Seeni

Software Engineer

Programming With Shell for Automation

Most of the programmers use programming languages such as C,C++, Python, Javascript. They use shell for many tasks . But most of them don’t know that they can program with shell for automating stuff that we find
ourselves doing over and over again.
Though shell is limited in number of tokens, its power lies within the
hands of the developer. He/She can write a program and install it and
can use it shell just like any other command. How awesome is that ? So
next time you need something like this, just make a coffee and start
coding….

Dive In

In this article , I will share some basic things about shell that is
necessary to program. Before That, Lets do a quick Review of most used
commands
ls — List the files and directories in the Current directory or in the given directory
pwd — Print the current working directory
mkdir — Make a directory with a name given in the current directory
rm — Remove specified files and folders
seq — Produce a sequence from start to stop with increment steps
cd — Change the current directory
cat — Prints the contents of the file
echo — Prints the string passed to it in the terminal
There are plenty of more commands and above are enough for enough to follow. If you need to learn more or study above commands in detail, Feel free to do so.

Lets Get Started

Let’s Answer this question
What are things needed in a programming Language for basic stuff ?
Answer is …
  1. Variables and Data types
  2. Control Statements [ if, while, for ..]
  3. Arithmetic Expressions and Boolean Expressions
Well, above are the things needed to do basic automation But Shell has also got
  1. Arrays
  2. Regular Expressions and related tools
  3. I/O Functions for interacting with the user and the environment
  4. Functions
  5. Others

Start Note

    shell programs are saved with .sh extension. They can be run with
    #shellname program.sh
    #Example
    sh pro1.sh
    There are many shells such as Z shell, C shell , bash, an lot of others. Most common is bash. Install any if you don’t have any. Ask your
    administrator for permission if you don’t have sufficient rights for
    executing programs.
    This article assumes that you are using bash and you have programming experience.

    Variables and Data types

    Variables in shell can be set by simply typing
    VARIABLE=VALUE
    Examples are
    numberVariable=3;
    realVariable=3.14;
    stringVariable_1=pi;
    stringVariable_2="pi";

    Control Statements [ if, while, for ..]

    If statement
    if[testCondition]
    then
         #codeblock
    fiORif[testCondition]
    then
        #code block
    else
       #codeblock
    fi
    Examples
    if [ $a -eq $b ] # -eq tests whether left operand is equal to the right operand
    then
       echo "a is equal to b"
    else
       echo "a and b are not equal"
    fi
    While Statement
    Like other programming Languages, syntax of while is shell is similar
    while [ testCondition ]
    do
        #code block
    done
    Examples
    i=0
    while [ $i -lt 10 ]
    do
      echo $i;
      i=$((i+1));   #increment the varaible i
    done
    For statement
    for statement syntax is
    for variable in sequence
    do
      #codeblock
    done
    variable is a usual variable whereas sequnce is a list of values separated by spaces.
    Example
    for i in 1 2 3 4 5 6
    do
        echo $i;
    done

    Arithmetic Expressions

    Usual Arithmetic expressions are possible in shell which includes operators +, -, *, / and % and usual hard-cored values and variables (prefixed with $). You should enclose the arithmetic expressions in $((exp)) or any other standard formats. See the code snippet below…
    i=5;
    echo $((1+2));
    echo $(($i-3));
    echo $((7/2));
    echo $((7%5));

    Boolean Expressions

    This is the area where shell differs from most of the languages. Boolean expression is of of the form operand -relop operand
    operand can be hard-core values or variables or themselves boolean expressions and relop can be one of the following:
    • -lt for less than operator ( “<” in other languages )
    • -le for less than or equal to operator ( “≤” in other languages )
    • -gt for greater than operator ( “>” in other languages )
    • -ge for greater than or equal to operator ( “≥” in other languages )
    • -eq for equal to operator ( “=” in other languages )
    • -ne for equal to operator ( “!=” in other languages )
    • -a for logical and operator ( “&&” in other languages )
    • -o for logical or operator ( “||” in other languages )
    • ! expression— for not operator ( “! expression “ in other languages)
    • Look at the example below
      if [ 3 -eq 3 -a 3 -ge 2 ]
      then
          echo "work begins";
      fi
      
      if [ ! 3 -ge 4 ]
      then 
          echo "! 3>4";
      fi
      That’s it for now. We can just take a moment and try to use it in a
      scenario. Next Part is my favourite. I always use that snippet for
      organizing Downloads Directory.
      Sample Program
      Lets us consider a simple scenario. Consider that you have all videos of every season of your favourite TV series in a single Folder and you want to organize them into seasons. Thanks to appropriate Naming (S0xE0x ) of episodes. Following snippet will do the job for you.
      for i in `seq 5` #consider that we have 5 seasons 
      do
          mkdir S$i;  # creating directory
          mv *S$i* S$i;  #moving the files
      done
      f this 4 line script can save that much work, Just think about what else you can do with it.

      End Note

      So if you find yourselves typing some commands or tasks repeatedly, try to make it as a shell program.
      Congratulations! You made it to the end. I hope you found this article useful.
      Check out my profile to find more articles.
      Thank You.
      Happy Coding with Shell ………

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Comments

September 3rd, 2019

Great start, thank you so much <3

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