’s capability) per board. For OneFeed’s boards, Custom Fields Power-Up is added, so that cards can have custom fields like “Bug ID” and “Feature ID”, which can be used for example in Git commit messages.

bug card

Also Trello has mobile apps for accessing from anywhere :)