env

env is a shell command for Unix and Unix-like operating systems. It is used to either print a list of environment variables or run another utility in an altered environment without having to modify the currently existing environment. Using env, variables may be added or removed, and existing variables may be changed by assigning new values to them.

In practice, env has another common use. It is often used by shell scripts to launch the correct interpreter. In this usage, the environment is typically not changed.

Examples[edit]

To clear the environment (creating a new environment without any existing environment variables) for a new shell:

env -i /bin/sh

To launch the X application xcalc and have it appear on a different display:

env DISPLAY=foo.bar:1.0 xcalc

Here is the code of a very simple Python script:

#!/usr/bin/env python2
print "Hello World."

In this example, /usr/bin/env is the full path of the env command. The environment is not altered.

Note that it is possible to specify the interpreter without using env, by giving the full path of the python interpreter. A problem with that approach is that on different computer systems, the exact path may be different. By instead using env as in the example, the interpreter is searched for and located at the time the script is run. This makes the script more portable, but also increases the risk that the wrong interpreter is selected because it searches for a match in every directory on the executable search path. It also suffers from the same problem in that the path to the env binary may also be different on a per-machine basis.

See also[edit]

• Set (command)

External links[edit]

• env: set the environment for command invocation – Commands & Utilities Reference, The Single UNIX® Specification, Issue 7 from The Open Group
• env -- manual page from GNU coreutils.
• env(1): run a program in a modified environment – OpenBSD General Commands Manual

source - https://en.wikipedia.org/wiki/Env

JavaScript Variables

JavaScript Variables

JavaScript variables are containers for storing data values.

In this example, x, y, and z, are variables:

From the example above, you can expect:

• x stores the value 5
• y stores the value 6
• z stores the value 11

Much Like Algebra

In this example, price1, price2, and total, are variables:

In programming, just like in algebra, we use variables (like price1) to hold values.

In programming, just like in algebra, we use variables in expressions (total = price1 + price2).

From the example above, you can calculate the total to be 11.

	JavaScript variables are containers for storing data values.

JavaScript Identifiers

All JavaScript variables must be identified with unique names.

These unique names are called identifiers.

Identifiers can be short names (like x and y), or more descriptive names (age, sum, totalVolume).

The general rules for constructing names for variables (unique identifiers) are:

• Names can contain letters, digits, underscores, and dollar signs.
• Names must begin with a letter
• Names can also begin with $ and _ (but we will not use it in this tutorial)
• Names are case sensitive (y and Y are different variables)
• Reserved words (like JavaScript keywords) cannot be used as names

	JavaScript identifiers are case-sensitive.

The Assignment Operator

In JavaScript, the equal sign (=) is an "assignment" operator, not an "equal to" operator.

This is different from algebra. The following does not make sense in algebra:

In JavaScript, however, it makes perfect sense: it assigns the value of x + 5 to x.

(It calculates the value of x + 5 and puts the result into x. The value of x is incremented by 5.)

	The "equal to" operator is written like == in JavaScript.

JavaScript Data Types

JavaScript variables can hold numbers like 100, and text values like "John Doe".

In programming, text values are called text strings.

JavaScript can handle many types of data, but for now, just think of numbers and strings.

Strings are written inside double or single quotes. Numbers are written without quotes.

If you put quotes around a number, it will be treated as a text string.

Declaring (Creating) JavaScript Variables

Creating a variable in JavaScript is called "declaring" a variable.

You declare a JavaScript variable with the var keyword:

After the declaration, the variable has no value. (Technically it has the value of undefined)

To assign a value to the variable, use the equal sign:

You can also assign a value to the variable when you declare it:

In the example below, we create a variable called carName and assign the value "Volvo" to it.

Then we "output" the value inside an HTML paragraph with id="demo":

	It's a good programming practice to declare all variables at the beginning of a script.

One Statement, Many Variables

You can declare many variables in one statement.

Start the statement with var and separate the variables by comma:

A declaration can span multiple lines:

Value = undefined

In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input.

A variable declared without a value will have the value undefined.

The variable carName will have the value undefined after the execution of this statement:

Re-Declaring JavaScript Variables

If you re-declare a JavaScript variable, it will not lose its value.

The variable carName will still have the value "Volvo" after the execution of these statements:

JavaScript Arithmetic

As with algebra, you can do arithmetic with JavaScript variables, using operators like = and +:

You can also add strings, but strings will be concatenated (added end-to-end):

Also try this:

	If you add a number to a string, the number will be treated as string, and concatenated.

source - http://www.w3schools.com/js/js_variables.asp

JavaScript Performance

How to speed up your JavaScript code.

Reduce Activity in Loops

Loops are often used in programming.

Each statement in a loop, including the for statement, is executed for each iteration of the loop.

Search for statements or assignments that can be placed outside the loop.

The bad code accesses the length property of an array each time the loop is iterated.

The better code accesses the length property outside the loop, and makes the loop run faster.

Reduce DOM Access

Accessing the HTML DOM is very slow, compared to other JavaScript statements.

If you expect to access a DOM element several times, access it once, and use it as a local variable:

Reduce DOM Size

Keep the number of elements in the HTML DOM small.

This will always improve page loading, and speed up rendering (page display), especially on smaller devices.

Every attempt to search the DOM (like getElementsByTagName) will benefit from a smaller DOM.

Avoid Unnecessary Variables

Don't create new variables if you don't plan to save values.

Often you can replace code like this:

With this:

Delay JavaScript Loading

Putting your scripts at the bottom of the page body, lets the browser load the page first.

While a script is downloading, the browser will not start any other downloads. In addition all parsing and rendering activity might be blocked.

	The HTTP specification defines that browsers should not download more than two components in parallel.

An alternative is to use defer="true" in the script tag. The defer attribute specifies that the script should be executed after the page has finished parsing, but it only works for external scripts.

If possible, you can add your script to the page by code, after the page has loaded:

Avoid Using with

Avoid using the with keyword. It has a negative effect on speed. It also clutters up JavaScript scopes.

The with keyword is not allowed in strict mode.

source - http://www.w3schools.com/js/js_performance.asp

HTML <script> defer Attribute

 HTML <script> tag

Definition and Usage

The defer attribute is a boolean attribute.

When present, it specifies that the script is executed when the page has finished parsing.

Note: The defer attribute is only for external scripts (should only be used if the src attribute is present).

Note: There are several ways an external script can be executed:

• If async is present: The script is executed asynchronously with the rest of the page (the script will be executed while the page continues the parsing)
• If async is not present and defer is present: The script is executed when the page has finished parsing
• If neither async or defer is present: The script is fetched and executed immediately, before the browser continues parsing the page

source - http://www.w3schools.com/tags/att_script_defer.asp

HTML <script> async Attribute

 HTML <script> tag

Definition and Usage

The async attribute is a boolean attribute.

When present, it specifies that the script will be executed asynchronously as soon as it is available.

Note: The async attribute is only for external scripts (and should only be used if the src attribute is present).

Note: There are several ways an external script can be executed:

• If async is present: The script is executed asynchronously with the rest of the page (the script will be executed while the page continues the parsing)
• If async is not present and defer is present: The script is executed when the page has finished parsing
• If neither async or defer is present: The script is fetched and executed immediately, before the browser continues parsing the page

source - http://www.w3schools.com/tags/att_script_async.asp