Shell Scripting

• Shell vs Bash
• Unix had the Bourne Shell, which was modified in Linux and is called Bash (Bourne Again SHell).
• FYI: Unix vs Linux
• Shell scripting crash course (beginners)

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• The # is called ‘sharp’ and the ! is called a ‘bang’, so together (#!) they’re called a ‘shebang’ or ‘hashbang’.
• A shebang is used to declare the dialect being used by the script.
• To find the location of the dailect, use the ‘which’ command. Eg: which bash Output: /usr/bin/bash

• Add this as the first line in every Shell script.

    #!/usr/bin/bash
  

• Running/executing a Shell script
  • Two ways
    • ./script.sh (Preferred, as the terminal can decide the interpreter to be used.)
    • sh script.sh
  • Difference between ./script.sh and sh script.sh.

• Output to terminal

    echo Selena Gomez
  

• Variables
  • Uppercase by convention
  • Allowed: letters, numbers and underscores

    NAME="Harsh Kapadia"  # no spaces between var name, '=' and var value allowed.
    echo "My name is $NAME"
    echo "My name is ${NAME}"
  

  • Strings in bash can be defined with ' and " delimiters, but they are not equivalent. Strings delimited with ' are literal strings and will not substitute variable values whereas " delimited strings will.

    $ name=selena
  
    $ echo 'Hey ${name}!'
    Hey ${name}!
  
    $ echo "Hey ${name}!"
    Hey Selena!
  	
    $ echo "Hey '${name}'!"
    Hey 'Selena'!
  

  • ${} and $() are different.
    • ${abc} is variable/parameter substitution.
    • $(abc) is command substitution and will execute whatever is inside the (), which means it is usually used to run a command or store the output of a command.
    • More info and good practices

• User input

    read -p "Enter your name: " NAME # 'p' stands for prompt
    echo "Hey $NAME, nice to meet you!"
  

• If statement

    if [ "$NAME" == "Selena Gomez" ] # The spaces are important!
    then
        echo "Hey Sel!"
    fi # ending if statement
  

• If-elif-else statement

    if [ "$NAME" == "Selena Gomez" ] # The spaces are important!
    then
        echo "Selly!"
    elif [ "$NAME" == "Selena" ]
    then
        echo "Yay it's Sel!"
    else
        echo "Hiya!"
    fi # ending if statement
  

• Comparisons
  • val1 -eq val2: Returns true if the values are equal
  • val1 -ne val2: Returns true if the values are not equal
  • val1 -gt val2: Returns true if val1 is greater than val2
  • val1 -ge val2: Returns true if val1 is greater than or equal to val2
  • val1 -lt val2: Returns true if val1 is less than val2
  • val1 -le val2: Returns true if val1 is less than or equal to val2

    NUM_1=22
    NUM_2=7
    NUM_3=1992
  
    if [ "$NUM_1" -gt "$NUM_2" ]
    then
        echo "$NUM_1 is greater than $NUM_2!"
    fi
  

• File conditions
  • -d file True if the file is a directory
  • -e file True if the file exists (note that this is not particularly portable, thus -f is generally used)
  • -f file True if the provided string is a file
  • -g file True if the group id is set on a file
  • -r file True if the file is readable
  • -s file True if the file has a non-zero size
  • -u True if the user id is set on a file
  • -w True if the file is writable
  • -x True if the file is an executable

    FILE="text.txt"
  
    if [ -f "$FILE" ]
    then
        echo "$FILE is a file!"
    else
        echo "$FILE is not a file : ("
    fi
  

• Case statements

    read -p "Are you 21 or over? Y/N " ANSWER
  
    case "$ANSWER" in
        [yY] | [yY][eE][sS]) # Accepts 'y', 'Y', 'yes', 'YES', 'Yes', 'yEs'...
            echo "You can have a beer ( :"
            ;; # ';;' is equivalent to a 'break' statement in C language
        [nN] | [nN][oO])
            echo "Sorry, no drinking"
            ;;
        *) # Default
            echo "Please enter y/yes or n/no"
            ;;
    esac
  

• For loop

    NAMES="Selena Felix Lucas Evelien Sean Elisabeth Robert Sonny"
  
    for NOME in $NAMES
        do
            echo "Hello $NOME! ( :"
    done
  	
    # touch file_1.md file_2.md file_3.md
    FILES=$(ls *.md)
  
    for FOLE in $FILES
        do
            echo "Renaming $FOLE to test_$FOLE"
            mv $FOLE test_$FOLE
    done
  

• While loop

    # Read from file, line by line
    LINE=1
  
    while read -r CURRENT_LINE
        do
            echo "$LINE: $CURRENT_LINE"
            ((LINE++)) # '(())': https://www.geeksforgeeks.org/basic-operators-in-shell-scripting
    done < "./test_f1.md"
  

• String manipulation
  • Delete the shortest substring match from the front of ${str}: ${str#substring}
  • Delete the shortest substring match from the back of ${str}: ${str%substring}
  • Delete the longest substring match from the front of ${str}: ${str##substring}
  • Delete the longest substring match from the back of ${str}: ${str%%substring}

    $ str="Hey Selena! Hey Lucas!"
  	
    $ echo ${str#*Hey}
    Selena! Hey Lucas!
  	
    $ echo ${str%Hey*}
    Hey Selena! 
  	
    $ echo ${str##*Hey}
    Lucas!
  	
    $ echo ${str%%Hey*}
  	
  

The asterisk (*) is a catchall to match any and all characters. (More info)

• Internal Field Separator (IFS)
  • Used to separate a string based on a certain character.
  • Default value: Three character string comprising a space, tab and newline.

    $ string="foo bar foobar"
  	
    $ for i in $string
    > do
    >    echo "'$i' is the substring"
    > done
    'foo' is the substring 
    'bar' is the substring
    'foobar' is the substring
  

  • Custom values can be set.
  • More info and use cases
  • Unset IFS - unexpected behaviour

• Functions

    function greet()
    {
        echo "Hello $1! Your age is $2!"
    }
    greet "Selena" "29"
  

• Create a folder and write to a file

    mkdir "hello"
    touch "hello/world.md"
    echo "# Hello World" >> "hello/world.md"
    echo "Created hello/world.md"
    echo "Contents:"
    cat "hello/world.md"