Conditional Logic

• Conditional logic in shell scripting is used to execute commands based on specific conditions. This allows scripts to make decisions, enabling dynamic and flexible execution paths.

Types of Conditional Statements

1. if Statement

2. if-else Statement

3. if-elif-else Statement

if Statement

• The if statement runs a block of commands if the condition evaluates to true.

• The condition is enclosed under a pair of square brackets [ ].

    if [ condition ]; then
        commands
    fi
  

if-else Statement

• The if-else statement adds an alternative set of commands to run when the condition is false.

    if [ condition ]; then
        commands_if_true
    else
        commands_if_false
    fi
  

if-elif-else Statement

• The if-elif-else statement is used for multiple conditions.

    if [ condition1 ]; then
        commands_if_condition1
    elif [ condition2 ]; then
        commands_if_condition2
    else
        commands_if_all_fail
    fi
  

Note:

• Comparison statements or conditions must be enclosed within a pair of square brackets [ ], and there should be at least one space between the brackets and the statement as well as between the operator and the values on either sides.

  [ string1 = string2 ]

  

• We can also use double pair of square brackets [[ ]] to specify the conditions but it only works in bash:

  [[ string1 = string2 ]]

  • This works similar to above and its an enhanced version and supports additional operations like matching patterns using expressions.

• AND (&&) and OR (||) Operators are there to combine multiple conditions:

  

• Below are some file-level operators:

  

• Use -z <variable> to check for command line argument.

    if [ -z $month ]; then                    # this checks if the $month is empty
            echo "No month number given"
    fi
  

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Loops

• The loop in shell scripting is used to iterate over a sequence of items, performing a set of commands for each item.

• There are 2 types of loops: for and while.

For Loop

• for loops is used when you want to run:

  • Same command multiple times.

  • Iterate through files

  • Iterate through lines within a file

  • Iterate through the output of a command

• Syntax:

    for variable in list; do
        commands
    done
  

  • variable: A placeholder that takes the value of each item in the list during iteration / gives the value of current iteration.

  • list: A sequence of items (e.g., numbers, strings, or file names).

  • commands: Commands to execute for each item in the list.

• Break and Continue:

  • break: Exit the loop immediately.

  • continue: Skip the current iteration and proceed to the next.

Ways to use for loop

1. Iterating Over a List of Words:

    #!/bin/bash
    for item in apple orange banana; do
        echo "Fruit: $item"
    done
   

2. Iterating Over Numbers:

    #!/bin/bash
    for num in 1 2 3 4 5; do
        echo "Number: $num"
    done
   

3. Using a Range of Numbers:

    #!/bin/bash
    for i in {1..5}; do
        echo "Count: $i"
    done
   

4. Using a Range with Step:

    #!/bin/bash
    for i in {1..10..2}; do
        echo "Step: $i"
    done
   

5. Iterate through list of files from the output of the ls command:

    #!/bin/bash
    for file in $(ls)
    do
     echo Line count of $file is $(cat $file | wc -l)
    done
   

6. Iterating Over Files:

    #!/bin/bash
    for file in *.txt; do
        echo "Processing $file"
    done
   

7. Iterate through list of packages stored in a file and installs them one by one:

    #!/bin/bash
    for package in $(cat install-packages.txt)
    do
     sudo apt-get –y install $package
    done
   

8. C-Style for Loop:

    #!/bin/bash
    for ((i=1; i<=5; i++)); do        # need to use double parentheses
        echo "Value: $i"
    done
   

While Loop

• While loop works same as the for loop except that it executes the loop as long as the condition is true.

• It is ideal for situations where the number of iterations is not predetermined and depends on dynamic conditions.

• Syntax:

    while [ condition ]; do
        commands
    done
  

  • condition: A condition that determines whether the loop should continue.

  • commands: The block of code to be executed repeatedly as long as the condition is true.

• Break and Continue:

  • break: Exit the loop immediately.

  • continue: Skip the current iteration and proceed to the next.

Ways to use while loop

1. Basic while Loop:

    #!/bin/bash
    count=1
    while [ $count -le 5 ]; do
        echo "Count: $count"
        ((count++))
    done
   

2. Reading a File Line by Line:

    #!/bin/bash
    filename="example.txt"
    while read -r line; do
        echo "Line: $line"
    done < "$filename"
   

   • The read -r command reads each line from the file.

   • The < "$filename" redirects the file's contents into the loop.

3. Infinite Loop:

    #!/bin/bash
    while true; do
        echo "Press Ctrl+C to stop"
        sleep 1
    done
   

4. Loop with a Condition:

    #!/bin/bash
    num=10
    while [ $num -gt 0 ]; do
        echo "Countdown: $num"
        ((num--))
    done
   

---

Case Statements

• The case statement in shell scripting is used to simplify the evaluation of multiple conditions.

• It is a more concise and readable alternative to if-elif-else structures when need to compare a single variable or expression against multiple patterns.

• Syntax:

    case variable in
        pattern1)
            commands
            ;;
        pattern2)
            commands
            ;;
        *)
            default_commands
            ;;
    esac
  

  • variable: The value to be evaluated.

  • pattern1, pattern2, ...: Patterns to match the variable's value.

  • *): The default case, executed if no patterns match.

  • ;;: Indicates the end of a case.

Ways to use case statement

1. Simple Menu:

    #!/bin/bash
    echo "Enter a number (1-3):"                # can also be written as:
    read num                                    # read -p "Enter a number (1-3):" num
   
    case $num in
        1)
            echo "You selected 1"
            ;;
        2)
            echo "You selected 2"
            ;;
        3)
            echo "You selected 3"
            ;;
        *)
            echo "Invalid selection"
            ;;
    esac
   

2. File Type Check:

    #!/bin/bash
    file="example.txt"
   
    case $file in
        *.txt)
            echo "This is a text file."
            ;;
        *.jpg | *.png)
            echo "This is an image file."
            ;;
        *.sh)
            echo "This is a shell script."
            ;;
        *)
            echo "Unknown file type."
            ;;
    esac
   

3. Day of the Week:

    #!/bin/bash
    day=$(date +%A)
   
    case $day in
        Monday | Tuesday | Wednesday | Thursday | Friday)
            echo "It's a weekday."
            ;;
        Saturday | Sunday)
            echo "It's the weekend!"
            ;;
        *)
            echo "Unknown day."
            ;;
    esac
   

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