Replacing Bash with Python?

Since I'm very new to Python, but already familiar with Bash, I asked google search if it is a good idea to use Python instead of Bash.

It seems that generally Bash can be replaced with Python, integrating Bash and Python functionality. It is generally said that programming in Python is easier and more convenient than programming in Bash.
However, there are cases when it is rationally recommended to stick with Bash instead of Python:

"I agree that bash's syntax is pretty awful except for the one thing it does very well, which is execute programs (and pipe their output into other programs). Its syntax is very optimized for that. And yes, implementing find(1) yourself in Python is more work than just using find directly. :)"

(Ted Mielczarek, comment on jlebar: Replacing Bash)

When migrating from Bash to Python while still learning Python, it is recommended to do it slowly, taking one step at a time, replacing only those pieces in your bash script, that are easier to do in Python than in Bash:

1. Replace AWK and PERL with Python. Leave everything else alone.
2. Look at replacing GREP with Python. This can be a bit more complex, but your version of GREP can be tailored to your processing needs.
3. Look at replacing FIND with Python loops that use os.walk. This is a big win because you don't spawn as many processes.
4. Look at replacing common shell logic (loops, decisions, etc.) with Python scripts.

(S.Lott, answer to "Can I use Python as a bash replacement?" at stackoverflow)

RedHat Magazine has an enjoyable article by Noah Gift about working your way through the learning process. When reading it, the whole migration seems so easy...

It is also a good idea to get familiar with some Python modules that can help in replacing Bash with Python:

• subprocess - an implemented module
• sh - a non implemented module
• plumbum - a library for shell script-like programs in Python

There is also an interactive Python shell that might replace the linux shell for you:

• IPython

Notes on Python for Informatics Chapter 3: Conditional execution

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What is Conditional execution?

Conditional execution is a building block of programs. A condition determines if a part of the program will run at all.

• conditional statement: A statement that controls the flow of execution depending on some condition.
• condition: The boolean expression in a conditional statement that determines which branch is executed.
• boolean expression: An expression whose value is either True or False.
• branch: One of the alternative sequences of statements in a conditional statement. 

Types of conditional execution

The most simple conditional statement has only one branch:

if x > 0:
    print 'x is positive'

More complex versions are:

• alternative execution: A conditional statement with two alternative branches.

if x%2 == 0 :
    print 'x is even'
else :
    print 'x is odd'

• chained conditional: A conditional statement with a series of alternative branches. 

if x < y:
    print 'x is less than y'
elif x > y:
    print 'x is greater than y'
else:
    print 'x and y are equal'

• nested conditional: A conditional statement that appears in one of the branches of another conditional statement. 

if x == y:
    print 'x and y are equal'
else:
    if x < y:
        print 'x is less than y'
    else:
        print 'x is greater than y'

What type of Conditions can be used?

• comparison operator: One of the operators that compares its operands:
• <        # strictly less than
• <=      # less than or equal
• >        # strictly greater than
• >=      # greater than or equal
• ==      # equal
• !=       # not equal. (<> was used in old version)
• logical operator: One of the operators that combines boolean expressions:
•  x or y      # if x is false, then y, else x
• This is a short-circuit operator, so it only evaluates the second argument if the first one is False.
• x and y     # if x is false, then x, else y
• This is a short-circuit operator, so it only evaluates the second argument if the first one is True.
• not x    # if x is false, then True, else False
• not has a lower priority than non-Boolean operators, so not a == b is interpreted as not (a == b), and a == not b is a syntax error.
• short circuit: When Python is part-way through evaluating a logical expression and stops the evaluation because Python knows the final value for the expression without needing to evaluate the rest of the expression.
• guardian pattern: Where we construct a logical expression with additional comparisons to take advantage of the short circuit behavior. 
• i.e. if we want to avoid zero division, we could put a !=0 condition on the divisor.

Compound Statements 

• compound statement: A statement that consists of a header and a body. The header ends with a colon (:). The body is indented relative to the header.
• body: The sequence of statements within a compound statement.

Examples:

• if statement is used for conditional execution
• while statement is used for repeated execution
• for statement is used for repeated execution
• try statement is used forexception handling

Syntax:

• Indenting and dedenting is a grammar rule of Python. 
• A compound statement's body is always indented relative to the header. 
• To set the end of the body, the code should be dedented. 
• When indenting, whitespace should be used consequently: it is required to use only one type of whitespace within a single code file (4 spaces or one tab for one level of indentation).

Handling (catching) exceptions

It is possible to resolve foreseen errors with the try statement.
Unresolved errors make the program quit with a traceback massage (if it was run from a script file).

• traceback: A list of the functions that are executing, printed when an exception occurs. 

inp = raw_input('Enter Fahrenheit Temperature:')
try:
    fahr = float(inp)
    cel = (fahr - 32.0) * 5.0 / 9.0
    print cel
except:
    print 'Please enter a number'

Reserving space in code structure for later usage

It is possible to leave parts of code to be written later by reserving the space in the code structure with the pass statement:

if x < 0 : 
    pass # need to handle negative values!

Notes on Python for Informatics Chapter 2: Variables, expressions and statements

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There are different value types. 

• type: A category of values. The types we have seen so far are integers (type int), floating-point numbers (type float), and strings (type str).
• value: One of the basic units of data, like a number or string, that a program manipulates.

The type of a value can be retrieved with the type() built-in function.
Examples:

• int - integer number i.e. 1, 2
• float - floating point number i.e. 1.0, 1.187694652
• str - string text i.e. "1", "Hello World!"
• bool - boolean (either true or false) i.e. True, False (note the case!)

Explanation:

• string: A type that represents sequences of characters.
• integer: A type that represents whole numbers.
• floating-point: A type that represents numbers with fractional parts.

Read more on Python's Built-in Types 
Read more on Python's Built-in Functions

Variables represent values.

• variable: A name that refers to a value.
• assignment: A statement that assigns a value to a variable.

Examples:

• x = 1
• x = 1.0
• x = "one"
• x = True

It is generally a good idea to use mnemonic variable names that refer to the content of the variable.

• mnemonic: A memory aid. We often give variables mnemonic names to help us remember what is stored in the variable.

Rules regarding variable names:

• They have to begin with a letter (exception: variable names can start with an underscore character but this for is generally used for library code.)
• They can contain letters (both upper and lowercase) and numbers and the underscore character (_)
• They are case sensitive (it is suggested to use only lowercase letters in variable names) - just like everything else in Python
• They can be arbitrarily long

The reserved words (keywords) of Python cannot be assigned as variable names.

• keyword: A reserved word that is used by the compiler to parse a program.

Read the full list of Python's keywords

Comments can be made on the program code.

• comment: Information in a program that is meant for other programmers (or anyone reading the source code) and has no effect on the execution of the program.

A comment starts with the # character, and lasts until the end of the line.
Examples:

• # This whole line is a comment
• x = 1 # The part before the "#" will run as a code, but the rest is a comment.

Statements

• statement: A section of code that represents a command or action.

Examples:

• x = 3.5 # assignment statement
• print x # print statement

Operators and Operands

• operator: A special symbol that represents a simple computation like addition, multiplication, or string concatenation. 
• operand: One of the values on which an operator operates.

Different operations can be run on differen types of values.
Examples of numeric type operators:

• + performs addition
• - performs subtraction
• * performs multiplication
• / performs division (always in Python 3) or floor division depending on the value type (in Python 2)
• // performs always floor division
• % performs division and results the remainder of the division
• ** performs exponentiation

Explanation:

• floor division: The operation that divides two numbers and chops off the fraction part.
• modulus operator: An operator, denoted with a percent sign (%), that works on integers and yields the remainder when one number is divided by another.

Examples of sequence type operators:

• + performs concatenation

• concatenate: To join two operands end-to-end.  I.e. "2" + "3" concatenates to "23"

Read more on numeric types and their operators.
Read more on sequence types and their operators.

Expressions

• expression: A combination of variables, operators, and values that represents a single result value. 

Examples:

• x # if x already has a value assigned
• x + 1 # if x already has a value assigned
• 1
• 20 + 32 * (x - 1) / (x * 60 ) + x * x / 60 * 5**2 - ( 5 + 9 ) * ( 15 - 7 )

• evaluate: To simplify an expression by performing the operations in order to yield a single value.

Examples, taken that x = 5

• x # will yield 5
• x + 1 # will yield 6
• 1 # will yield 1
• 20 + 32 * (x - 1) / (x * 60 ) + x * x / 60 * 5**2 - ( 5 + 9 ) * ( 15 - 7 ) # will yield -92 because of the rules of precedence.

• rules of precedence: The set of rules governing the order in which expressions involving multiple operators and operands are evaluated.

For mathematical operators, Python follows mathematical convention. The acronym PEMDAS is a useful way to remember the rules:

• Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first, 2 * (3-1) is 4, and (1+1)**(5-2) is 8. You can also use parentheses to make an expression easier to read, as in (minute * 100) / 60, even if it doesn’t change the result.
• Exponentiation has the next highest precedence, so 2**1+1 is 3, not 4, and 3*1**3 is 3, not 27.
• Multiplication and Division have the same precedence, which is higher than Addition and Subtraction, which also have the same precedence. So 2*3-1 is 5, not 4, and 6+4/2 is 8, not 5.
• Operators with the same precedence are evaluated from left to right. So in the expression 5-3-1 is 1, not 3 because the 5-3 happens first and then 1 is subtracted from 2.

User input

The user can be prompted to input data through the keyboard with the raw_input() built-in function.

Reserved words of Bash

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!         case      esac      in        while
[[        do        fi        select   
]]        done      for       then   
{         elif      function  time   
}         else      if        until   

Reserved words of Python

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and del from not while as elif global or with assert else if pass yield break except import print class exec in raise continue finally is return def for lambda try

Notes on Python for Informatics Chapter 1: Why should you learn to write programs?

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What does a programmer do:

• knows a programming language (i.e. Python) - "words", "sentences", "paragraphs", etc.
• "tells a story": combine the text elements is a way that provides a general "idea" to the "reader". In programming, our program is the “story” and the problem you are trying to solve is the “idea”.

Computer hardware architecture

The high-level definitions of these parts are as follows:

• The Central Processing Unit (or CPU) is that part of the computer that is built to be obsessed with “what is next?”. If your computer is rated at 3.0 Gigahertz, it means that the CPU will ask “What next?” three billion times per second. You are going to have to learn how to talk fast to keep up with the CPU.
• The Main Memory is used to store information that the CPU needs in a hurry. The main memory is nearly as fast as the CPU. But the information stored in the main memory vanishes when the computer is turned off.
• The Secondary Memory is also used to store information, but it is much slower than the main memory. The advantage of the secondary memory is that it can store information even when there is no power to the computer. Examples of secondary memory are disk drives or flash memory (typically found in USB sticks and portable music players).
• The Input and Output Devices are simply our screen, keyboard, mouse, microphone, speaker, touchpad, etc. They are all of the ways we interact with the computer.
• These days, most computers also have a Network Connection to retrieve information over a network. We can think of the network as a very slow place to store and retrieve data that might not always be “up”. So in a sense, the network is a slower and at times unreliable form of Secondary Memory

The building blocks of programs

These constructs are part of every programming language from machine language up to the high-level languages.

• input:
  Get data from the “outside world”. This might be reading data from a file, or even some kind of sensor like a microphone or GPS. In our initial programs, our input will come from the user typing data on the keyboard.
• output:
  Display the results of the program on a screen or store them in a file or perhaps write them to a device like a speaker to play music or speak text.
• sequential execution:
  Perform statements one after another in the order they are encountered in the script.
• conditional execution:
  Check for certain conditions and execute or skip a sequence of statements.
• repeated execution:
  Perform some set of statements repeatedly, usually with some variation.
• reuse:
  Write a set of instructions once and give them a name and then reuse those instructions as needed throughout your program.

General types of errors:

• Syntax errors:
  These are the first errors you will make and the easiest to fix. A syntax error means that you have violated the “grammar” rules of Python. Python does its best to point right at the line and character where it noticed it was confused. The only tricky bit of syntax errors is that sometimes the mistake that needs fixing is actually earlier in the program than where Python noticed it was confused. So the line and character that Python indicates in a syntax error may just be a starting point for your investigation.
• Logic errors:
  A logic error is when your program has good syntax but there is a mistake in the order of the statements or perhaps a mistake in how the statements relate to one another. A good example of a logic error might be, “take a drink from your water bottle, put it in your backpack, walk to the library, and then put the top back on the bottle.”
• Semantic errors:
  A semantic error is when your description of the steps to take is syntactically perfect and in the right order, but there is simply a mistake in the program. The program is perfectly correct but it does not do what you intended for it to do. A simple example would be if you were giving a person directions to a restaurant and said, “... when you reach the intersection with the gas station, turn left and go one mile and the restaurant is a red building on your left.”. Your friend is very late and calls you to tell you that they are on a farm and walking around behind a barn, with no sign of a restaurant. Then you say “did you turn left or right at the gas station?” and they say, “I followed your directions perfectly, I have them written down, it says turn left and go one mile at the gas station.”. Then you say, “I am very sorry, because while my instructions were syntactically correct, they sadly contained a small but undetected semantic error.”.

Terminology:

• high-level language:
  A programming language like Python that is designed to be easy for humans to read and write.
• machine language / low-level language:
  A programming language that is designed to be easy for a computer to execute; also called “machine code” or “assembly language.” The lowest level language for software which is the language that is directly executed by the central processing unit (CPU).
• compile:
  To translate a program written in a high-level language into a low-level language all at once, in preparation for later execution.
• interpret:
  To execute a program in a high-level language by translating it one line at a time.
• portability:
  A property of a program that can run on more than one kind of computer.
• program:
  A set of instructions that specifies a computation.
• source code:
  A program in a high-level language.
• bug:
  An error in a program.
• interactive mode:
  A way of using the i.e. Python interpreter by typing commands and expressions at the prompt.
• parse:
  To examine a program and analyze the syntactic structure.
• prompt:
  When a program displays a message and pauses for the user to type some input to the program.
• semantics:
  The meaning of a program.