Linux & Server Administration

Finding files

$ grep -rnw '/path/to/somewhere/' -e 'pattern'

• -r or -R is recursive,
• -n adds line numbers,
• -w stands for match the whole word,
• -l (lower-case L) can be added to just give the file name of matching files

Ref: How do I find all files containing specific text on Linux? - https://stackoverflow.com/questions/16956810/how-do-i-find-all-files-containing-specific-text-on-linux

Network

Show ports based on PID of a process

$ sudo netstat -tulpn | grep ${PID}

Show information about CPU

$ cat /proc/cpuinfo

Check the connection to the server and open port on a specific remote computer

$ telnet SERVER_ADDRESS PORT

Connect to a remote server, the best way through the command 'ssh'

$ ssh USER@SERVER_ADDRESS

Less

Less is a program similar to more (1), but which allows backward movement in the file as well as forward movement. Useful commands when browsing logs:

• Waiting for latest lines at EOF - SHIFT + F
• Finish of waiting at EOF - CTRL + C
• Search for a specific text - /${TEXT}
• Leave or finish - Q

Nano

GNU nano is a small and friendly text editor. Besides basic text editing, nano offers many extra features like an interactive search and replace, go to line and column number, auto-indentation, feature toggles, internationalization support, and filename tab completion.

• Search for a specific text - CTRL + W

Grep

grep is a command-line utility for searching plain-text data sets for lines that match a regular expression. Its name comes from the ed command g/re/p (globally search a regular expression and print), which has the same effect: doing a global search with the regular expression and printing all matching lines. Grep was originally developed for the Unix operating system, but later available for all Unix-like systems.

grep -E option is for extended regexp. If you use the grep command with -E option, you just need to use | to separate multiple patterns for the or condition.

$ ps aux | grep -E 'ssh|PID'

It is useful for the showing the title of listing processes when filter for a specific command.

Copying on Mac OS X

To copy files on Mac OS X on a remote server can be used FileZilla program, because it uses a secure FTPS protocol. Another good alternative to browsing the file system is e.g. Double Commander (similar to Total Commander) and Midnight Commander.

• https://filezilla-project.org/
• https://doublecmd.sourceforge.io/
• https://midnight-commander.org/

Naming conventions

Many-to-many relationship = joining tables

Coming up with a good name for a table that represents a many-to-many relationship makes the relationship easier to read and understand. Sometimes finding a great name is not trivial but usually it is worth to spend some time thinking about.

An example: Reader and Newspaper.

A Newspaper has many Readers and a Reader has many Newspapers

You could call the relationship NewspaperReader but a name like Subscription might convey better what the table is about.

The name Subscription also is more idiomatic in case you want to map the table to objects later on.

The convention for naming many-to-many tables is a concatenation of the names of both tables that are involved in the relation. ColourShape would be a sensible default in your case.

Ref: StackOverflow: What should I name a table that maps two tables together? - http://stackoverflow.com/questions/1813321/what-should-i-name-a-table-that-maps-two-tables-together

JPA & Hibernate

Lazy loading problem

Also beware of the n+1-problem. Hibernate will not actually load all children when you access the collection. Instead, it will load each child individually. When iterating over the collection, this causes a query for every child. In order to avoid this, you can trick hibernate into loading all children simultaneously, e.g. by calling parent.getChildren().size().

public void iterateOverChildren() {
  Set<Child> children = parent.getChildren(); // Still contains proxy
  children.size(); // Force initialization

  for (Child child : children) {
    child.doSomething();
  }
}

or fetching a collection in HQL query:

Query query = session.createQuery("from Parent p join fetch p.Children ch");

Ref: StackOverflow: What is lazy loading in Hibernate? - http://stackoverflow.com/questions/2192242/what-is-lazy-loading-in-hibernate

Sometimes a proxy or collection needs to be initialized before closing the Session. You can force initialization by calling cat.getSex() or cat.getKittens().size(), for example. However, this can be confusing to readers of the code and it is not convenient for generic code.

Ref: Hibernate docs: Chapter 20. Improving performance - https://docs.jboss.org/hibernate/orm/3.5/reference/en/html/performance.html#performance-fetching-initialization

Multiple fetches with EAGER type in Hibernate with JPA

Error log:

    Caused by: org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags

The root cause of the problem is that when Hibernate fetches SQL query results there is no simple way to tell which child element belongs to which collection. o summarize you have following workarounds:

• Load each collection separately using subselect @Fetch(FetchMode.SELECT)
• Force usage of list instead of bag by adding index column @IndexColumn(name="LIST_INDEX")
• Use unordered collection like Set

Ref: StackOverflow: Multiple fetches with EAGER type in Hibernate with JPA - http://stackoverflow.com/questions/17566304/multiple-fetches-with-eager-type-in-hibernate-with-jpa

Zend Framework

How to create a Zend project using command lines. Firstly, go to project home directory and do the following:

1. zf show version
2. zf create project your-project
3. zf enable layout
4. zf create controller AlbumController
5. zf create db-table Albums albums
6. zf create form Album
7. zf create action login Auth

Ref: ZendFramework - Manual: Using Zend_Tool On The Command Line - http://framework.zend.com/manual/1.12/en/zend.tool.usage.cli.html