Bash Shell Basics

• The default shell in the WebIDE is Zsh, we'll be writing Bash scripts in this lab. Bash (Bourne Again SHell) is the most common and widely used shell in Unix-like systems. Zsh (Z Shell) is an extended version of Bash with some improvements and features.

• In Shell, if we don’t see any output that generally means the command is successful.

• #!/bin/bash : This is call ‘shebang’ line. This is to tell the system to interprete as bash shell script.

  Below is the 1st Bash Shell program.

    #This is for cheating the hello.sh file
    labex:project/ $ touch hello.sh 
  
    ##Inside the hello.sh file we write the below shell script which is our 1st shell script
  
    #!/bin/bash
    echo 'Hello, World!'
  
    ## Below is the command to execute it.
    #This is to give execute permission for that file via chmod and x is execute
    labex:project/ $ chmod +x hello.sh
  
    ##Below is the way to run the hello.sh file
    labex:project/ $ ./hello.sh 
  
    Hello, World! #This is the output
  
    ## cat is for see the content of the file
    labex:project/ $ cat hello.sh
    #!/bin/bash
    echo 'Hello, World!'
  

##Writing variables in shell
labex:project/ $ touch /home/labex/project/variables.sh
labex:project/ $ chmod +x /home/labex/project/variables.sh

##Inside variable.sh
#!/bin/bash

PRICE_PER_APPLE=5
MyFirstLetters=ABC
greeting='Hello        world!'

echo "Price per apple: $PRICE_PER_APPLE"
echo "My first letters: $MyFirstLetters"
echo "Greeting: $greeting"

##Let's execute it.
labex:project/ $ ./variables.sh 
Price per apple: 5
My first letters: ABC
Greeting: Hello        world!

##Use escaping charachter and preserving white spaces
#!/bin/bash

PRICE_PER_APPLE=5
MyFirstLetters=ABC
greeting='Hello        world!'

# Escaping special characters
echo "The price of an Apple today is: \$HK $PRICE_PER_APPLE"

# Avoiding ambiguity
echo "The first 10 letters in the alphabet are: ${MyFirstLetters}DEFGHIJ"

# Preserving whitespace
echo $greeting
echo "$greeting"

##Output
labex:project/ $ ./variables.sh
The price of an Apple today is: $HK 5
The first 10 letters in the alphabet are: ABCDEFGHIJ
Hello world!
Hello        world!

Command Substitution: See the below examples to understand.

# Command substitution
CURRENT_DATE=$(date +"%Y-%m-%d")
echo "Today's date is: $CURRENT_DATE"

FILES_IN_DIR=$(ls)
echo "Files in the current directory:"
echo "$FILES_IN_DIR"

UPTIME=$(uptime -p)
echo "System uptime: $UPTIME"

##Below are the output of the above
Today's date is: 2025-10-22
Files in the current directory:
birthdate.sh
variables.sh
System uptime: up 33 weeks, 16 hours, 40 minutes

• $(date +"%Y-%m-%d") runs the date command and captures its output.

• $(ls) runs the ls command and captures its output.

• $(uptime -p) runs the uptime command with the -p option and captures its output.

labex:project/ $ touch /home/labex/project/arithmetic.sh
labex:project/ $ chmod +x /home/labex/project/arithmetic.sh

##Inside arithmatic.sh

#!/bin/bash

X=10
Y=5

# Addition
SUM=$((X + Y))
echo "Sum of $X and $Y is: $SUM"

# Subtraction
DIFF=$((X - Y))
echo "Difference between $X and $Y is: $DIFF"

# Multiplication
PRODUCT=$((X * Y))
echo "Product of $X and $Y is: $PRODUCT"

# Division
QUOTIENT=$((X / Y))
echo "Quotient of $X divided by $Y is: $QUOTIENT"

# Modulus (remainder)
REMAINDER=$((X % Y))
echo "Remainder of $X divided by $Y is: $REMAINDER"

# Increment
X=$((X + 1))
echo "After incrementing, X is now: $X"

# Decrement
Y=$((Y - 1))
echo "After decrementing, Y is now: $Y"

##Below is the output
labex:project/ $ ./arithmetic.sh 
Sum of 10 and 5 is: 15
Difference between 10 and 5 is: 5
Product of 10 and 5 is: 50
Quotient of 10 divided by 5 is: 2
Remainder of 10 divided by 5 is: 0
After incrementing, X is now: 11
After decrementing, Y is now: 4

Using Environment variables such as $HOME, $LOGNAME, $SHELL, $PATH etc.

Note: export is used for creating new enviroment variable and unset is used for removing the environment variable.

labex:project/ $ touch /home/labex/project/environment.sh
labex:project/ $ chmod +x /home/labex/project/environment.sh

##Inside environment.sh
#!/bin/bash

# Displaying some common environment variables
echo "Home directory: $HOME"
echo "Current user: $LOGNAME"
echo "Shell being used: $SHELL"
echo "Current PATH: $PATH"

# Creating a new environment variable
export MY_VARIABLE="Hello from my variable"

# Displaying the new variable
echo "My new variable: $MY_VARIABLE"

# Creating a child process to demonstrate variable scope
bash -c 'echo "MY_VARIABLE in child process: $MY_VARIABLE"'

# Removing the environment variable
unset MY_VARIABLE

# Verifying the variable is unset
echo "MY_VARIABLE after unsetting: $MY_VARIABLE"

##below is the output
labex:project/ $ ./environment.sh 
Home directory: /home/labex
Current user: labex
Shell being used: /usr/bin/zsh
Current PATH: /usr/lib/jvm/java-11-openjdk-amd64/bin:/usr/lib/jvm/java-11-openjdk-amd64/bin:/usr/lib/jvm/java-11-openjdk-amd64/bin:/usr/lib/jvm/java-11-openjdk-amd64/bin:/usr/lib/code-server/lib/vscode/bin/remote-cli:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/home/labex/.local/bin:/usr/local/go/bin:/home/labex/golang/bin:/usr/share/maven/bin:/usr/local/bin:/usr/sbin/nodejs/bin:/usr/sbin/yarn/bin:/home/labex/.local/bin:/usr/local/go/bin:/home/labex/golang/bin:/usr/share/maven/bin:/usr/local/bin:/usr/sbin/nodejs/bin:/usr/sbin/yarn/bin
My new variable: Hello from my variable
MY_VARIABLE in child process: Hello from my variable
MY_VARIABLE after unsetting:

Passing argument in bash Shell

#Inside arguments.sh
## In the below script $0, $1, $2 and $3 are arguments or parameters

#!/bin/bash

# Your code will go here
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "Third argument: $3"

##Output
labex:project/ $ chmod +x arguments.sh

##Below command where we pass the parameters, observe the output
labex:project/ $ ./arguments.sh hello world example 
Script name: ./arguments.sh
First argument: hello
Second argument: world
Third argument: example

Passing argument with condition

#!/bin/bash

##Below '$#' is the special variable which contains the 'number of argument'
if [ $# -eq 0 ]; then
  echo "No arguments provided."
elif [ $# -eq 1 ]; then
  echo "One argument provided: $1"
elif [ $# -eq 2 ]; then
  echo "Two arguments provided: $1 and $2"
else
  echo "More than two arguments provided:"
  echo "First argument: $1"
  echo "Second argument: $2"
  echo "Third argument: $3"
  echo "Total number of arguments: $#"
fi

##Output
labex:project/ $ ./arguments.sh 
No arguments provided.
labex:project/ $ ./arguments.sh one
One argument provided: one
labex:project/ $ ./arguments.sh one two
Two arguments provided: one and two
labex:project/ $ ./arguments.sh one two three four
More than two arguments provided:
First argument: one
Second argument: two
Third argument: three
Total number of arguments: 4

• $# is a special variable that contains the number of arguments passed to the script.

• [ $# -eq 0 ] is a condition that checks if the number of arguments is equal to 0.

• elif is short for "else if". It allows you to check multiple conditions in sequence.

• The -eq operator means "equal to". There are other operators like -lt (less than), -gt (greater than), etc.

Loop through all arguments

#!/bin/bash

echo "Total number of arguments: $#"
echo "All arguments:"
## $@ is a special variable contains all the arguments, then there is a for loop
count=1
for arg in "$@"; do
  echo "Argument $count: $arg"
  count=$((count + 1))
done

##Output
labex:project/ $ ./arguments.sh apple banana cherry date
Total number of arguments: 4
All arguments:
Argument 1: apple
Argument 2: banana
Argument 3: cherry
Argument 4: date